Day 1: BMS Fundamentals, Drawings & Control Panels

By the end of this module, candidates should be able to explain what a Building Management System is, identify the main building services typically connected to a BMS, and understand the relationship between the BMS and the mechanical and electrical systems within a building.

A Building Management System (BMS) is a computer-based control and monitoring system used to manage the mechanical and electrical services within a building.

You may also hear the terms:

• BEMS — Building Energy Management System
• BAS — Building Automation System
• Building Controls

Although terminology varies, the fundamental purpose is the same.

The BMS receives information from equipment and sensors throughout the building, processes that information using programmed control strategies, and then controls equipment to achieve the required building conditions.

In simple terms:

The BMS monitors what is happening within the building, makes control decisions and operates equipment accordingly.

A BMS can therefore:

Consider an office that needs to be maintained at 21°C.

A room temperature sensor measures the actual room temperature and sends this information to the BMS.

The BMS compares the measured temperature against the required temperature setpoint.

Example

Room temperature: 18°C

Required temperature: 21°C

The BMS determines that additional heating is required.

It may then command a heating valve to open.

Hot water passes through the heating coil and the room temperature begins to increase.

As the room approaches 21°C, the BMS reduces the heating demand and begins closing the valve.

The control process can be represented as:

The BMS continuously repeats this process.

This basic principle applies to many systems throughout a building.

A BMS performs several important functions.

Monitoring

The BMS receives information from the building.

Examples include:

• Room temperature
• Duct temperature
• Water temperature
• Humidity
• CO₂ concentration
• Differential pressure
• Flow
• Fan running status
• Pump running status
• Equipment fault conditions
• Valve position
• Damper position
• Energy consumption

These become BMS points.

Control

The BMS can operate equipment automatically.

Examples include:

• Starting and stopping fans
• Starting and stopping pumps
• Opening and closing valves
• Modulating control valves
• Opening and closing dampers
• Controlling fan speed
• Controlling pump speed
• Enabling boilers
• Enabling chillers
• Enabling heat pumps
• Controlling room temperature
• Maintaining duct pressure
• Maintaining water differential pressure

Scheduling

Building services do not normally need to operate continuously.

The BMS can automatically operate plant according to occupancy schedules.

Schedules can reduce unnecessary plant operation and energy consumption.

Alarming

The BMS monitors equipment and building conditions for abnormal operation.

Typical alarms include:

• Fan failed
• Pump failed
• High temperature
• Low temperature
• Frost condition
• Dirty filter
• High pressure
• Low pressure
• Plant fault
• Sensor failure
• Communications failure

These alarms can be displayed to the building operator and recorded for investigation.

Trending

A BMS can record values over time.

This is known as trending.

For example, the BMS could record a room temperature every 15 minutes.

An engineer can then review how the temperature changed throughout the day.

Trending is extremely useful for:

• Fault finding
• Commissioning
• Investigating complaints
• Energy analysis
• Proving plant performance

A modern BMS can interact with a large number of building-services systems.

Heating Systems

The BMS may control and monitor:

• Boilers
• Air Source Heat Pump
• Heating pumps
• Heating control valves
• LTHW systems
• Buffer vessels
• Heating circuits
• Underfloor heating

Cooling Systems

The BMS may interact with:

• Chillers
• Chilled-water pumps
• Cooling valves
• Cooling circuits
• CRAC units
• Fan-coil units

Ventilation

Ventilation is one of the most common areas of BMS control.

Systems can include:

• Air Handling Units (AHUs)
• Supply fans
• Extract fans
• VAV boxes
• CAV boxes
• MVHR units
• Dampers
• Heating and cooling coils
• Heat recovery systems

Room & Zone Control

The BMS can also control individual spaces.

Examples include:

• Fan-coil units
• VAV systems
• Temperature control
• CO₂-based ventilation
• Occupancy-based control

Other Building Systems

The BMS may also monitor or interface with:

• Electrical meters
• Heat meters
• Water meters
• UPS systems
• Generators
• Fire alarm systems
• Packaged plant
• Leak detection systems
• Refrigeration equipment
• Lighting control systems
• Specialist mechanical equipment

An important distinction must be understood:

The BMS does not necessarily control every system connected to it.

Some equipment may be fully controlled by the BMS.

Other equipment may only provide information to the BMS for monitoring.

Understanding this distinction is fundamental to BMS engineering.

Monitoring

The BMS receives information but does not directly operate the equipment.

For example:

The pump provides a fault signal to the BMS.

The BMS monitors that signal and may generate an alarm.

Control

The BMS sends a command to another device.

For example:

The BMS sends a signal requesting the pump to operate.

Control and Monitoring

Many items of plant have signals travelling in both directions.

A typical pump could have:

If the pump uses a Variable Speed Drive, there could additionally be:

This distinction becomes extremely important during pre-commissioning because every one of these signals needs to be individually proved.

A BMS engineer needs to understand mechanical systems.

You do not need to be a mechanical designer, but you must understand what the equipment you are controlling is supposed to achieve.

Consider an Air Handling Unit.

The mechanical system may contain:

• Supply fan
• Extract fan
• Heating coil
• Cooling coil
• Filters
• Dampers
• Temperature sensors
• Pressure sensors

The BMS connects these individual components together through a control strategy.

For example, the BMS could:

1. Receive a command to start the AHU.
2. Open the fresh-air & Supply dampers
3. Open the Extract and Exhaust dampers
4. Start the supply and extract fans.
5. Prove that the fans are running.
6. Monitor supply-air temperature.
7. Modulate the heating or cooling valve based on temperature control.
8. Adjust fan speed to maintain pressure.
9. Monitor filter condition.
10. Stop the system and generate an alarm if a critical fault occurs.

The mechanical equipment performs the physical work.

The BMS coordinates and controls its operation.

The BMS also relies heavily on electrical systems.

A BMS installation can include:

• 230 VAC power supplies
• 24 VAC power supplies
• 24 VDC power supplies
• Control relays
• Contactors
• Variable Speed Drives
• Digital signals
• Analogue signals
• Network communications

The BMS may issue a command, but electrical equipment ultimately provides power to the plant.

The BMS engineer therefore needs to understand both the control signal and the electrical circuit through which that signal operates.

This is why electrical fault finding is an important part of BMS pre-commissioning.

One of the most important concepts for a new BMS engineer is that a BMS sits in the middle of several different disciplines.

A problem seen on the BMS is not automatically a BMS fault.

For example, the BMS may correctly command a heating valve to 100%, but the room remains cold.

Possible causes could include:

• The BMS output is incorrect.
• The actuator is faulty.
• The actuator has been installed incorrectly.
• The valve is stuck.
• There is no heating water available.
• Poor balancing issues, resulting in poor flow
• The pump is not operating.
• An isolation valve is closed.
• The heating system has another mechanical fault.

The BMS engineer must therefore learn to distinguish between:

BMS faults

Electrical faults

Mechanical faults

Installation faults

Network/communications faults

Plant faults

This is a fundamental part of pre-commissioning.

A BMS commissioning engineer cannot properly commission software and control strategies if the physical installation has not first been proved.

For example, if the BMS commands a heating valve to open but the actuator has been wired incorrectly, the software cannot correct the physical installation.

Likewise, if a temperature sensor is connected to the wrong controller input, the BMS may be receiving a valid temperature — but from the wrong location.

Pre-commissioning establishes confidence that:

The correct device is connected to the correct point, the signal operates correctly, and the physical equipment responds as expected.

This is why pre-commissioning is much more than checking that something appears on a BMS graphic.

At the end of this module candidates should understand five fundamental principles:

1. A BMS monitors and controls building services.

2. Sensors and plant provide information to the BMS.

3. The BMS uses programmed logic to make control decisions.

4. The BMS sends commands to equipment to achieve the required building conditions.

5. Successful BMS engineering requires an understanding of the BMS, electrical installation and mechanical plant — not just the controller.

By the end of this module, candidates should be able to identify the main components that make up a Building Management System, explain the basic function of each component and understand how information and control commands move through the BMS.

Candidates should understand the relationship between:

Candidates should also understand that not every BMS installation requires a central supervisor or head-end.

A smaller standalone installation may simply consist of:

The purpose of this module is to give candidates an understanding of the overall BMS architecture before looking in more detail at I/O types, control panels, drawings and electrical circuits later in the day.

BMS architecture describes how the different components of a Building Management System are arranged, connected and communicate with each other.

A modern BMS can contain hundreds or even thousands of individual points distributed throughout a building.

These points may be associated with equipment such as:

• Temperature sensors
• Pressure sensors
• Flow sensors
• Valves
• Dampers
• Fans
• Pumps
• Variable Speed Drives
• Energy meters
• Packaged plant
• Boilers
• Chillers
• Heat pumps
• Fan-coil units
• VAV units

The BMS requires a structured way of connecting all of this equipment together.

On a larger installation, a simplified architecture may be represented as:

Mechanical & Electrical Plant

However, this does not mean that every BMS installation requires a supervisor.

A smaller standalone system may have no central supervisor at all.

For example, a school boiler room may have a dedicated BMS control panel containing a controller and I/O modules together with a local HMI mounted on or adjacent to the panel.

The architecture may therefore be:

The local HMI allows an operator or engineer to view and interact with the system without requiring a separate central supervisor.

Whether a supervisor is provided depends on the size of the installation, the project specification, the level of central monitoring required and the way the BMS has been designed.

