Underfloor heating failed or just RCD

[DavidM]

Brother in law asked me to take a look at his bathroom undefloor heating - the RCD supplying it keeps tripping off, but only after it's been on for 30-45 minutes or so.

I disconnected the load (heating cable) from the back of the control (timer/thermostat) but left the temp sensor connections in place. The RCD still trips.

I then disconnected the temp sensor wires and the RCD doesn't trip.

There's no CPC connection in/to the control unit itself, and no provision for one - it's all plastic, but there are two CPCs linked together by a terminal block in the back box (note not 3 CPCs as I might have expected - supply, load and sensor cables).

So it seems to me that it's either the temp sensor cable gone faulty (in which case the heating is a write off - underneath tiled floor) or the RCD has gone faulty - over sensitive maybe. I noticed when I push and hold the test button on the RCD it buzzes, even after it has tripped off. I don't have the kit to test the RCD properly, but it might be worth just replacing it and see what happens - it's just mounted in normal surface box outside the bathroom, and also has fuse in it's faceplate.

Any thoughts?

[cynic]

SNIP

Doesn't your own logic narrow this down somewhat? Time to get some money out and use the correct test equipment on the sensor circuit.

[Tim]

It's possible that the sensor might be replaceable. I seem to recall that our sensor was placed in a tube in the tile cement specifically so that it could be replaced.

Tim

[Tabby]

There's no reason a dud sensor would write off a heating system. If its the sensor, you can simply run the system without one by reducing the power to it, or add a temp sensor on the surface.

NT