Testing and calibrating a BMS installation: test gear, insulation tests and sensor checks

A BMS is part of the building’s electrical installation, so it has to be tested like one. It’s also a measurement system, so its sensors need checking against something you can trust. Both jobs need care, and both need proper records.

Electrical testing without killing the electronics

The BMS wiring and panels need the same inspection and testing as any other part of the installation: continuity, polarity, earthing, insulation resistance and the rest, carried out by someone competent and recorded properly.

The catch is insulation resistance testing. IR testers apply a high DC test voltage, and that can damage controllers, sensors, drives and other electronic equipment. Before IR testing:

• disconnect or isolate sensitive equipment, such as controllers, sensors, VSDs and anything with electronics, or test with them disconnected from the circuit under test
• check the manufacturer’s instructions for what can and can’t be tested in place
• where equipment can’t be disconnected, use an approach BS 7671 allows, such as linking the live conductors together and testing only between them and earth, and follow the equipment manufacturer’s guidance
• reconnect everything afterwards and record what was disconnected and reconnected

When a controller turns out to be “dead on arrival” on a new installation, an IR test is one of the first things worth asking about.

Test equipment and its calibration

The readings you take are only as good as the instruments you take them with. A basic BMS test kit usually includes:

• a multimeter (volts, current, resistance, continuity)
• a reference thermometer with suitable probes
• a humidity reference instrument
• a manometer for air pressures
• a loop calibrator for 4–20 mA and 0–10 V signals
• a multifunction installation tester for the electrical testing

Anything you use as a reference should have a current calibration certificate from a recognised lab, showing its accuracy and uncertainty. Keep a register of your test equipment with serial numbers, calibration due dates and certificate copies. Clients and auditors will ask for it.

Checking sensors against a reference

Calibrating a BMS sensor usually means comparing it, in place, with a calibrated reference:

1. Let things settle. Put the reference probe as close to the sensor as possible, and give both time to stabilise, especially in ducts and pipes.
2. Compare the readings and note the difference.
3. Check against the tolerance. Is the difference within the accuracy you’d expect from the sensor and the reference combined?
4. Find the cause before adjusting anything. A big error usually has a reason: the wrong sensor type configured, cable resistance, a poor location or a failed sensor.
5. Adjust if appropriate. Apply an offset in the controller only once you’re sure the sensor and wiring are right. On some sensors, a two-point check (at a low and a high value) shows whether you need to correct the slope as well.
6. Record it. Note the before and after readings, the reference instrument used and its calibration status.

There’s more on the sources of sensor error in BMS sensors.

Calibration curves and lookup tables

Some inputs need more than a simple offset. Non-standard sensors or transducers sometimes need a lookup table or a custom curve in the controller, to convert the raw signal into the right engineering units. If you set one up, document the source (usually the manufacturer’s datasheet) and test it at several points across the range, not just one.

Environmental conditions

Accuracy can depend on where and when you measure. Duct sensors read differently with fans off; humidity sensors are sensitive to nearby moisture sources; outdoor sensors react to sun and wind. Test under the conditions the sensor normally works in, and note anything unusual when you record the results.

The paperwork matters

At the end of testing and calibration, the client should have:

• electrical test results for the BMS installation
• sensor calibration records, showing before and after values and the reference used
• the test equipment used and its calibration status
• any offsets or lookup tables applied, and why
• a list of any defects found and what was done about them

These records protect you as much as the client. They show the system was right at handover.