A BMS that has commanded an AHU fan to run is not necessarily looking at a running fan. The digital output may be energised, the contactor may have pulled in — but a tripped overload, a blown control fuse, or a seized motor bearing means the fan isn't moving. Without the right monitoring device wired back to the BMS controller, the system sits there believing the air handling is working, running its sequences against a building that's getting no fresh air. This is a real failure mode, and it's one that the wrong type of current monitoring device will fail to catch.
Both Current Transformers (CTs) and Current Switches (CSs) are installed on load conductors and both respond to electrical current flow — but they answer fundamentally different questions. Specifying the wrong device means either missing the diagnostic detail you need or generating misleading signals when something goes wrong.
A Current Transformer is a measurement device. It clamps around a live conductor and induces a proportional secondary current — typically 0–5 A — that reflects the actual current flowing through the primary cable. That secondary signal is either fed directly into an energy meter or passed through a transducer that converts it to a 0–10 V or 4–20 mA signal, which is then wired to an analogue input (AI) on the BMS controller. The result is a real-time numerical reading of motor or load current.
The value of this is diagnostic. A CT on an AHU supply fan allows the BMS to trend motor current over time and compare it against the design nameplate figure. A gradual increase in current over weeks or months is the signature of a building-up blockage — a dirty filter, a closing damper, a developing bearing fault — long before the motor trips or the fault reaches the operator's attention. A CT on a chiller compressor allows the controls system to calculate real-time power draw and, when combined with metered thermal output, to calculate COP continuously. On pumps driven by VSDs, current trending reveals hydraulic changes in the circuit — blocked strainers, partially closed isolators, cavitation — that would otherwise go undiagnosed until something fails.
CTs are installed on critical or energy-intensive plant where performance visibility and early fault detection justify the analogue input cost. AHU fans and extract fans, primary and secondary pumps, chillers, boilers, and electrical sub-metering panels are all natural CT applications.
A Current Switch monitors the same conductor but answers a much simpler question: is current flowing above a set threshold? The output is a volt-free contact — normally open or normally closed — that changes state when the current exceeds the trip point. That contact is wired to a digital input (DI) on the BMS controller and appears as a binary value: equipment running or not running.
This is exactly what is needed for proof of operation. Where the BMS needs to know whether a fan has actually started, a pump is circulating, or a motor is drawing current, a current switch provides a clean, reliable answer. It is faster to install, cheaper than a CT-transducer combination, and requires no analogue input or signal calibration. For the majority of FCU fan motors, small circulation pumps, and secondary plant items where confirmation rather than measurement is what's needed, a current switch is the correct specification.
Current switches come in fixed set-point versions — factory set at 0.5 A, 5 A, or other common thresholds — and adjustable versions where a potentiometer dial sets the trip point across a range of roughly 1.5 A to 200 A. The adjustable type is more flexible and allows the threshold to be tuned after installation, which matters on variable-speed-driven plant where startup and running currents differ significantly from nameplate full-load values. For retrofits and live upgrades, split-core current switches snap onto existing cables without disconnection — a practical advantage when working on energised panels. Solid-core versions require the conductor to be threaded through during initial installation and are marginally more accurate but rarely worth the extra disruption on a retrofit.
On critical plant — primary AHU fans, main chilled water pumps, condensers — the sensible approach is to use both: a current switch wired to a digital input for fast, clean run confirmation and interlocking, and a CT wired to an analogue input for performance trending and diagnostics. The run signal from the current switch tells the BMS the plant is operating; the current value from the CT tells it how hard. In a well-designed control panel, this is standard practice and adds modest cost compared with the diagnostic value it delivers over the plant's lifetime.
BS 7671:2018 governs the electrical installation of both devices — cable sizing, termination standards, earthing arrangements, and panel protective device coordination all apply. Critically, secondary wiring from CTs must not be left open-circuit while the primary conductor is energised — open-circuit secondary voltage can reach dangerously high levels and will damage the CT. Where BMS panels are being installed or upgraded, all current monitoring wiring should be documented on as-built drawings with device ratings, trip-point settings, and input assignments clearly noted. This is the information a future commissioning or maintenance engineer needs to verify the system is functioning correctly, and it is routinely missing from older installations that were specified quickly.
The practical decision comes down to what the building actually needs from each monitoring point. For proof of operation on secondary plant — FCU fans, small pumps, extract fans on a fresh air unit — a current switch is the right answer: simple, reliable, low installation cost. For primary or energy-intensive plant where fault diagnosis, performance trending, or energy metering is required — supply fans, main pumps, chillers, heat pump compressors — a CT gives you the measurement data that a current switch cannot. For safety-critical interlocking, a current switch's volt-free contact is preferable to relying on analogue signal processing. For compliance with ESOS, MEES, or sub-metering requirements, only a CT feeding a properly calibrated energy meter provides the measurement accuracy and audit trail those frameworks require. BS EN 61869-2 — the standard for current transformers — classifies CTs by accuracy class (0.1, 0.2, 0.5, 1, 3, 5) relative to rated current; BMS energy metering applications typically require Class 1 or Class 0.5 — Class 3 or 5 CTs, which are cheaper and more common in older installations, introduce measurement errors that make sub-metering data unreliable for energy management purposes.
Getting the specification right at design stage is significantly cheaper than retrofitting the device you should have fitted in the first place. Alpha Controls specifies current monitoring devices to suit the control strategy and long-term operational requirements of each building — not just what's quickest to install. Contact us to discuss your BMS project, or see our BMS installation services and commissioning pages.
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