The BMS controllers and I/O modules are usually installed within BMS control panels distributed around the building.

These panels may be located within plantrooms, electrical cupboards, risers or other suitable locations close to the equipment being controlled.

Some devices are physically wired back to a BMS control panel and connected to the controller or I/O modules.

Other intelligent devices communicate with the BMS through a communications network.

Understanding these different arrangements is an important part of understanding BMS architecture.

On larger BMS installations, a supervisor or head-end may be provided.

It may also be referred to as the:

• Head-End
• Supervisor
• BMS Server
• Operator Workstation

The supervisor provides a central interface between the BMS and the people operating the building.

It can allow an operator or commissioning engineer to view information from multiple BMS controllers, panels, floors or plant areas from one location.

Typical supervisor functions include:

• Plant graphics
• Current temperatures and pressures
• Plant running status
• Equipment faults
• Alarm management
• Historical trends
• Setpoint adjustment
• Plant enable/disable commands
• System status
• Network status

For example, an operator may open an AHU graphic and see:

• Supply fan running
• Extract fan running
• Supply-air temperature
• Duct pressure
• Heating-valve position
• Cooling-valve position
• Filter condition
• Damper positions
• Current alarms

The supervisor receives information from controllers located throughout the building.

A Supervisor Is Not Always Required

Not every BMS installation requires a central supervisor.

A smaller standalone installation may consist of:

BMS Controller & I/O

Local HMI ↕ BMS Controller & I/O

Field Devices and Plant

For example, a school boiler room may have a standalone BMS panel with a local HMI used to view:

• Temperatures
• Plant status
• Faults
• Setpoints
• Plant enable commands
• Basic alarms

There may be no requirement for a separate server, workstation or central supervisor.

Whether a supervisor is provided depends on the project design, building size and operational requirements.

The Supervisor and the Pre-Commissioning Engineer

For this course, candidates only need to understand where the supervisor sits within the overall BMS architecture and what its purpose is.

The supervisor itself would not normally form part of the BMS pre-commissioning engineer's field testing activities.

Detailed commissioning of the supervisor, including graphics, alarms, trends, schedules, network integration and supervisory functions, would normally be undertaken by the BMS commissioning engineer.

The pre-commissioning engineer's focus is primarily on the physical installation and field-level operation, including:

• Field devices
• Field wiring
• Panel terminations
• BMS I/O
• Relays
• Actuators
• Plant interfaces
• Physical plant response

Understanding the supervisor is still useful because it allows candidates to understand how the local controllers and field devices fit into the complete BMS system.

The Supervisor Is Not Normally Controlling Every Point Directly

Where a supervisor is provided, it does not normally make every control decision within the building.

The actual control logic for plant is generally located within the BMS controllers.

For example, an AHU controller may contain the logic required to:

1. Receive an enable command.
2. Open the required dampers.
3. Start the fans.
4. Prove the fans are running.
5. Monitor supply-air temperature.
6. Modulate the heating or cooling valve.
7. Control fan speed.
8. Monitor plant faults.
9. Stop the AHU if a critical condition occurs.

The supervisor allows the operator or commissioning engineer to monitor and interact with the system, while the local controller performs the plant control.

BMS controllers are the main control devices within the system.

They are installed throughout the building and are responsible for controlling the mechanical and electrical plant connected to them.

A controller receives information from the building, processes that information using programmed control logic and then sends commands to equipment.

The basic process is:

For example, a controller may receive:

Room Temperature = 18°C

The programmed control logic may compare this against:

Room Temperature Setpoint = 21°C

The controller determines that heating is required and sends a command to the heating valve.

As the room temperature increases, the controller adjusts the valve position accordingly.

The controller is therefore performing the decision-making function discussed in Module 1.

Where Are BMS Controllers Installed?

BMS controllers are usually installed within dedicated BMS control panels.

Depending on the building and the system design, these panels may be installed within:

• Mechanical plantrooms
• Electrical cupboards
• Service risers
• Floor-level control cupboards
• Dedicated BMS panel locations

On larger buildings, there may be many BMS control panels distributed across different floors and plant areas.

Locating controllers close to the plant and field devices they serve can reduce the amount of field wiring required and allows the BMS to be distributed throughout the building.

What Can One Controller Control?

The amount of equipment connected to a controller depends on the design of the BMS.

A controller may control a single item of plant, such as an AHU.

Alternatively, one controller may control several systems.

For example, a controller could be responsible for:

• Heating pumps
• Heating valves
• Differential-pressure control
• Water temperatures
• Plantroom ventilation
• Energy monitoring

Other controllers may be dedicated to room or zone control.

The BMS design determines how the building is divided between controllers.

Controllers Contain Programmed Logic

The controller contains software that determines how the plant should operate.

This may include:

• Start/stop sequences
• Temperature control
• Pressure control
• Pump control
• Valve control
• Damper control
• Duty/standby rotation
• Alarm generation
• Interlocks
• Safety conditions

Pre-commissioning engineers do not necessarily need to write this software.

However, they need to understand what the controller is expected to do so that the physical installation can be correctly tested.

The term I/O means:

Input / Output

I/O provides the interface between the BMS controller and the physical devices installed around the building.

Examples of physical devices connected through BMS I/O include:

• Temperature sensors
• Pressure sensors
• Flow switches
• Differential-pressure switches
• Fan status contacts
• Pump fault contacts
• Relays
• Valve actuators
• Damper actuators
• Plant enable signals

Some BMS controllers have I/O connections built directly into the controller.

Other systems use separate I/O modules connected to the controller.

Additional I/O modules allow the BMS to connect to a larger number of field devices.

A simplified arrangement may therefore look like:

Field Device

For example:

For an output operating plant through a panel-mounted relay:

Where Are I/O Modules Installed?

I/O modules are usually installed within the BMS control panel alongside the controller.

Depending on the BMS architecture, additional or remote I/O modules may also be installed in other panels or locations closer to the equipment they serve.

The field wiring from sensors, switches, actuators and plant interfaces is typically brought back to the BMS control panel.

Inside the panel, the field cables are normally terminated onto terminals before being connected to the appropriate BMS I/O.

This creates an important physical path that the pre-commissioning engineer needs to understand.

For a direct connection:

Where a panel-mounted relay is used:

Later in the day, candidates will be shown photographs of BMS control panels and their internal components, allowing them to identify the controllers, I/O modules, terminals and other equipment typically found within a BMS panel.

At this stage, candidates only need to understand where the controller and I/O sit within the overall BMS architecture and what their purpose is.

The different I/O types — DI, DO, AI, AO and Universal I/O — will be covered in detail in the next module.

Field devices are the devices installed around the building that allow the BMS to interact with the physical environment and mechanical plant.

They can broadly be divided into devices that provide information to the BMS and devices that respond to commands from the BMS.

Devices Providing Information

Examples include:

• Temperature sensors
• Humidity sensors
• CO₂ sensors
• Pressure sensors
• Differential-pressure sensors
• Flow switches
• Differential-pressure switches
• Frost thermostats
• Plant run contacts
• Plant fault contacts

These devices allow the BMS controller to understand what is happening within the building or plant.

Devices Responding to BMS Commands

Examples include:

• Valve actuators
• Damper actuators
• Relays
• Contactors
• Variable Speed Drives
• Pump enable circuits
• Fan enable circuits
• Packaged-plant enable circuits

These devices allow the BMS controller to operate or influence the mechanical and electrical plant.

For example:

The temperature sensor tells the controller what is happening.

The controller processes this information using its programmed control logic.

The controller then sends a command to the valve actuator.

The actuator physically moves the valve, changing the flow of water through the mechanical system.

The same principle can be applied to a damper actuator:

This relationship between field devices, controllers and physical plant is fundamental to BMS operation.

A large building may contain many BMS control panels and controllers distributed across different floors and plantrooms.

Where a central BMS architecture is required, these controllers may communicate with each other and with a supervisor through a BMS network.

A simplified larger-site example may look like:

AHU BMS Panel — Controller 4

The network allows information to be transferred around the BMS.

For example, information from a controller within a plantroom may be made available elsewhere within the wider BMS system.

Networks may also allow information to be shared between controllers.

On a small standalone installation, there may be no site-wide BMS network or supervisor.

Instead, the controller may only communicate with a local HMI and any intelligent devices connected directly to that system.

Common BMS Communication Methods

During their career, candidates are likely to encounter communication technologies and protocols including:

• Ethernet
• IP networks
• BACnet
• Modbus

At this stage it is not necessary to understand how these protocols work in detail.

The important principle is:

A network allows multiple intelligent devices to exchange information without requiring a separate physical cable for every individual point.

Detailed communications and integration testing will be introduced later in the course.

Not every BMS point reaches the controller in the same way.

Two common methods are:

Hardwired connections

and

Network communications

Understanding the difference is important when fault finding.

Hardwired Device

A hardwired device has a physical electrical connection between the field device and the BMS I/O.

The field cable will commonly return to a BMS control panel.

For example, an input may follow this route:

BMS Input

For an output controlling a pump through a relay, the route may be:

Pump / Plant Interface

The relay is typically located within the BMS control panel and allows the BMS output to switch the external plant interface.

Each required signal normally has its own physical wiring arrangement.

If a pump requires:

• Start command
• Run status
• Fault status

these may be three separate hardwired signals.

Each signal can therefore have its own wiring fault.

Networked Device

Some equipment contains its own intelligent controller and can communicate directly with the BMS.

