In Building Management Systems (BMS), knowing whether equipment has been commanded to run is not the same as knowing whether it is actually running.
To verify real operation, BMS systems rely on electrical current monitoring — most commonly using Current Transformers (CTs) and Current Switches (CSs).
Although they are often confused or used interchangeably, CTs and CSs serve very different purposes within a control system. Choosing the wrong one can limit diagnostics, cause false alarms, or hide genuine faults.
This article explains what each device does, how they are used, and when each should be specified.
A Current Transformer (CT) is a measurement device that senses the actual current flowing through a conductor and converts it into a proportional, reduced signal suitable for a BMS controller or metering device.
Unlike simple run-status devices, CTs provide real numerical data.
A CT allows the BMS to:
CT outputs are typically:
These signals are connected to analogue inputs (AI) on the controller.
CTs are used where performance and efficiency matter, including:
In short: CTs are used when the building needs insight, not just confirmation.
A Current Switch (CS) — also known as a current sensing switch — is a device that monitors electrical current and changes state when the current exceeds a predefined threshold.
It does not measure how much current is flowing. It simply provides a Go / No-Go indication.
A current switch answers one question only:
"Is this equipment drawing current?"
The output is typically:
This makes CSs ideal for proof of operation.
Current switches are widely used across HVAC and building services due to their simplicity and reliability.
Typical applications include:
In many cases, CSs replace complex mechanical flow switches or auxiliary relays with a far simpler solution.
| Feature | Current Transformer (CT) | Current Switch (CS) |
|---|---|---|
| Measures actual current | Yes | No |
| Provides analogue value | Yes | No |
| Go / No-Go indication | No | Yes |
| Energy monitoring | Yes | No |
| Performance diagnostics | Yes | No |
| Typical BMS input | Analogue Input (AI) | Digital Input (DI) |
| Complexity | Moderate | Low |
In a well-designed BMS panel:
CTs are installed around the load cable and wired to:
CSs are installed around the load cable and wired to:
It is common — and good practice — to use both:
A simple rule used on Alpha Controls projects:
Specifying the correct device at design stage avoids:
Current Transformers and Current Switches may look similar on a drawing, but in practice they deliver very different levels of information.
Understanding when to use each ensures:
At Alpha Controls, current monitoring devices are selected based on how the building should behave, not just how quickly it can be installed.
Need help specifying current monitoring for your BMS project? Contact Alpha Controls for expert guidance.
Our team of building automation specialists is ready to help you optimise your building's performance and efficiency.
Get in Touch
