Underfloor Heating Problem

[Jack07]

Hey,

The builder on a job I'm on recently installed an underfloor heating system in a clients bathroom, which is tripping the RCD when it's powered up.

I've done the usual ohms test and continuity test on the cold tails and the readings seem fine

I'm getting 99ohms between L&N (97 in manual) and L&E N&E are giving a reading of >500

but the RCD is tripping as soon as the thermostat is switched on.

With the RCD tripping and not the MCB there must be a problem between N&E ?(or earth leakage)

Anyone any ideas why the readings from the cold tails  seem fine but the RCD keeps tripping?

Thanks in advance

 

[Doc Hudson]

What are your test readings on the circuit before it reaches the cold tails, CU to thermostat controller. Is the RCD dedicated only for the floor heating, or is it shared with other circuits. If shared what are the test readings on those circuits. Is it supplied via a fused spur? trapped wire in back of spur could be a possibility. Have you proved the RCD is operating correctly and is not over sensitive, what are trip times and ramp test values.

Doc H.

 

[ProDave]

I presume you mean >500M hms  not >500  hms  ?

Is it a new circuit?  If so is neutral in the right N block?

 

[Jack07]

Thanks for the reply.

I'm back at the job tomorrow so I was going to try the heating circuit on the other RCD.

Its a shared RCD. Yeah it's supplied via a fused spur.

I'll have more time tomorrow to have a better look and take more readings. I only had about 30mins

on Friday to have a look and carry out basic tests.

I presume you mean >500M hms  not >500  hms  ?

Is it a new circuit?  If so is neutral in the right N block?

Sorry, yes >500 ohms

It's a new circuit. Neutral is definitley in the correct side.

 

[Doc Hudson]

You should have already carried out your tests on the supplying circuit and RCD before attempting to energise the floor mat, so you should not need to 'try the other RCD'.

Doc H.

 

[Murdoch]

Have you tested the cable from the FCU to the controller separately? Who did the install? how deep is the back box for the controller? If its not deep enough its really easy to catch a cable!

 

[Evans Electric]

Has anything been fixed to the floor , bath legs, toilet pan, towelrail ?

 

[ProDave]

Thanks for the reply.

I'm back at the job tomorrow so I was going to try the heating circuit on the other RCD.

Its a shared RCD. Yeah it's supplied via a fused spur.

I'll have more time tomorrow to have a better look and take more readings. I only had about 30mins

on Friday to have a look and carry out basic tests.

Sorry, yes >500 ohms

It's a new circuit. Neutral is definitley in the correct side.

If it's 500 ohms, not 500 Megohms, then there is your problem.

 

[jonny the spark]

I'm interested to know how this situation ended,

did you find the fault?

if so, did the whole floor come up?

did you just leave it disconnected and explain to the customer?

Had a similar fault myself.

 

[pewter]

One of my good customers had an extension and the builder and his electrician installed under floor heating.

A year later the Matt started to cause the RCD to trip. When I tested the Matt to Earth the reading was low (<1Mohm). I disconnected it and they now have an electric radiator.

 

[SLIPSHOD & SLAPDASH]

We have had faults caused by tilers repaired, costs a fortune (i think they use x-ray, thermal & hi tech volt sticks. You also have to have replacement tile(s) for where the damage is.