Examples may include:

• Variable Speed Drives
• Energy meters
• Heat meters
• Packaged AHUs
• Boilers
• Chillers
• Heat pumps
• UPS systems

Instead of individually wiring every piece of information, the BMS may communicate with the device over a network.

For example:

The same network connection may provide multiple values such as:

• Voltage
• Current
• Power
• Energy consumption
• Frequency
• Device status

This is fundamentally different from individually hardwiring every value back to the BMS.

Why Does This Matter for Pre-Commissioning?

The method used to connect the device changes how the engineer investigates a problem.

For a hardwired device, the engineer may need to investigate:

For an output using a panel-mounted relay, the engineer may need to follow:

For a networked field device, the physical connection method will be different from a conventional hardwired point.

The pre-commissioning engineer needs to understand how the field device is connected and what should physically happen when it operates.

One of the easiest ways to understand BMS architecture is to follow a single piece of information from the field device towards the controller.

Consider a room temperature sensor.

The sensor measures:

Room Temperature = 22.5°C

The field-level path is:

BMS Controller

This is the part of the architecture that is most relevant to the pre-commissioning engineer.

The engineer needs to prove that the correct physical device is connected to the correct BMS point and that the expected signal reaches the controller.

If the controller is not receiving the correct value, possible causes may include:

• Sensor problem
• Field wiring problem
• Incorrect termination
• Wrong cable connected
• I/O problem
• Incorrect I/O configuration

The engineer should work through the signal path systematically rather than guessing.

Where a supervisor or HMI is installed, the controller information may later be presented through that interface, but detailed commissioning of those supervisory functions is outside the scope of the pre-commissioning role.

Commands also travel from the controller towards the physical plant.

Consider a heating valve.

The controller determines that heating is required.

The field-level sequence may be:

Heating Water Flow Increases

The important point is that the BMS command is only one part of the process.

A controller may correctly command a valve to open, but this does not automatically prove that:

• The correct output has operated.
• The signal has reached the correct panel terminal.
• The correct field cable is connected.
• The correct actuator received the command.
• The actuator physically moved.
• The valve moved with the actuator.
• Heating water is available.
• Water is flowing through the valve.
• The mechanical system responded correctly.

This is why practical pre-commissioning must prove the physical response, not simply the software command.

Don't assume it works. Prove it.

An important feature of many BMS installations is that local controllers can operate independently of a central supervisor.

In some installations, there may be no central supervisor at all.

Consider a boiler-room controller installed within a standalone BMS control panel.

The controller may contain all of the logic required to:

• Enable boilers or heat pumps
• Start pumps
• Control valves
• Monitor water temperatures
• Monitor differential pressure
• Generate alarms
• Operate duty/standby plant
• Apply time schedules
• Stop plant following critical faults

A local HMI may be fitted to the panel to allow an operator or engineer to view and interact with the system locally.

In this arrangement:

Local HMI = Operator Interface

BMS Controller = Local Control

On a larger installation, a supervisor may also be connected to the controller.

The important principle for candidates is that the controller performs the local plant control, while an HMI or supervisor provides an interface to the system.

Detailed commissioning of the supervisor is the responsibility of the BMS commissioning engineer rather than the pre-commissioning engineer.

There is no single BMS architecture that applies to every building.

A large building may contain:

• Main heating plant
• Cooling plant
• AHUs
• Floor-by-floor ventilation
• Fan-coil units
• Energy meters

and multiple BMS control panels distributed throughout the site.

A larger-site architecture may be:

Mechanical & Electrical Plant

A smaller standalone installation may instead be:

Alongside hardwired field devices, controllers may also communicate with intelligent equipment:

VSDs / Meters / Packaged Plant

The exact arrangement depends on the project.

The important point for the pre-commissioning engineer is to understand the field-level architecture of the system being tested.

A pre-commissioning engineer needs to understand the complete physical route between the BMS controller and the equipment installed in the field.

When investigating a point, the engineer should be thinking:

What is the field device?

Where is the device physically installed?

Which BMS control panel does it return to?

Which controller is responsible for it?

Which I/O or network connection is being used?

Is the point hardwired or networked?

What should the controller be seeing?

What should the controller be commanding?

What should physically happen in the field?

This allows the engineer to work through problems logically rather than guessing.

For a hardwired output:

Where a panel-mounted relay forms part of the circuit:

For an input:

This field-level route is the main focus of the pre-commissioning engineer.

A useful fault-finding approach is:

Start with what you know.

Identify what should happen next.

Prove whether it happened.

Move to the next stage.

This approach will be developed throughout the course.

1. BMS architecture describes how the different parts of the Building Management System are connected and communicate.
2. Not every BMS installation requires a central supervisor or head-end.
3. A smaller standalone system may consist of a BMS control panel, controller, I/O modules and a local HMI.
4. Where provided, the supervisor gives a central operator and commissioning interface to the wider BMS.
5. Detailed commissioning of the supervisor is normally undertaken by the BMS commissioning engineer, not the pre-commissioning engineer.
6. BMS controllers are usually installed within BMS control panels distributed around the building.
7. Controllers perform local control by receiving information, running programmed logic and sending commands to plant.
8. I/O modules provide the interface to hardwired field devices.
9. Field devices allow the BMS to measure conditions and physically interact with mechanical and electrical plant.
10. Valve actuators and damper actuators are field devices that physically respond to commands from the BMS.
11. Devices may be connected using hardwired signals or network communications.
12. Where a panel-mounted relay is used, the relay forms part of the output path between the BMS output and the field wiring.
13. The pre-commissioning engineer's primary focus is the physical route between controller/I/O, panel, field wiring, device and plant response.
14. A command or value within the BMS does not prove that the physical installation is operating correctly.

Don't assume it works. Prove it.

By the end of this module, candidates should be able to identify the main types of BMS inputs and outputs, understand what information they carry and relate each I/O point to the physical device or plant that it represents.

Candidates will be introduced to:

• Digital Inputs — DI
• Digital Outputs — DO
• Analogue Inputs — AI
• Analogue Outputs — AO
• Universal Inputs / Universal I/O — UI / UIO
• Common BMS signal types
• Command versus status
• Physical device versus BMS point

The purpose of this module is not to teach detailed electrical testing.

The objective is for candidates to be able to look at a BMS point, understand what it represents and begin to determine what type of input or output should be associated with it.

In Module 2, we established that the BMS controller needs a way of receiving information from the field and sending commands back to plant.

This is the purpose of BMS I/O.

I/O = Input / Output

An input brings information into the BMS.

An output sends a command out from the BMS.

A simple way of remembering this is to think from the position of the BMS controller:

For example:

The temperature is information coming into the controller. This is an input.

Alternatively:

The BMS is sending a command to the actuator. This is an output.

This distinction is fundamental.

Before deciding whether something is a DI, DO, AI or AO, first ask: Is information coming into the BMS, or is the BMS sending something out?

A Digital Input (DI) tells the BMS that something is in one of two states.

• On / Off
• Run / Stop
• Healthy / Fault
• Open / Closed
• Flow / No Flow
• Normal / Alarm

A digital input does not normally tell the BMS how much of something exists. It tells the BMS whether a particular condition is present.

Typical BMS digital inputs include:

• Pump run status
• Pump fault
• Fan run status
• Fan fault
• Flow switch
• Differential-pressure switch
• Filter differential-pressure switch
• Frost thermostat
• Fire alarm interface
• Boiler fault
• Chiller fault
• High-level alarm
• Low-level alarm
• Damper end switch
• Valve end switch

For example:

Pump Run Status

The BMS does not need to know a numerical value. It needs to know Pump Running or Pump Not Running. Therefore, this is a Digital Input.

Where Does a Digital Input Come From?

A digital input will commonly be provided by a contact.

The contact changes state when the plant condition changes. The BMS detects that change.

The exact electrical arrangement will be covered later in the course.

DI = A two-state condition coming INTO the BMS.

A Digital Output (DO) allows the BMS to issue a two-state command.

• Start / Stop
• Enable / Disable
• Open / Close
• On / Off

Typical BMS digital outputs include:

• Pump start
• Fan start
• Boiler enable
• Chiller enable
• Heat pump enable
• Packaged plant enable
• Lighting enable
• Motorised valve open/close command
• Motorised damper open/close command

For example:

BMS Pump Start Command

The controller decides that the pump is required. The BMS output changes state.

Where a panel-mounted relay is used, the physical route may be:

DO = A two-state command going OUT from the BMS.

An Analogue Input (AI) provides the BMS with a variable value rather than simply an ON/OFF condition.

• Temperature
• Pressure
• Differential pressure
• Humidity
• CO₂
• Valve position feedback
• Damper position feedback
• VSD speed feedback

Typical displayed values may include:

21.4°C

85 kPa

650 ppm CO₂

48% RH

6.2 l/s

72% Speed

Unlike a digital input, an analogue input can represent many values across a range.

For example, a room temperature sensor may measure temperatures between approximately 10°C and 30°C.

The controller converts the electrical signal from the sensor into an engineering value that can be used by the control strategy.

At this stage, candidates do not need to understand the detailed conversion process.

AI = A variable measured value coming INTO the BMS.

Different field devices can provide analogue information to the BMS in different ways.

Common signals candidates are likely to encounter include:

• Thermistor resistance
• 0–10 V
• 4–20 mA

Thermistors

Thermistors are commonly used for temperature measurement. The electrical resistance of the sensor changes as the temperature changes.

