RCD tripping- earth neutral fault

Gixxerdan · May 31, 2010

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

Sintra · May 31, 2010

I would tend to agree with the neutral / earth fault diagnosis. However before bypassing the RCD he should have disconnected all appliances from the sockets and then tried to reset RCD as it is more than likely a fault on an appliance that is causing the problem.

Deleted member 9648 · May 31, 2010

Personally I would not bypass the RCD,it is tripping because there is a fault on the circuit which should be rectified....A N-E fault seems most likely and is unlikely to be dangerous....but as apparently no tests were carried out there may be a more serious problem,get it checked properly ASAP.

bugsy · May 31, 2010

I agree with the last post , not a good idea to bypass RCD I/R test required to determine fault

baldsparkies · May 31, 2010

I also agree with the other posts. Even if this guy had no test equipment on board he should at very least attempted to isolate the problem by removing any plugged in appliances. To remove the rcd from service is not the way to go. Please have this rectified asap as the neutral to earth fault is only guesswork at this stage.

davelerave · May 31, 2010

yep -never bypass an rcd

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

Gixxerdan · Jun 1, 2010

what is a n-e fault thou? Am i right in saying bypassing an rcd can mean that impedance could no be too high for fault current to flow to earth so fault current keeps flowing to earth and could cause fires? As far as i know, all appliances were disconnected when he tried resetting rcd

Deleted member 9648 · Jun 1, 2010

A N-E fault is simply a low resistance between N and Earth somewhere on the circuit,either on the wiring or a load on the circuit. As N and E are at the same potential such a fault is unlikely to present a hazard,and before RCD's became the norm such a fault would have gone un-noticed unless an IR test was carried out.
However as no tests were carried out it could also be a Live-earth fault,with too high a resistance to trip the overload protection,but sufficient to cause a hazard. This is why the RCD should not be bypassed unless the person bypassing it has verified the nature of the fault by testing,the RCD is there for a reason and is doing it's job....protecting the end user.

billy110 · Jun 1, 2010

The guy should have carried out basic tests and N to E faults usual suspects for RCD tripping but not always, the RCD could be defective. I have replaced 3 nearly new Wylex units for different clients in the last 2 months alone.

Bandula · Sep 17, 2010

This is the last thing I want to happen to me in the middle of the night (this of course happens in the middle of the night, I have explained later)
You may be already knowing that if there is a live earth fault there it is easy to isolate it by switching off all the MCBs and switching them on again one by one while the RCD is on position. As soon as you switch on the faulty MCB the RDC goes to off. Then you can Isolate the problem in that section and repair it.

But earth neutral fault you can never be isolated like that as the neutral is common and originates from a common hub just after the RCD.

How to Diagnose earth to neutral fault? as I said it is difficult, if you can not isolate a Live to Earth fault and RCD trips while all the MCBs are off position; two things are possible 1) Malfunctioning RCD 2) Neutral to Earth fault. you can confirm a malfunctioning RCD by removing both live and neutral of the MCB side if the RCD still goes to off while above both connections are disconnected that means RCD faulty, If you can turn on the RCD while both wires (of MCB side ) are disconnected and the connect only live or neutral separately one by one. You will see that RCD goes to off with the neutral.

Causes of neutral to earth fault? Most possible cause rat bitting and short circuiting a three core wire with Live Neutral and Earth running together (rat bites and short circuit Neutral and Earth) (there is a chance that affected people may have experience tripping in the middle of the night several times before they start full black out one day) (why ? as and when rat bites the live wire the RCD goes off saving the life of the rat).

That is why this is most of the time happening in the middle of the night!
Isolating the place is very difficult? think about possible places vulnerable to rat attack! Specially did you recently block a rat entry hall close to a three core wire?
other method need expert on this field. Turn off main switch take a multimeter and measure the resistance between Earth and Neutral of a normal plug point
(normally it is around 2-3 ohms if there is a Neutral to Earth fault ) check all the other plug points you will get minimum resistance value as and when you are closer to the fault. It is a very costly time taking procedure as you are looking for a wire damage which may even be smaller than 1 inch !

Rats cut off my Satellite LBN wire for blocking an entry point to a rat hole? can they think like humans?