Strange RCD tripping fault, any ideas!?

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Had a look at some electric underfloor heating tonight causing RCD to trip.
It has only recently been installed and all metered ok when fitted a couple months ago. Has been working fine since, the place is still undergoing building work.
RCD trips instantly when heating is turned on on the stat. Meggered to start with at 250v and had 200M to earth. All ok I thought. Tried it at 500V, and got 0.00M. Tried again at 250V and still now at 0.00M.
The chippy there said that one tack nail when fitting the wooden flooring was put in the wrong place, so he showed me which nail it was, it was near where the UFH element runs. Put megger to the nail and to the live conductors, got 1.3M. Other nails near by up to around 70M.
Thinking the nail may have hit the element, we took up the floor, no damage was visible, however now the reading L-E and N-E is back to 200M at 250V.
Tried plugging it back in without the thermostat just straight on a plug top, and RCD tripped again.

Continuity of the element is at 16ohms.

Any ideas?
 
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Did you disconnect the elements from the controller when meggering ? ( I believe you would have ) If not then something in the controller may be the culprit. OK at 230 volts but becomes conductive at 500 volt and remains conductive for a while afterwards. Maybe a protective semiconductor device on the controller's output has gone faulty.

Could be another nail that is just piercing the element and depending on who is standing where on the floor the nail either touches or does not touch the element.
 
Yes had the element disconnected from the controller for testing, and the last time I powered up it was just the element straight on with a temporary plug top so no controller isn't the issue. No one else was on the floor when testing, and it's only a small floor! Strange one which has got me scratching my head!
 
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Also has some lighting and sockets on the RCD. All wiring is new, however some power tools would have been plugged in to the sockets.
 
No I haven't but then why would the element go from 0.00M back to 200M? I'm going back there tomorow so could megger everything else anyway.
 
My father had under floor heating fitted and I came in to find the builder had hit the cable and had used insulation tape to join the cable. I took photos and waited to see him. Sad to say this covering up of a fault is common and it causes all sorts of odd readings.
 
Went back this morning and tried meggering the whilst someone else walked around on the floor but nothing changed, stayed at 200M on 250v but dropped to 0.00M on 500V
Eric - I know it's nothing like that as the builder is the customer of mine (who happens to be a friends father) and he is only the one to have worked there (apart from me!)
I wondered if it could be the joint between the flex and where it joins the element but reluctant to take apart as this will void the warranty.
I spoke to the manufacturers and for the cost of around £300 they will come out and pin point the fault and repair. Obviously if it's a manufacturing default it's free.
 
Went back this morning and tried meggering the whilst someone else walked around on the floor but nothing changed, stayed at 200M on 250v but dropped to 0.00M on 500V.
You may be interested in this thread which I started in relation to an experience I had at my daughter's house - as with you, I was getting very high IR readings at 250V and low ones at 500V (although they reverted to being high immediately I went back down to 250V). In that case, I eventually discovered that the cause had been a previously 'unknown' (and 'dead') RCD still wired into the circuit that was hidden away in a cupboard!

Kind Regards, John
 

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