The BMS controller measures the resistance and converts it into a temperature value.

Examples include:

• Room temperature sensors
• Duct temperature sensors
• Outside-air temperature sensors
• Water temperature sensors

Candidates may encounter different thermistor characteristics depending on the manufacturer and specification.

A thermistor changes resistance with temperature.

The correct sensor type must be configured for the BMS input. A temperature sensor being physically connected does not automatically mean the BMS will display the correct temperature.

0–10 V Inputs

Some sensors provide a variable voltage signal. A common example is 0–10 V.

The voltage represents a value across a defined range. For example, a pressure sensor may be configured so that:

0 V = 0 Pa

10 V = 500 Pa

A signal of approximately 5 V would therefore represent approximately half of the configured range. The exact scaling depends on the device.

4–20 mA Inputs

Some sensors use a current signal rather than a voltage signal. A common industrial signal is 4–20 mA.

For example, a pressure transmitter may be configured so that:

4 mA = 0 kPa

20 mA = 160 kPa

Again, the controller converts the electrical signal into the required engineering value.

Detailed measurement, scaling and electrical testing of these signals will be covered later.

For now, candidates should recognise that an analogue value shown by the BMS originates from a physical electrical signal or communicated value.

An Analogue Output (AO) allows the BMS to send a variable command rather than simply ON or OFF.

A common BMS analogue output signal is 0–10 V.

Typical applications include:

• Heating valve position
• Cooling valve position
• Damper position
• VSD speed demand
• Fan speed demand
• Pump speed demand

For example, a modulating heating valve may operate approximately as:

0 V = 0%

5 V = 50%

10 V = 100%

The exact relationship depends on the actuator and its configuration.

A BMS controller may calculate Heating Demand = 60% and provide a corresponding analogue output to the valve actuator.

The actuator responds by moving to the required position.

AO = A variable command going OUT from the BMS.

Digital

Digital signals represent two states.

• Pump Running / Pump Stopped
• Fault / Healthy
• Flow / No Flow
• Enabled / Disabled

Typical BMS types: DI or DO

Analogue

Analogue signals represent a variable value.

• Temperature = 21.5°C
• Pressure = 85 kPa
• Valve Position = 62%
• Fan Speed Demand = 75%

Typical BMS types: AI or AO

A simple method for candidates is:

Does the BMS need to know YES/NO or ON/OFF? Think Digital.

Does the BMS need to know HOW MUCH? Think Analogue.

Not every BMS controller has completely separate terminals labelled DI, AI, DO and AO.

Modern controllers may use Universal Inputs (UI) or Universal I/O (UIO).

A Universal Input can often be configured in software to accept different types of input.

Depending on the controller, this may include:

• Digital input
• Thermistor input
• 0–10 V input
• Other resistance inputs
• 4–20 mA input, where supported

Some controllers also provide Universal I/O points that can be configured as either inputs or outputs.

The exact capability depends on the BMS manufacturer and controller.

Therefore, a terminal being labelled UI or UIO does not by itself tell the engineer exactly what signal is connected.

The engineer may need to refer to:

• BMS drawings
• Points schedule
• Controller documentation
• Device documentation
• Software configuration

Universal I/O is configurable — its actual function depends on how the point has been set up.

Candidates should begin to associate common BMS points with their likely I/O types.

These are typical examples. The exact I/O arrangement can vary depending on the project design, controller and equipment being used.

The engineer should always work from the project information rather than assuming.

A common mistake for new engineers is to think:

One piece of equipment = one BMS point.

This is often not the case.

Consider a VSD-driven pump. The BMS may have:

• Pump Start — DO
• Pump Run Status — DI
• Pump Fault — DI
• Pump Speed Demand — AO
• Pump Speed Feedback — AI, depending on the design

That is several separate BMS points associated with one physical pump.

Likewise, a modulating valve may have Valve Position Demand — AO and, where feedback is provided, Valve Position Feedback — AI.

A damper may have Damper Position Demand — AO and Damper Position Feedback — AI, or end switches such as Damper Open Status — DI and Damper Closed Status — DI.

The number and type of points depends on the project design.

The value displayed by the BMS and the electrical signal in the field are not necessarily the same thing.

For example, the BMS may display Duct Pressure = 250 Pa, but the physical signal arriving at the BMS input may be 5 V.

The controller has been configured to convert the electrical signal into an engineering value.

Another example: the BMS may display Heating Valve Demand = 60%, but the electrical output to the actuator may be approximately 6 V.

This distinction becomes important during pre-commissioning.

The engineer may need to prove both what the BMS is commanding or reading and what electrical signal actually exists at the field connection.

Detailed practical testing of these signals will be covered later in the course.

During pre-commissioning, the engineer is primarily carrying out point-to-point checks between the field installation and the BMS control panel/controller.

The purpose is to prove that each field device has been installed, wired and connected to the correct BMS I/O point.

Typical issues found during point-to-point checking include:

Incorrect Field Device

The device installed in the field may not be the device shown on the BMS drawings or equipment schedule.

• Incorrect sensor type
• Incorrect actuator
• Incorrect pressure sensor range
• Incorrect differential-pressure switch

The engineer should confirm that the installed device is suitable for the intended BMS point.

Incorrect Cable

The field cable may have been incorrectly identified or connected to the wrong device.

For example, Room Temperature Sensor 01 may have accidentally been connected to the cable intended for Room Temperature Sensor 02.

Both devices may produce believable temperature readings, but the point-to-point installation is still incorrect.

This is why every point must be individually proven.

Incorrect Panel Termination

The correct field cable may enter the BMS panel but be terminated onto the wrong terminal.

The engineer should prove the complete route:

Incorrect I/O Point

A cable may be correctly installed and terminated within the panel but connected to the wrong controller or I/O channel.

For example, Duct Temperature Sensor may be connected to UI-05 when the BMS design requires UI-06.

The point-to-point check should identify this before commissioning begins.

Incorrect Signal Type

The installed field device must be compatible with the BMS input or output.

• Thermistor
• 0–10 V
• 4–20 mA
• Volt-free contact
• 24 V control signal

The pre-commissioning engineer should identify what signal is expected and confirm that the installed device and wiring arrangement are suitable.

Incorrect Polarity

Some BMS field devices and signals are polarity sensitive.

Where polarity is required, incorrect connection can prevent the device from operating or result in an incorrect signal.

The engineer should check the manufacturer's wiring information and the BMS drawings where required.

Open Circuit or Short Circuit

Field wiring faults may include:

• Loose terminals
• Disconnected cables
• Shorted conductors
• Damaged cables

These faults can prevent the BMS from receiving the expected signal.

Output Connected to the Wrong Plant

An output cable may be connected to the wrong item of plant.

For example, BMS Pump 1 Enable could physically operate Pump 2.

The fact that something operates does not prove that the point-to-point installation is correct.

The engineer must prove that the correct BMS point is connected to the correct physical item of plant.

Point-to-Point Principle

For every hardwired BMS point, the pre-commissioning engineer should be working towards proving:

For outputs, this extends to proving that the correct physical device or plant interface is connected:

The objective of pre-commissioning is to leave the installation in a condition where the commissioning engineer can carry out the final BMS commissioning and functional testing without first having to find basic installation and point-to-point faults.

Pre-commissioning proves the installation.

Commissioning proves the system operation.

1. I/O means Input / Output.

2. An input brings information into the BMS.

3. An output sends a command out from the BMS.

4. A Digital Input represents a two-state condition such as run, fault, flow or alarm.

5. A Digital Output provides a two-state command such as start, stop, enable or disable.

6. An Analogue Input provides a variable measured value such as temperature, pressure, CO₂ or humidity.

7. An Analogue Output provides a variable command such as valve position or VSD speed demand.

8. Common analogue signals include thermistors, 0–10 V and 4–20 mA.

9. Universal I/O can be configured for different functions depending on the controller.

10. Command and status are not the same thing.

11. One item of plant may have several different BMS I/O points.

12. A BMS engineering value and the physical electrical signal are not necessarily expressed in the same units.

13. The exact I/O arrangement must be confirmed against the project drawings, I/O schedule and equipment requirements.

14. A believable value does not prove that the correct device has been connected.

15. The pre-commissioning engineer should be able to follow the complete physical route of an I/O point.

Don't assume it works. Prove it.

By the end of this module, candidates should be able to identify the main components typically found within a BMS control panel, understand their basic purpose and follow the physical route of common BMS inputs and outputs through the panel.

Candidates will be introduced to:

• BMS control panels
• Controllers and I/O modules
• Power supplies and transformers
• Panel terminals
• Standard, fused and blade-disconnect terminals
• Relays and contactors
• MCBs, fuses and circuit protection
• Hand / Off / Auto controls
• Indicator lamps
• Local HMIs
• Network connections
• Internal panel wiring and field wiring
• Following DI, DO, AI and AO circuits through the panel

The purpose of this module is not to teach candidates how to design or manufacture BMS control panels.

The objective is for candidates to recognise the main components and understand how a field device is physically connected through the panel to the correct BMS I/O point.

A BMS control panel provides the central location for the equipment used to monitor and control building-services plant.

Depending on the project, a BMS panel may control or monitor:

• Heating systems
• Chilled-water systems
• Pumps
• Air Handling Units — AHUs
• Fans
• Valves
• Dampers
• Variable Speed Drives — VSDs
• FCUs
• VAVs
• Packaged plant

A project may have one BMS panel or many panels distributed throughout the building.

