Under floor heating, RCD tripping

[junior-sparky]

Hello Sparky's,

So today was one of my work experience days that I do once a week, our last visit/job was to a house that a builder had installed under floor heating prior to getting any help from any electrician in the new bathroom he just installed. I've seen this builder on a few jobs and seems an ok guy but not qualified to do any electrics.

So what's happened... the builder has installed the system max 16A and spurred it from the main house ring, both upstairs and downstairs sockets apart from kitchen are on the same 32A MCB. the system at first looks to be working fine but as soon as you plug something like a hair dryer or when they use ther hoover the only RCD in the board trips NOT the mcb when the under flooring heating is not connected everything works fine.

As it was a flying visit to look at it my mentor was not able to go into any depth with me about possible causes but what we did was just disconnect it and hey presto nothing trips anymore.

So straight away I'm thinking that 16A plus the rest of the load of the house (only 2 people living in a 3 bedroom house) will trip the MCB due to overload but as before the only RCD on the board trips.

Could you fine people give me something off the top of your heads an explanation of why it's happening.

 

[PEG]

HI dude,if the RCD is feeding that circuit,and has been tested as working,then earth leakage on one or more of the circuits fed by said RCD. Or,one or more of a dozen other things.....testing required....That will be £60 please...:28:

 

[junior-sparky]

Would it be safe to say that somehow when he connected it under the tiles he has not earthed it properly, not sure how you'd earth it under the tiles but the cpc is deff connected to the the to ring but again I'm Not sure how you'd go about testing it when you can't get to it.

How ow would you go about proving earth leakage ?

Would that explain why when the floor heating is connected and anything like hair dryer or hoovering is connected the rcd trips?

 

[Richard Burns]

An RCD will trip because of an imbalance in the current line to neutral, this leakage current is usually going to earth.
A possible scenario is that the heating mat was damaged when it was put down, all heating mats now tend to have a metal mesh grid associated with the mat that is used as the earth connection, if the heating cables were damaged such that they are just touching the earth grid then the RCD would trip, this fault would be found by performing an insulation resistance test from the mat cable to the earth connection cable, a low value would indicate that the mat is damaged and that it would need replacement or at the very least repair.
However if this were the case the RCD would trip as soon as the mat was switched on to heat.

From your description of the fault it is only when there is another load on the circuit that the RCD trips.
(One question is is it the RCD for the ring final that trips?)
This may mean that the fault is a slight neutral to earth fault (or possibly that the is a shared neutral in the board, but since that should not have changed unlikely) without a heavy load the majority of the current is going via its normal route with a small percentage (<30mA) leakage to earth, once there is a higher current demand then the percentage of current leaking exceeds 30mA and the RCD trips.
Same test on the mat to check the fault.