RCD fault help required

[tbs1983]

Advice required as my fault finding experience isn't as vast as many on this forum.

Called to a house last night, RCD tripping. 16th ed board. Garage, Cooker, Sockets & Shower on RCD.
RCD holds with all MCB's on bar the sockets.
Once sockets MCB is turned on sometimes it holds for 10 mins or so then randomly trips, other times it trips straight away.
There is ring continuity (0.71,0.71,1.08) although on lifting floorboards its clear that there are several spurs from JB's on this circuit.
Insulation resistance - L-N >200, L-E >200, N-E 0.71
Based on the above I believe ghe neutral-earth low IR is causing the fault,
1st question - is 0.71 mega ohms low enough to trip RCD.
2nd question - Would a Ramp test be of any help (don't have this function
on my MFT, but could borrow one).
3rd question - if it is this N-E fault why does the MCB have to be on for it to trip?
Thanks in advance

 

[pjcomp]

Make sure everything is unplugged from the sockets circuit (hidden things behind chairs and in cupboards ...) and see if it still trips, as it's more likely to be an appliance than the wiring (assuming you've asked the usual questions about recent work, nails in walls, floors etc). Time-delay tripping suggests something that switches itself on and off, like an old freezer or fridge, or the boiler. And don't forget to check for anything outside that could have got water in it.

pj

 

[sparkstu]

was there a tumble dryer or washing machine on at the time could be at certain point through there cycly causing problems and the same as above make sure everything is unplugged

 

[Lucien Nunes]

Agree with the above, but to answer your specific question

1st question - is 0.71 mega ohms low enough to trip RCD.

No, it isn't, if it is the true insulation resistance, especially from N-E with typically only a few volts between them. You can get an approximate idea from ohms law, say six volts drop in each of L & N of the final and a few at the origin, call it a maximum of 10V across the suspected 'fault'. 10/710,000 =14 microamps, around a thousand times lower than you would need to be causing occasional tripping. But as mentioned above your reading might not be representative of what is going on when the circuit is energised. If you are lucky the lower IR might at least lead you to the fault, even though 0.71 megohms won't cause tripping by itself. FWIW if an RCD trips at 20mA, even from line to earth you need around 230/0.02 = 0.012 megohms to directly cause a trip, if there is no other source of leakage.

 

[telectrix]

too much brain work for a saturday. :behead:

 

[terminator]

You need to identify everything on that circuit - I would be looking to see what's on those spurs off the JB's. You need to check out every accessory that is connected to that circuit and isolate each. As in post #2 - has the customer had any work done recently that preceded this problem? TV booster in loft recently fitted, etc etc.

 

[jolly]

Had a similar fault before. Believed at 1st that it was an iron that was faulty, though the woman said it took roughly 10mins before tripped. After carrying out Zs on sockets one was particular higher than rest. Again thought was spur. So opened the back of the socket to check visually and found that in was a ring and the connections in the back were not secure and were just making connection. So re terminated and got the Zs down to the same ad others. Then got told by owner that she only uses that socket when ironing. Hope this helps .

 

[Octopus]

Unplug everything you can and see what happens.

Did you do a ramp test? Could be a knackered RCD.

 

[Deleted member 9648]

Are the IR results for the ring only?....if you havent done a global IR on all the circuits off the RCD you should do. I know you say it only trips when the ring MCB is on but without testing the other circuits as well you may be misled. Eliminate all other circuits from your enquiries before taking it as gospel the ring is at fault.