RCD tripping

[Dambo]

Hey,

Just after some advice on a CU problem I came across today

I replaced an old rewireable fuseboard with a new dual RCD CU. The RCDs tripped when switched on. When working my way through each MCB to try and find the circuit(s) causing the problem it came to light that the RCDs were tripping when none of the MCBs were on i.e. only the main switch and RCDs were on.

Has anyone come across this before?

Cheers

 

[Manator]

Looks like you need to test more before, and during any board change. It sounds to me like a borrowed neutral, or the IR is far too low.

You will need to test to find the problem.

 

[ProDave]

Or a N-E short on one of the circuits.

What testing did you do on each circuit?

 

[Dambo]

I tested the IR before and it was fine. I'm back in the morning to investigate further. A borrowed neutral could be the answer. The occupant mentioned today that, from what they understand, the previous owner did some of the electrical work himself so that would add up.

 

[boltonsparky]

Prime for this is one of the socket or switch screws has clipped the neutral cable thus shorting earth to neutral which trips the RCD without the MCBs being on.

EDIT

However this should have shown up in the IR test so probably not the cause.

 

[NozSpark]

Dambo, you should really be carrying out the prescribed testing before you power up after a CU change,,,, and maybe even do some basic testing when you look at the job.

Anyway as to your fault,,, like Manator says it'll be either a borrowed neutral or a low N-E Insulation resistance

 

[Marvo]

2-Way lighting systems are one of the main suspects for borrowed neutrals. Could be a crossed neutral in the CU as well. You should be able to localise the fault to one or even two circuits by disconnecting all the neutrals and testing them one at a time to find the circuit with either low IR or find the crossed neutral. For future it's better to test all circuits and establish if there's any crossed/borrowed neutrals before the RCD's are installed, that way you can include it on quote and get paid for the time involved.

 

[Andy™]

Hey,Just after some advice on a CU problem I came across today

I replaced an old rewireable fuseboard with a new dual RCD CU. The RCDs tripped when switched on. When working my way through each MCB to try and find the circuit(s) causing the problem it came to light that the RCDs were tripping when none of the MCBs were on i.e. only the main switch and RCDs were on.

Has anyone come across this before?

Cheers

in other words, you just swapped the board and switched it on without doinhg any form of testing, and hoping for the best. which in this occasion, didnt happen.

as for the fault of RCD tripping with no MCB's on, yes. been to those faults many times. doesnt take long with an IR tester to identify the problem & offending circuit.

---------- Post Auto-Merged at 20:16 ---------- Previous post was made at 20:15 ----------

What testing did you do on each circuit?

breaker test. it failed, now he needs our advice to find out why.

 

[Doc Hudson]

Hey,Just after some advice on a CU problem I came across today

I replaced an old rewireable fuseboard with a new dual RCD CU. The RCDs tripped when switched on. When working my way through each MCB to try and find the circuit(s) causing the problem it came to light that the RCDs were tripping when none of the MCBs were on i.e. only the main switch and RCDs were on.

Has anyone come across this before?

Cheers

Is this a students college question? as I would not expect any competent person would be energizing circuits they have not fully tested whilst the circuits are dead. An RCD will detect an imbalance, that is current flowing from Line to earth OR Neutral to earth at any part of the circuits that come after the RCD. When in dual RCD boards this typically means multiple circuits could be the possible cause. And as N-E faults can cause RCDs to operate clearly everyone knows that the MCBs do not have to be on.

Doc H.

---------- Post Auto-Merged at 20:29 ---------- Previous post was made at 20:29 ----------

thread moved.

 

[springcrocus]

Hey,Just after some advice on a CU problem I came across today

I replaced an old rewireable fuseboard with a new dual RCD CU. The RCDs tripped when switched on. When working my way through each MCB to try and find the circuit(s) causing the problem it came to light that the RCDs were tripping when none of the MCBs were on i.e. only the main switch and RCDs were on.

Has anyone come across this before?

Cheers

I find that hard to believe. RCD's don't trip when there is no current at all going through them.

 

[Andy™]

I find that hard to believe. RCD's don't trip when there is no current at all going through them.

yes, they do. if you have a N-E fault, with some power coming from the earth into the neutral, then it will trip, even without any MCB's being on. rare, but possible and something i have found whilst fault finding and more than once

 

[Doc Hudson]

I find that hard to believe. RCD's don't trip when there is no current at all going through them.

You are correct but clearly, you are applying a level of basic electrical science that Dambo has failed to recognise. As it is a dual RCD board I suspect Dambo is saying that all MCB's on one of the RCD's are off, but is missing the obvious of a cross connection from a final circuit off the other RCD's protected circuits. I find it hard to believe than anyone competent enough to change a CU cannot carry out essential basic tests to either prevent this sort of fault or identify the cause quickly if it does occur. Clearly training standards within this industry are slipping and I find this very worrying.

Doc H.

 

[kme]

I`m sorry - its a bit of a grey area as to whether the thread SHOULD have been moved Doc........

Dambo - you`ve been here long enough - you`re accepted as a scam member - you`ve seen this type of question time & time again.

Yet you`re out there, doing the exact thing you ought not to be doing - and you know it, FCOL!

I assume this isn`t one of the jobs you were going to show the assessor?

I`m saying no more - else I`ll have to ban myself or summat!

 

[Andy™]

I assume this isn`t one of the jobs you were going to show the assessor?

as long as the membership payment clears, it doesnt matter if he does show this job or not, he will pass anyway

 

[lineman]

As said N fault. Drop the neutrals, meg em out to earth, find faulty circuit, trace it from there. Bell out neutral bars to each other to look for a borrowed N. easy. If both RCDs are tripping you could have dropped a ball and put one side of a ring on each RCD. I don't reckon it will be any more than the three faults mentioned above..

 

[steptoe]

You are correct but clearly, you are applying a level of basic electrical science that Dambo has failed to recognise. As it is a dual RCD board I suspect Dambo is saying that all MCB's on one of the RCD's are off, but is missing the obvious of a cross connection from a final circuit off the other RCD's protected circuits. I find it hard to believe than anyone competent enough to change a CU cannot carry out essential basic tests to either prevent this sort of fault or identify the cause quickly if it does occur. Clearly training standards within this industry are slipping and I find this very worrying.Doc H.

thats a debatable point given the type of fault he is now asking us to fix.

 

[springcrocus]

[quote name='Andy