Typical examples include a main plant panel, AHU panel, floor control panel, boiler-room panel or energy-centre panel.

The exact panel arrangement depends on the project design, but candidates are likely to encounter:

• BMS controllers
• I/O modules
• Relays
• Contactors
• Transformers
• 24 V AC or 24 V DC power supplies
• MCBs and fuses
• Numbered panel terminals
• Network connections
• Ethernet switches, where required
• Local HMI, where provided
• Hand / Off / Auto controls
• Indicator lamps
• Panel isolator
• Internal panel wiring
• Field cable terminations

Not every panel will contain every item.

The pre-commissioning engineer should be able to look at the panel drawings and panel photographs and begin to identify the main components and their relationship to the field installation.

A BMS panel may contain more than one voltage.

The incoming panel supply may be 230 V AC or 400 V AC depending on the panel design and the equipment being supplied.

Lower control voltages are then commonly produced within the panel, including:

• 24 V AC
• 24 V DC

This means a BMS panel can contain both mains-voltage circuits and extra-low-voltage control circuits.

Candidates must not assume that a circuit is 24 V simply because it is associated with the BMS.

The correct voltage should always be confirmed from the BMS panel drawings and the equipment requirements.

Safe isolation and the correct electrical safety procedures must always be followed.

Transformers and power supplies are used to provide the control voltages required by BMS controllers and field devices.

A typical 24 V AC arrangement may be:

230 V AC

Transformer

24 V AC Control Supply

A typical 24 V DC arrangement may be:

230 V AC

DC Power Supply

24 V DC Control Supply

During pre-commissioning, the engineer may need to establish:

• What voltage does the field device require?
• Where is the supply generated?
• Which terminal provides the supply?
• Is the correct supply reaching the field device?

The BMS controller performs the local control functions for the plant connected to the panel.

Depending on the controller, it may include:

• Onboard I/O
• Connections to separate I/O modules
• Ethernet connections
• RS485 connections
• Power terminals
• Status LEDs
• Service or engineering ports

Inputs

BMS Controller / Control Logic

Outputs

The pre-commissioning engineer is not expected to design or write the control software.

However, they should understand where the physical BMS I/O connects and be able to follow the point-to-point route through the panel.

I/O modules provide the physical connection between the BMS controller and field devices.

As covered in Module 3, these may include:

• Digital Inputs — DI
• Digital Outputs — DO
• Analogue Inputs — AI
• Analogue Outputs — AO
• Universal Inputs — UI
• Universal I/O — UIO

The I/O may be built into the controller or provided on separate modules.

A typical input route is:

Field Device

Numbered Panel Terminal

I/O Module

BMS Controller

Panel terminals connect the field wiring to the internal panel wiring.

Field cables normally terminate onto individually numbered terminals. Typical numerical references may include:

• 101
• 102
• 103
• 376
• 378
• 380

The terminal numbers appear on the BMS panel drawings and identify exactly where the field cables should connect.

For example, a temperature sensor may be shown as terminating on Terminals 101 & 102.

A typical input route is:

Field Device

Field Cable

Numbered Panel Terminal

Internal Panel Wiring

BMS I/O

A typical output route is:

BMS Output

Relay, where required

Numbered Panel Terminal

Field Cable

Field Device / Plant Interface

Standard Terminals

A standard terminal provides a straight connection between the internal panel wiring and the field wiring.

It is used where no local isolation or individual circuit protection is required at the terminal.

Fused Terminals

A fused terminal incorporates a replaceable fuse into the terminal assembly.

Fused terminals are commonly used where an outgoing supply to a field device requires individual circuit protection.

Examples may include 24 V supplies to:

• Sensors
• Actuators
• Other powered field devices

The purpose of the fused terminal is to:

• Protect the individual outgoing circuit
• Allow a field fault to operate the individual fuse rather than affect other devices on the same control supply
• Assist fault finding by separating individual field circuits

24 V Supply

Field Cable

If a powered field device is not operating, the engineer may need to check:

• Is the correct supply entering the fused terminal?
• Is the fuse intact?
• Is the correct supply leaving the terminal?
• Does the supply reach the correct field device?

A blown fuse should not simply be replaced repeatedly without identifying the cause of the fault.

Blade Disconnect Terminals

A blade-disconnect terminal allows the circuit to be electrically separated at the terminal without physically removing the field wiring.

The terminal contains a disconnecting blade that can be opened to break the circuit.

This can be useful for:

• Point-to-point testing
• Fault finding
• Circuit isolation
• Separating field wiring from the BMS I/O

Field Device

Blade Disconnect Terminal

Opening the blade separates the field circuit from the BMS input. This can help the engineer determine whether an issue is on the field side or the panel/BMS side.

The engineer must understand the circuit and follow the correct electrical safety procedures before disconnecting or testing.

Why Terminal Types Matter

Standard Terminal = Connection

Fused Terminal = Connection + Circuit Protection

Blade Disconnect Terminal = Connection + Disconnect / Test Facility

During point-to-point checking, the engineer should establish:

• What field device is the cable connected to?
• Which numbered terminals should it connect to?
• What type of terminal is being used?
• Does the physical termination match the BMS panel drawing?
• Where does the circuit go internally?

The engineer should use the approved panel drawings and should not infer a terminal's function from the number alone.

Relays are commonly used to interface BMS outputs with external plant control circuits.

A typical arrangement is:

BMS Digital Output

Relay

Numbered Panel Terminal

Field Cable

Pump / Fan / Plant Interface

The BMS output energises the relay coil. The relay contact then changes state and switches the external circuit.

During point-to-point checking, the engineer should be able to identify the BMS output, relay, relay contact, numbered terminal, field cable and plant interface.

Relays may be used to:

• Provide electrical isolation between circuits
• Provide volt-free contacts
• Switch a different control voltage
• Increase switching capability
• Provide multiple contacts
• Interface with external plant or control circuits

The important principle is:

The BMS output operates the relay. The relay contact interfaces with the external circuit.

Relay contacts are commonly identified as:

• C — Common
• NO — Normally Open
• NC — Normally Closed

Normally refers to the state of the relay contact when the relay coil is not energised.

Normally Open — NO:

• Relay de-energised: contact open
• Relay energised: contact closes

Normally Closed — NC:

• Relay de-energised: contact closed
• Relay energised: contact opens

The engineer should always check the BMS panel drawing and relay markings rather than assuming which contact has been used.

Contactors are generally used for switching higher electrical loads than small control relays.

In a BMS panel, the BMS may control a relay which then operates a contactor or motor-starter control circuit.

BMS Output

Contactor / Starter Control Circuit

Candidates should understand the difference between a relay used within a control circuit and a contactor used to switch a higher electrical load.

BMS panels contain circuit protection to protect panel equipment, control circuits and outgoing field supplies.

Candidates may encounter:

• MCBs
• Fuses
• Fused terminals
• Other circuit-protection devices

Separate protection may be provided for:

• BMS controller
• I/O modules
• 24 V control supply
• Actuator supplies
• Field-device supplies
• Auxiliary circuits

If protection has operated, the engineer should not simply reset or re-energise the circuit without considering why it operated.

Some BMS panels include Hand / Off / Auto controls for plant such as pumps and fans.

Hand — the plant may be manually commanded independently of the normal BMS automatic command.

Off — the plant is prevented from operating.

Auto — the plant is available for normal BMS control.

During pre-commissioning, the selector position can directly affect whether an output appears to operate correctly.

The engineer should therefore understand the selector state before deciding that a BMS output or plant interface is faulty.

BMS panels may include local indicator lamps.

Typical examples include:

• Power On
• Plant Running
• Plant Failed
• Low Flow
• Common Alarm

These lamps provide local indication of plant or panel status and may be associated with relay contacts, status signals or alarm circuits.

A local lamp can be useful during fault finding, but it does not replace proving the correct BMS point and physical circuit.

Some BMS panels may have a local Human Machine Interface — HMI.

This may be installed on the front of the panel or nearby.

The HMI may contain graphics for the local plant controlled by that panel. For example:

• Air Handling Units — AHUs
• Boiler-room plant
• Heating and chilled-water plant
• Pumps
• Valves and dampers
• Other local mechanical plant

From the graphics, the operator or engineer may be able to view the status of:

1. Temperatures
2. Pressures
3. Plant status
4. Faults / alarms
5. Plant enables

Setpoints may also be displayed and, depending on the system design and user access level, some setpoints may be adjustable.

A small standalone BMS installation may use a local HMI without having a central BMS supervisor.

For the pre-commissioning engineer, the HMI may provide useful local visibility of controller information and I/O while carrying out point-to-point checks.

Detailed commissioning of HMI graphics, software, setpoints and control functions remains part of the BMS commissioning function.

The key distinction is:

Physical point-to-point pre-commissioning

versus

Software / graphics commissioning

BMS panels may also contain network connections used to communicate with:

• Other BMS controllers
• Supervisor / Head-End, where provided
• Remote I/O
• VSDs
• Energy meters
• Packaged plant
• Other intelligent devices

Common physical network connections may include Ethernet and RS485.

An Ethernet switch may also be installed within the panel where required.

Candidates should understand the difference between hardwired field I/O and network communications.

Detailed network commissioning will be covered later in the course.

It is important to distinguish between internal panel wiring and field wiring.

