RCD tripping with underfloor heating

[bonn1ett]

Can anyone offer some advice......

Customer recently moved into property, underfloor heating not working but looks like quite a new install. First problem i found was the switched FCU was faulty that supplies the underfloor heating - replaced that, switched on still nothing illuminating on the thermostat screen? Removed thermostat cover to find all the wires were disconnected form the thermostat terminals (alarm bells ringing now). I tested continuity of underfloor element - was 63ohms which is what it should be according to the manual.

I connected everything up, switched on FCU and the rcd tripped in CU. Disconnected all wires from back of thermostat controller including earth, switched on and rcd stayed on. I then connected the live and neutral incoming only to the back of thermostat control (element and thermostat disconnected still), switched on and rcd tripped? I carried out a ramp test on the rcd and tripped at 18mA........could it be the rcd is too sensitive?

 

[telectrix]

you n eed to do IR at the RCD output. testing L-E and N-E. first with the mat dissed, and then IR test the mat.

 

[bonn1ett]

ok, but the mat is not affecting the rcd as its tripping with the mat diconnected? With only L and N connected (no load) to thermostat its tripping. When i disconnect everything, cables in free air, the rcd stays on, wouldn't this imply that the cable from the CU through the FCU and up to the thermostat is ok?

 

[telectrix]

just double check in the CU. could be that the neutral for the U/F is in the wrong bar.

 

[bonn1ett]

yes, i will check that, good point! I just thought that perhaps the rcd could be too sensitive (tripping at 18mA) and perhaps replace it?

 

[telectrix]

18mAis a bit low, but there could be a bit of leakage somewhere in the house. ramp test the RCD with no circuits/loads connected. if it still trips at 18mA, i suggest replacement.

 

[bonn1ett]

yes will try that. Ran out of time to carry out more tests, advised customer that probable cause is the rcd and may need replacing but will double check everything else first. Don't want to make customer hand over £70-£80 for a replacement rcd if that's not the problem! Thanks the advice.

 

[telectrix]

you're welcome. just click on the thanks button, bottom left of the post/s.

 

[Engineer54]

yes will try that. Ran out of time to carry out more tests, advised customer that probable cause is the rcd and may need replacing but will double check everything else first. Don't want to make customer hand over £70-£80 for a replacement rcd if that's not the problem! Thanks the advice.

That's a dammed expensive RCD, who's make is it??

 

[telectrix]

£70 including labour ain't excessive. E54.

 

[bonn1ett]

That's a dammed expensive RCD, who's make is it??

The make is MK and looking in my TLC catalogue it comes in at £62.16 (inc VAT). Thats for a 30mA 63Amp RCD