If your carbon film heater is tripping the breaker, the first step is to identify which protective device is tripping. A circuit breaker tripping and an RCD tripping point to different problems. In this article we explain the causes of each and the steps to solve them.
Warning: Fault finding and repairs must be carried out by a qualified electrician. Do not keep switching the system back on until the problem has been fixed.
Which Device Is Tripping?
| Device Tripping | Meaning | Possible Causes |
|---|---|---|
| Circuit breaker (MCB) | Overcurrent or short circuit | Overloaded circuit, short circuit, loose connection |
| Residual current device (30 mA RCD) | Current leaking to earth | Moisture, damaged film, uninsulated end or connection |
If the Circuit Breaker Trips
1. Overloaded Circuit
The film power plus other appliances on the same circuit may exceed the breaker rating. Check total current = total power ÷ 230 V. Solution: a dedicated circuit with a suitable breaker for the heater. See our sizing and power calculation article.
2. Short Circuit
The film may have been cut lengthways, the two busbars may be touching, or a clamp may be fitted incorrectly. Check the connections with the system de-energised.
3. Thermostat or Contactor Problem
An undersized thermostat’s contacts can weld or overheat. The thermostat current rating should be at least 1.25 times the load.
If the RCD Trips
1. Moisture or Water
Spills, wet cleaning, rising damp or condensation causes earth leakage when it reaches connections and cut ends.
2. Uninsulated Ends and Connections
Failing to seal the copper busbars at the film ends and the clamps on both sides with insulating tape is one of the most common causes. See our cutting and connecting guide for the correct method.
3. Mechanical Damage
Film pierced by nails, staples, screws or heavy objects loses its insulation.
4. Other Appliances on a Shared Circuit
Leakage from another appliance on the same RCD can also trip it.
Step-by-Step Fault Finding
- Note which protective device tripped.
- Disconnect the thermostat and other appliances and test the film on its own.
- Disconnect the strips one by one to find the zone with the problem.
- Measure the resistance of each strip; a very low value indicates a short circuit.
- Measure film-to-earth insulation resistance with an insulation tester; a low value shows where the leakage is.
- Open up, dry and re-insulate the connections and ends of the faulty strip, or replace the strip.
Preventive Advice
- Supply the heater from a dedicated circuit with a suitable breaker and a 30 mA RCD.
- Always carry out an insulation test after installation.
- Do not wet-clean the carpet in place.
- Use a moisture barrier under the film if the floor is damp.
Frequently Asked Questions
Can I bypass the RCD?
Absolutely not. The RCD is essential for personal safety; when it trips, it is warning you of a problem.
Why does the breaker only trip in cold weather?
In cold weather the system runs longer at full load, so a circuit already near its limit can be exceeded, or condensation can cause earth leakage.
Conclusion
If the circuit breaker trips, investigate load and short circuits; if the RCD trips, investigate moisture and insulation. For heating failures, see carbon film heater not heating, or contact us for fault finding and service.