Internal panel wiring connects components within the panel, such as:

• Controller
• I/O modules
• Relays
• Terminals
• Power supplies
• Switches
• Indicator lamps

Field wiring leaves the BMS panel and connects to equipment installed around the building, such as:

• Sensors
• Actuators
• Pumps
• Fans
• VSDs
• Switches
• Thermostats
• Plant interfaces

The numbered panel terminal is commonly the boundary between the internal panel wiring and the field wiring.

Internal Panel Wiring

Field Wiring

Clear panel labelling is essential for pre-commissioning and fault finding.

Candidates may see labels identifying:

• Controllers
• I/O modules
• Relays
• MCBs
• Transformers
• Power supplies
• Network equipment
• Plant references
• Cable numbers
• Individual numerical terminal numbers

For example, a relay may be identified as R1 and an MCB as MCB3, while panel terminals are identified by their individual numerical terminal numbers such as 101, 102 and 103.

The engineer should use the panel drawings and component labels together to trace the circuit.

Consider a pump run-status input.

Pump / Starter / VSD

Run Contact

Field Cable

Numbered Panel Terminal

Internal Panel Wiring

BMS Digital Input

Controller

The pre-commissioning engineer should prove that the correct physical run contact reaches the correct BMS input.

For example, Pump 1 Run must not appear as Pump 2 Run.

Every point must be individually proven.

Consider a pump-start command.

BMS Controller

Digital Output

Relay

Numbered Panel Terminal

Field Cable

Pump / Starter / VSD Enable

The pre-commissioning engineer should prove that the correct BMS output reaches the correct physical plant interface.

Pump 1 Start must operate Pump 1, not Pump 2.

The fact that something operates does not prove that the point-to-point installation is correct.

Consider a duct-pressure sensor.

Duct Pressure Sensor

Field Cable

Numbered Panel Terminal

Internal Panel Wiring

BMS AI / UI

Controller

The physical signal may be 0–10 V, while the BMS displays an engineering value such as 250 Pa.

The engineer should understand that the displayed value and the physical electrical signal are not necessarily expressed in the same units.

Consider a modulating valve actuator.

BMS Controller

Analogue Output

Numbered Panel Terminal

Field Cable

Valve Actuator

A direct analogue output does not necessarily use a relay.

The engineer should follow the actual panel drawing and field wiring arrangement rather than assuming every output circuit is the same.

During point-to-point pre-commissioning, the engineer should establish:

• Correct BMS panel
• Correct field device and plant reference
• Correct field cable and cable identification
• Correct numbered panel terminal
• Correct controller / I/O point
• Correct signal type
• Correct field-device supply
• Correct polarity, where applicable
• Correct relay and relay contact, where applicable
• Correct field plant interface
• Secure terminations
• No obvious damage
• Panel components and installation match the drawings

The purpose is to prove the physical installation before final commissioning.

Typical issues discovered during pre-commissioning include:

• Field cable terminated in the wrong terminal
• Cross connections in the field
• Incorrect I/O channel
• Loose termination
• Incorrect polarity
• Incorrect control voltage
• Incorrect field-device supply
• Incorrect relay contact used
• Missing cable identification
• Incorrect sensor or actuator
• Device not powered
• Panel installation differing from the drawings

The purpose of systematic point-to-point checking is to identify these issues before final commissioning.

1. A BMS control panel provides the physical interface between the BMS controller and the field installation.
2. A panel may contain controllers, I/O modules, power supplies, relays, contactors, circuit protection, terminals, network equipment and local controls.
3. BMS panels can contain different voltages. The correct circuit voltage must always be confirmed.
4. Numbered panel terminals provide the link between the BMS panel drawings and the physical field installation.
5. Standard terminals provide a connection, fused terminals provide connection plus circuit protection, and blade-disconnect terminals provide connection plus a disconnect/test facility.
6. Relays are commonly used to interface BMS outputs with external plant control circuits.
7. The correct relay contact — Common, Normally Open or Normally Closed — must be confirmed from the drawings.
8. Hand / Off / Auto controls can affect whether plant responds to a BMS command.
9. A local HMI may contain graphics for local plant and may show live status, alarms, schedules, enables and adjustable setpoints where permitted.
10. Detailed HMI graphics and software commissioning remains part of the BMS commissioning function.
11. Hardwired field I/O and network communications are different types of BMS connection.
12. The numbered panel terminal is commonly the boundary between internal panel wiring and field wiring.
13. Every DI, DO, AI and AO should be treated as a complete physical route through the installation.
14. A plant item operating does not prove that the correct BMS point has been connected.
15. Systematic point-to-point checking should identify panel and termination issues before final commissioning.

Pre-commissioning proves the installation.

Commissioning proves the system operation.

Don't assume it works. Prove it.

By the end of this module, candidates should be able to navigate a typical BMS control-panel drawing set, identify the information required for pre-commissioning and use the drawings to trace a field device through the panel to the correct BMS I/O point.

Candidates are not expected to become BMS panel designers.

The purpose of this module is to teach candidates how to use BMS drawings as a practical tool for point-to-point pre-commissioning.

• Navigate a BMS panel drawing set.
• Use the drawing legend.
• Recognise the Panel / Field demarcation line.
• Identify numerical terminal references.
• Identify field devices and plant references.
• Recognise DI, DO, AI, AO and UI points.
• Follow drawing and page cross-references.
• Follow relay references, including individual relay poles.
• Identify power, control and signal wiring.
• Trace inputs from the field into the BMS.
• Trace outputs from the BMS into the field.
• Compare the drawing against the physical installation.
• Use drawings systematically during point-to-point testing.

The drawing represents how the circuit should be installed.

Pre-commissioning proves whether the physical installation actually matches it.

The drawing tells you where the circuit should go. Point-to-point testing proves that it actually goes there.

A pre-commissioning engineer does not need to understand every circuit on every page before beginning work.

They do need to be able to use the drawings to answer practical questions:

• What device am I proving?
• What type of point is it?
• Where is the device installed?
• Which cable is associated with it?
• Which panel terminals should it connect to?
• Which BMS I/O point is associated with it?
• Is there a relay or other interface within the circuit?
• Which relay pole/contact is being used?
• What signal or voltage should I expect?
• Where does the circuit cross from the panel into the field?
• Does the physical installation match the drawing?

A BMS panel drawing is rarely one single drawing.

A typical control-panel drawing set may contain separate pages for:

• Drawing legend
• Incoming electrical supply
• Control supplies
• Safety circuits
• Plant power circuits
• Pumps
• Fans
• VSD interfaces
• Valves and actuators
• Sensors and switches
• Relays
• Digital inputs
• Digital outputs
• Analogue inputs
• Analogue outputs
• Controller and I/O modules
• Network connections
• External panel layouts
• Internal panel layouts

Before attempting to trace circuits, locate the drawing legend.

The legend explains the symbols and references used within that particular drawing set.

Typical references include: R# — Relay; MCB# — Miniature Circuit Breaker; F# — Fuse; TX# — Transformer; DO# — Digital Output; AO# — Analogue Output; PAGE # / SHEET # — Drawing cross-reference.

Drawing conventions can vary between manufacturers and panel builders.

Do not assume every drawing uses exactly the same symbols or references. Check the legend first.

One of the most important features for a pre-commissioning engineer is the Panel / Field demarcation line.

The demarcation separates the circuitry contained within the manufactured control panel from the wiring and equipment installed in the field.

Panel Side

• BMS controller
• I/O modules
• Internal wiring
• Relays
• Contactors
• MCBs and fuses
• Power supplies
• Control circuits
• Numbered terminals

Field Side

• Field cables
• Temperature sensors
• Pressure sensors
• Differential pressure switches
• Valve actuators
• Damper actuators
• VSD interfaces
• Pumps
• Fans
• Packaged plant interfaces

──────── PANEL / FIELD DEMARCATION ────────

For an input, the signal travels in the opposite direction:

──────── PANEL / FIELD DEMARCATION ────────

Why Is the Demarcation Line Important?

The demarcation line provides a useful testing and fault-finding boundary. If the correct output signal is present at the numbered panel terminal but is not reaching the field device, the investigation continues on the field side. If the expected signal is not reaching the terminal, the investigation continues back through the panel side. The same principle applies in reverse to an input.

Use the demarcation line to split the circuit: Panel Side or Field Side.

Numbered panel terminals form the physical connection between the panel side and field side of the installation.

The terminal number provides the link between the drawing, the panel and the field installation. Never determine the purpose of a terminal from its number alone. Use the approved panel drawing.

A BMS panel drawing can initially look complicated. Do not attempt to understand the whole page at once. Select one point, such as Supply Fan Run Status, and follow it systematically.

1. Where does the status originate?
2. What type of signal is it?
3. What field cable carries it?
4. Where does it cross the Panel / Field demarcation?
5. Which numbered terminal does it land on?
6. Where does it go internally?
7. Which BMS input receives it?

Start with one device. Follow one circuit. Prove one point.

BMS drawings identify the equipment associated with each circuit.

• Supply Fan
• Extract Fan
• Temperature Sensor
• Pressure Sensor
• Differential Pressure Switch
• Frost Thermostat
• Valve Actuator
• Damper Actuator
• VSD

The device description tells the engineer what they should physically be looking for in the field.

Prove the correct field device connected to the correct BMS point.

• DI — Digital Input
• DO — Digital Output
• AI — Analogue Input
• AO — Analogue Output
• UI — Universal Input

Dedicated I/O pages allow the engineer to connect the physical field installation to the correct BMS controller channel.

