RCD tripping- earth neutral fault

[Gixxerdan]

Hi guys.

I have a problem. Last night my partners dads lights went out at his home. He called out a guy who had a look round and found that the RCD feeding two MCBs for lights and sockets kept tripping. He has bypassed this and the lights and sockets now works. He has said it is due to a earth neutral fault. He couldn't carry out any more work as unqualified im guessing so has put a job card in for someone to come round and have a look. I was just curious as to what an earth neutral fault is? My first instinct when he told me what had happened was that it could be a faulty RCD and that a new one should be tried first but i'm not sure as to how prone they are to failing?? thanks for any advice/comments

 

[ringer]

The mcb only switches the line. The neutral is still connected to the neutral bar. So with current flowing through other circuits that are covered by the same rcd, some of that current can flow along the neutral bar, up the neutral of the circuit you believe to have been isolated, through the neutral of the plug/socket where the faulty appliance is plugged in, up the neutral of the faulty equipment and then down the earth of that equipment via the N-E fault within that equipment.

 

[Alan Marriott]

Can I ask stupid question in this subject please? Could a Live/Earth fault cause tripping RCD also? If you had a rcd protected board is it possible for the rcd to trip and not the mcb on a live/earth fault? Thanks

 

[Avo Mk8]

Can I ask stupid question in this subject please? Could a Live/Earth fault cause tripping RCD also? If you had a rcd protected board is it possible for the rcd to trip and not the mcb on a live/earth fault? Thanks

Yes. The RCD trips when the current flowing in L and N are different by more than 20ish mA.
So if there is a L to E leakage of 10's of mA you would expect the RCD to trip, but not the MCB.

it sometimes happens that appliances with a heating element (immersion heater, washing machine, kettle, oven, etc) develop a leakage to earth, which will trip an RCD, but not the MCB.
Also lots of loads that individually have EMI (interference) filters in them, all connected to the same circuit, can result in excessive leakage to earth, which can cause nuisance tripping of an RCD.

 

[Alan Marriott]

Yes. The RCD trips when the current flowing in L and N are different by more than 20ish mA.
So if there is a L to E leakage of 10's of mA you would expect the RCD to trip, but not the MCB.

it sometimes happens that appliances with a heating element (immersion heater, washing machine, kettle, etc) develop a leakage to earth, which will trip an RCD, but not the MCB.

Thanks for that. I had a db which main rcd had tripped. I turned all circuits off,reset rcd and turned on one at a time turns out it was an immersion heater. So was curious what may have caused the rcd to trip and not mcb. Thanks again