problem with RCD

che gomez · Dec 17, 2011

I Have a problem with a job i am going to have a look at soon.The customer has a small garage/workshop that was recently wired and contains basic stuff some sockets and lights but does plan to plug things like compressor, heaters and other machinery to ringmain. The workshop is feed from a 6mm armored from existing fuse board in house on 32a breaker whitch is then protected by a 30ma RCD. The problem is when to many thing are running in the workshop its tripping the RCD in the house (not the MCB).
It sounds like over load but its not tripping the individual MCBs in the workshop or the house. Im thinking it might be a faulty device being plunged in but he says when he runs an extension lead off the house circuit it doesn't happen. It could be The wiring but all the circuits work fine its just seems to be when there is to much load on the ring main and if there was an wiring fault (damage or short) the RCD would not come back on at all......would it?

ringer · Dec 17, 2011

copied from other thread....

I assume there is a garage board because you mention mcbs in the garage. Is there not a rcd in the garage board? If it was recently wired then I would expect that board to have a rcd (proabably rcd main switch). I would also expect the 32A mcb in the main fuseboard to be not on a rcd - particularly if garage is fed by swa. Methinks dicrimination problem. You need to explain the circuits in both boards better for us to help further (RCD or not, how many circuits etc)

Deleted member 9648 · Dec 17, 2011

Possible poor IR to earth is always the first thing to eliminate with RCD tripping....Thoroughly IR test before looking for other causes.

GLENNSPARK · Dec 17, 2011

it aint overcurrent sunshine........if the RCD is tripping due to applied loads then it aint owt to do with overload......will be a line-earth fault or a neutral-earth fault....maybe on one or more of the loads or accumilated earth leakage due to applied loads......IR test the circuits and check all the terminations on this install....could be that as more load/s are appled ant as a consequence the earth leakage increases.......then it just gets to the point where the RCD lets go.......

ringer · Dec 17, 2011

There are of course many ways of supplying a garage/workshop, but the way it has been done is, in my opinion, poor design. Personally I would have put the 6mm swa feed (using 6mm suggest that the installer was aware of heavy loading) on its own 32A breaker which would not be protected by rcd at the main fuseboard, then installed a rcd board in the garage. Now this does not help you solve the current problem, but you could consider it as a way to overcome the problem.

Again, not related to current problem, but I would also be checking the earthing of the armour on the swa and what is used as cpc for the garage board - swa armour or a core? Is there a cert available with test results for the garage install?

Anyways, yes, agree with the above, is most likely accumulated leakage. When the customer uses an extension lead successfully, he is probably only using one or two pieces of equipment, not the whole lot. The way forward is earth leakage clamp meter (if you can get your hands on one) and IR testing all circuits

Octopus · Dec 17, 2011

My suggestion is that you need to visit the customer and come back with a few firm facts then you'd get more constructive responses. I'm guessing you are a sparky with the relevent experience and test equipment?