──────── PANEL / FIELD DEMARCATION ────────

During point-to-point testing, prove that the correct physical status reaches the correct BMS input.

A valid signal connected to the wrong point is still a fault.

──────── PANEL / FIELD DEMARCATION ────────

• Which DO is being tested?
• Which relay should operate?
• Which relay pole/contact is used?
• Which terminal should change state?
• Which field cable leaves the panel?
• Which item of plant should receive the command?

Relays may appear throughout BMS panel drawings. A relay may be identified as R5. This identifies the physical relay.

The individual poles or contact sets associated with that relay may then appear elsewhere on the drawing as:

R5/1 R5/2 R5/3

The number after the / identifies which pole/contact set of that relay is being used at that particular point in the circuit.

• R5/1 = Relay R5, Pole 1
• R5/2 = Relay R5, Pole 2
• R5/3 = Relay R5, Pole 3

One physical relay may perform several functions through its separate poles.

• R5/1 → Plant Enable
• R5/2 → Enable Lamp
• R5/3 → BMS Status / Interlock

If a drawing shows a lamp circuit connected through R5/2, the lamp is being switched using Pole 2 of Relay R5. If a status circuit contains R5/3, the status is connected through Pole 3 of Relay R5.

The /1, /2, /3 does not identify another relay. It identifies which pole/contact set of the same physical relay is being used.

This is particularly useful because the relay coil and the contacts belonging to that relay may appear in different positions or even on different pages of the drawing.

• What energises this relay?
• Which pole/contact of the relay is being used?
• What does that pole operate or provide a status to?

Pre-Commissioning Check

Do not simply prove that Relay R5 energises. Prove that the correct pole/contact changes state and operates the correct associated circuit.

──────── PANEL / FIELD DEMARCATION ────────

Establish the signal type, for example 0–10 V or 4–20 mA. The drawing tells you where the signal should go; the device information tells you what electrical signal should be expected.

──────── PANEL / FIELD DEMARCATION ────────

An analogue output does not necessarily require a relay.

Do not assume every BMS output passes through a relay. Follow the actual drawing.

Candidates do not need to design an HOA circuit, but they do need to understand its effect.

• HAND — Plant may operate independently of the normal BMS automatic command.
• OFF — Plant is prevented from operating.
• AUTO — Plant is available for normal BMS control.

When a BMS output appears not to operate the plant, check the HOA selector before concluding that the BMS point is faulty.

A BMS panel drawing may contain both power circuits and control circuits. The BMS may provide a control command to a VSD or starter; it does not necessarily supply the motor power.

When following a field device, do not only follow the signal conductor. Some devices also require a power supply.

• 24 V Supply
• 0 V Common
• 0–10 V Signal

A device can be correctly connected to the BMS signal terminal but still fail because its supply or common is missing.

• Where does the device get its power?
• Where is the common?
• Where is the signal?

A circuit may continue onto another page. References such as PAGE 11, SHEET 12, AO1 or DO1 are instructions to continue tracing the circuit elsewhere.

1. Note the reference.
2. Locate the referenced drawing.
3. Find the corresponding circuit.
4. Continue following the point.

Not everything shown on a BMS drawing is hardwired I/O. Drawings may also contain BACnet and Modbus communication networks.

Detailed network commissioning is covered later in the course. At this stage, recognise when a point is networked rather than hardwired.

Before using a drawing for pre-commissioning, check its drawing status and revision.

• Preliminary
• For Approval
• Construction Issue
• As-Fitted / As-Built

Ensure you are working from the correct approved drawing revision for the work being undertaken.

The drawing represents the intended or recorded installation. The physical installation must still be proved.

• Field cable terminated in the wrong terminal
• Cross connections in the field
• Incorrect I/O channel
• Loose termination
• Incorrect polarity
• Incorrect control voltage
• Incorrect field-device supply
• Incorrect relay contact used
• Missing cable identification
• Incorrect sensor or actuator
• Device not powered
• Panel installation differing from the drawings

If the drawing and physical installation differ, the discrepancy needs to be identified and recorded.

The drawing should become the pre-commissioning engineer's roadmap. For every point, establish:

1. What is the field device?
2. What signal are we proving?
3. Is it an input or output?
4. What cable is associated with it?
5. Where does it cross the Panel / Field demarcation?
6. Which numbered terminals should it land on?
7. Where does it go internally?
8. Is there a relay or other interface?
9. If there is a relay, which pole/contact is being used?
10. Which BMS I/O point should receive or generate the signal?
11. What should physically happen when the point is tested?

Use an anonymised extract from a typical AHU control drawing containing fan enable, fan fault, HOA control, airflow proving, inlet damper, damper end switch, 0–10 V speed control, BMS I/O references, relay references, individual relay poles, numerical terminals and the Panel / Field demarcation.

• Where is the field side?
• Where is the panel side?
• Where is the Panel / Field demarcation?
• Which terminals form the boundary?
• Which signal enables the fan?
• Which relay is involved?
• Which poles of that relay are used elsewhere?
• Where is the HOA selector?
• Where does the fault signal return?
• How is airflow proven?
• Where is the 0–10 V speed-control signal shown?

Teach candidates to break the drawing down and follow one circuit at a time.

1. A BMS drawing set contains multiple linked drawings.
2. The drawing legend explains the symbols and references used within that drawing set.
3. The Panel / Field demarcation line identifies the boundary between the manufactured panel and field installation.
4. The demarcation line is an important point-to-point and fault-finding reference.
5. Numerical terminal references link the drawing to the physical BMS panel.
6. Field-device descriptions identify the equipment being controlled or monitored.
7. DI, DO, AI, AO and UI references identify the BMS I/O associated with the field point.
8. Digital outputs may pass through relays before reaching the field.
9. R5 identifies the relay, while R5/1, R5/2 and R5/3 identify the individual poles/contact sets of that relay.
10. One relay may use separate poles for plant enable, indication, status or interlocking.
11. Proving that a relay energises is not enough; the correct pole/contact and associated circuit must also be proved.
12. Analogue outputs may connect directly to field actuators.
13. HOA controls can affect whether plant responds to a BMS command.
14. Power circuits and BMS control circuits perform different functions.
15. Powered devices may require a supply, common and signal.
16. Drawing cross-references allow a circuit to be followed between pages.
17. BMS drawings may contain both hardwired I/O and network communications.
18. The engineer must check the drawing status and revision.
19. A complicated drawing becomes easier when the engineer follows one circuit at a time.
20. The physical installation must always be checked against the drawing.

Complete Point-to-Point Routes

The drawing tells you where the circuit should go.

The Panel / Field demarcation tells you where the installation changes sides.

The relay reference tells you which relay and which pole is being used.

Point-to-point testing proves that the circuit actually goes where the drawing says it should.

DON'T JUST READ THE DRAWING. USE IT TO PROVE THE INSTALLATION.

By the end of this module, candidates should understand the electrical principles commonly encountered during BMS pre-commissioning and be able to apply those principles when testing and fault-finding BMS field wiring, control circuits, field devices and analogue signals.

This module is not intended to teach electrical installation or electrical design. It concentrates on the electrical knowledge required to understand and prove BMS circuits.

Taking a measurement is only useful if you understand what you are measuring, what you are measuring it against, and what result you should expect.

BMS systems can contain several different voltages within the same control panel and installation.

• 400 V AC three-phase supplies
• 230 V AC supplies
• 24 V AC control supplies
• 24 V DC control supplies
• 0–10 V DC control signals
• Low-level resistance and electronic signals

A BMS pre-commissioning engineer must never assume that a circuit is safe simply because it forms part of a BMS installation.

Before testing a circuit, establish what voltage should be present, whether it is AC or DC, where the circuit originates, what result is expected and whether it is safe and appropriate to carry out the test.

Never assume. Test and prove.

Where electrical isolation is required, the circuit must be correctly identified and isolated before work begins.

Safe isolation procedures shall be carried out in accordance with BS 7671 and relevant site procedures by a competent person.

Never assume a circuit is isolated. Prove it.

One of the first things a pre-commissioning engineer must establish is whether the circuit being tested is AC or DC.

A measurement of 24 V on its own is not enough information. The engineer needs to establish: 24 V AC or 24 V DC?

The BMS drawings and field-device information should establish what supply or signal should be present before testing begins.

Voltage is the electrical potential difference between two points.

Rather than thinking “There is 24 V on this wire,” the engineer should think “There is 24 V between these two points.”

This becomes particularly important when testing analogue circuits because the reference used for the measurement determines what the voltage actually means.

What am I measuring against?

Continuity testing is used to establish whether an electrical path exists between two points.

• Field cables
• Panel wiring
• Switch contacts
• Relay contacts
• Terminal-to-terminal connections
• Device contacts

If the BMS drawing shows a field device connected to a particular numbered panel terminal, continuity testing can help prove that the physical wiring follows the intended circuit.

Continuity testing must only be carried out on circuits that have been safely isolated and are suitable for continuity testing.

Never carry out continuity or resistance testing on an energised circuit.

Many BMS control and monitoring circuits operate by changing between an open and closed state.

An open circuit does not provide a complete electrical path. A closed circuit provides a complete electrical path.

• Run contacts
• Fault contacts
• Flow switches
• Differential pressure switches
• Safety interlocks
• Relay contacts
• Plant status contacts

During pre-commissioning, the engineer needs to prove that the contact changes state correctly when the associated device or plant condition changes.

Contacts are commonly identified as NO — Normally Open, NC — Normally Closed and COM — Common.

