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

 

[nickblake]

Squirrels

 

[tenheads]

This rat thing is making me giggle but i have had on two occasions cables being damaged by rodents nibbled right into the cable its especially frustrating when you have just done a board change and circuits are now rcd prot and you have to start looking for a little mouse bite in a cable little blighters.

 

[davelerave]

they cause a bit of bother alright

one bit clean through a 3*1.5 flex a while back on a job
plenty of bother with rats+mice when i worked on ind. maintenance-caused various breakdowns

 

[Bandula]

he must live in a rough area infested with rats

so as Mr Obama?
Rats in white house !! Rat in White House Rose Garden? - ABC News

 

[Bandula]

Neutral to Earth fault (short circuit) plastic outer cover bitten and damaged, short circuiting the Neutral to Earth. The RCD trips (even if you keep all the MCBs off). No way to get power back .. either you find the fault and repair it or bypass the RCD (not recommended)

 

[Bandula]

and the fault was......??????

Neutral to Earth fault (short circuit) plastic outer cover bitten and damaged, short circuiting the Neutral to Earth. The RCD trips (even if you keep all the MCBs off). No way to get power back .. either you find the fault and repair it or bypass the RCD (not recommended)

 

[skip]

an R C D works on balance ; for example if you have 10 amps going through the live you must have 10 amps returning through the neutral if their is any difference it can only be going to one place EARTH the R C D will detect this and trip. unplug all appliances . then plug them back in one by one until it trips most likely white goods agree with everyone else never bypass an R C D a bad move

 

[chrisreid186]

yep -never bypass an rcd

N-E fault usually trips rcd when you plug in a class2 appliance on a protected circuit

can you explain why this happens???

i came across a similar situation but class one equipment (earthed) tripped the RCD even with the ring mcb off
and class 2 equipment worked fine with the mcb on/off

pretty sure its down to a earth / neutral fault but cant figure out how class one trips the RCD as soon as its plugged in?????

 

[telectrix]

it trips with the mcb off because you have zero current flowing in the line. anneutral/earth fault wilcause current to flow throgh the neutral to earth, thus causing inbalance. with class 2 equipment, the earth is not connected to the appiance, so no fault current. sssssssimple.

 

[WayneL]

it trips with the mcb off because you have zero current flowing in the line. anneutral/earth fault wilcause current to flow throgh the neutral to earth, thus causing inbalance. with class 2 equipment, the earth is not connected to the appiance, so no fault current. sssssssimple.

If you have zero current flowing in the Line, how is there anything flowing from neutral to earth to cause an imbalance - I thought the neutral was 0v and earth was 0v.....so no flow.

What difference does it make if it's Class 2 and no earth - the fault current doesn't have to flow back down the 'earth' (or should I say CPC) - it can flow anywhere and still cause an imbalance between L & N.......it could flow through you!!...which is kind-of the whole point of RCD protection.

So....maybe not that sssssssimple.

 

[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