“Normal” generally refers to the condition of the contact when the relay or device is in its normal or de-energised state.

The engineer should not assume which contact should be used. The intended arrangement should be established from the BMS drawing and device information.

Volt-free contacts may also be referred to as dry contacts or potential-free contacts.

A volt-free contact does not provide a voltage. It simply changes between an open and closed state.

• Pump run
• Pump fault
• VSD run
• VSD fault
• Boiler fault
• Chiller common fault
• Flow switch
• Packaged plant status

The voltage used to monitor the contact may originate from the BMS panel or another control circuit.

A volt-free contact switches a circuit. It does not provide the voltage.

The previous module introduced relay references such as R5, R5/1, R5/2 and R5/3.

The pre-commissioning engineer needs to prove the actual relay contact rather than simply observing that the relay has energised.

1. What energises Relay R5?
2. Which pole/contact is being used?
3. What should its normal state be?
4. Does the contact change state when R5 operates?
5. Does the correct downstream circuit respond?

Seeing a relay operate does not prove the circuit.

24 V control circuits are used extensively within BMS installations. Candidates are likely to encounter both 24 V AC and 24 V DC.

• Where does the supply originate?
• Is it AC or DC?
• What is its intended common/reference?
• What equipment does it supply?
• Does polarity matter?
• What should be measured at the field device?

This becomes particularly important when dealing with electronic sensors, actuators and analogue signals.

To understand analogue signals correctly, candidates first need to understand what an electrical reference is.

The 0 V provides the reference against which the +24 V is measured.

Now consider a pressure sensor with three connections: +24 V — Supply; 0 V — Common / Reference; 0–10 V — Signal.

If the sensor is producing a 5 V signal, it is not simply “Signal = 5 V”. It is:

A voltage signal only has meaning relative to its reference.

A 24 V supply is described as floating when its control circuit has no intentional connection to earth.

A floating supply does not mean there is no reference within the control circuit. It means there is no intentional reference to earth.

Floating does not mean faulty.

Consider a pressure sensor connected to a conventional 0–10 V BMS analogue input. The pressure sensor produces its signal relative to its common.

The controller must be able to measure that signal against the correct reference.

The complete control circuit can remain floating relative to earth.

The controller does not need the signal referenced to earth. It needs the signal referenced to the correct common.

Now consider what happens if the field signal reaches the controller but the intended reference does not.

The result may be an incorrect, unstable or otherwise unreliable reading.

The signal can be present while the complete signal circuit is still incorrect.

SIGNAL + CORRECT REFERENCE = MEANINGFUL MEASUREMENT

• Protective Earth
• 0 V
• 24 V Common
• Signal Common
• Controller Common
• Cable Screen / Drain

A design may intentionally connect some of these together at a defined point, but that does not make them interchangeable throughout an installation.

0 V is not automatically earth.

Floating circuits can produce confusing readings when measured against earth.

Because the circuit has no intentional relationship with earth, measurements to earth can be affected by connected electronics, filtering, capacitance and other high-impedance paths.

What two points should I actually be measuring between?

For the circuits covered in this course, the important principle is that an analogue voltage must be measured against the correct reference.

The common does not need to be connected to earth simply to make the measurement possible.

What If the Drawing Shows Something Different?

Not every controller or field device is wired in exactly the same way. The pre-commissioning engineer should therefore not add, remove or alter common/reference connections based on assumption.

If these do not clearly establish the required connection, refer the issue back to the BMS engineer/designer before altering the circuit.

The signal and its reference work together. Prove both.

Consider a duct pressure sensor that is powered but the BMS displays an incorrect or unstable value.

Test 1 — Prove the Supply

The supply appears correct, but this does not prove the analogue circuit.

Test 2 — Prove the Field Signal

The sensor appears to be producing a sensible output.

Test 3 — Prove the Signal at the Controller

If this does not produce approximately the same 5.2 V, investigate:

“The sensor has 24 V” is not a complete diagnosis.

A missing reference can cause problems. However, too many reference paths can also cause problems. The objective is not simply to connect everything together. The circuit should be connected as it was designed.

Example 1 — Correct Reference

Example 2 — Missing Reference

Example 3 — Multiple Reference Paths

There may now be two conductive paths between the two ends of the circuit: Path 1 — Intended Common / Reference; Path 2 — Additional Earth Path.

If there is a small electrical potential difference between the two earth points, unwanted current may flow through the additional path. This can influence sensitive analogue circuits.

• Unstable analogue readings
• Small measurement errors
• Drifting values
• Different readings between the field and panel
• Faults that appear when other equipment operates

This type of problem may be referred to as a ground loop or earth loop.

Example 4 — Screen Connected at Both Ends

Where a particular design requires the screen to be terminated at only one end, incorrectly connecting it at both ends may create an additional conductive path.

This does not mean screens must always be earthed at one end. The correct screen arrangement depends on the circuit, equipment and manufacturer's requirements.

Terminate screens in accordance with the approved design and manufacturer's requirements. Do not assume that connecting more earths or commons is better.

The Important Difference

Candidates should understand the distinction between MISSING REFERENCE and MULTIPLE / UNINTENDED REFERENCES.

The solution to a missing reference is not to randomly connect commons or earths together. The engineer must first establish how the circuit was intended to operate.

Polarity is particularly important on DC circuits.

• 24 V DC sensor supplies
• 0–10 V signals
• 4–20 mA circuits
• Electronic field devices
• DC-powered controllers and I/O

Reversing polarity can result in no operation, incorrect signals, faults or potentially equipment damage. The required polarity should be established before connection or testing.

0–10 V DC is one of the most common analogue signals encountered within BMS systems.

• Valve position commands
• Damper position commands
• VSD speed references
• Pressure sensors
• Differential pressure sensors
• Other analogue transmitters

A 0–10 V signal is measured as a DC voltage between the signal conductor and its intended common/reference.

This may represent approximately 75% of the configured signal range, although the actual engineering value depends on the configuration.

Do not automatically use earth as the reference when testing a 0–10 V signal.

4–20 mA is another common analogue signal used by field transmitters. Unlike voltage, current is measured through the circuit.

For a 0–10 V measurement, the meter is normally connected across the signal and common. For a direct 4–20 mA measurement, the meter must be correctly connected in series with the current loop.

Incorrectly connecting a multimeter while configured for current measurement can short the circuit or operate the meter's internal protection.

4 mA = bottom of the normal measurement range

Zero or abnormally low current can indicate a supply, wiring or circuit problem rather than simply representing a zero process value.

Resistance is measured in ohms (Ω). BMS temperature sensors commonly use thermistors whose resistance changes with temperature.

The BMS controller measures this resistance and converts it into an engineering value such as 21.5°C.

Resistance measurement can help establish whether the sensor is connected, whether the circuit is open or shorted, whether the resistance is reasonable for the current temperature and whether the correct sensor type may have been installed.

The manufacturer's resistance/temperature information should be used where verification is required.

Resistance testing must only be carried out on a de-energised circuit suitable for resistance testing.

A multimeter is one of the most important tools available to a BMS pre-commissioning engineer. However, obtaining a number from the meter is not the objective.

• What am I expecting to measure?
• Should it be AC or DC?
• What should the approximate value be?
• What am I measuring against?
• Is the circuit safe to test?
• What will this measurement prove?

Typical functions include AC voltage, DC voltage, continuity, resistance and DC current where appropriate.

Don't chase numbers. Know what the number should mean.

Consider a valve actuator that is not responding to a 0–10 V command. Do not immediately replace the actuator.

1. Is the actuator correctly supplied?
2. Is the supply AC or DC as required?
3. Is the correct common/reference present?
4. Is the BMS producing the expected 0–10 V command?
5. Is the signal present at the BMS panel terminal?
6. Is the same signal present at the actuator?
7. Is the actuator wired correctly?
8. Does the actuator physically respond?

Electrical testing gives the pre-commissioning engineer a practical way of using the Panel / Field Demarcation introduced in Module 5.

If the correct signal is present at the panel boundary but does not reach the field device, investigate towards the field side. If the expected signal is not present at the boundary, investigate towards the panel side.

The same principle can be applied in reverse to an input circuit.

Find the boundary. Test the boundary. Decide which direction to go.

• 24 V is present because a device appears powered
• 24 V means DC
• 0 V means earth
• Common means earth
• A floating supply is faulty
• A sensor is faulty because the BMS reading is wrong
• A relay circuit is correct because the relay energises
• A 0–10 V signal is correct simply because voltage exists
• The controller and field device share the correct reference
• A correct supply voltage proves the signal circuit
• Two different control circuits should simply be commoned together
• Connecting additional earths or commons will solve an analogue problem

Candidates should leave this module understanding the fundamental electrical principles required for BMS pre-commissioning: AC and DC voltage, continuity, open and closed circuits, NO/NC/COM contacts, volt-free contacts, relay operation, polarity, analogue voltage, current and resistance.

Most importantly, they should understand the relationship between signal and reference.

Voltage is always measured between two points.

0 V/common is not automatically earth.

A floating circuit is not automatically faulty.

A floating circuit can have a perfectly valid control reference while remaining floating relative to earth.

A 0–10 V signal only has meaning when measured against the correct reference.

The controller does not necessarily need an earth reference. It needs the reference the signal was designed to use.

A missing reference can cause problems. Multiple unintended reference paths can also cause problems.

When the intended arrangement is unclear:

The signal and its reference work together. Prove both.