Fault on ring main

[Durksy]

My in laws recently returned from holiday to find that their downstairs ring main was off.when I looked the mcb for this circuit was in the odd position and on trying to switch on it tripped the main rcd switch for this set of circuits.after first unplugging any appliances and this not resolving the problem I started to look at hard wiring,I disconnected one leg of the ring from the mcb switched on and this cleared the fault.However when testing voltage at each socket it produces strange results. Live - cpc 14v , live - neutral 240v , neutral - cpc 240v. I get the same reading back at the board on the disconnected leg but on the leg still connected I get what you would expect ie live - cpc 240v, live - neutral 240v and neutral - cpc 0v.
Any ideas why this would be? And why(although I only been told this by them) the circuit has tripped once since supposedly getting rid of the fault.

 

[Darkwood]

Welcome to the Forum Durksy..

What other tests have you done and what were the results?

 

[Leesparkykent]

Maybe a Disconnected neutral somewhere.

 

[Darkwood]

With those results it would suggest the cpc is close to mains potential hence we get the voltage between N-E but very little between L-E or polarity issues hence I'm asking what were the results of the dead tests on the circuit.

 

[Durksy]

With those results it would suggest the cpc is close to mains potential hence we get the voltage between N-E but very little between L-E or polarity issues hence I'm asking what were the results of the dead tests on the circuit.

at this point no actual testing done was simply a quick test with voltage/continuity testers to prove that I still had power going to the circuit.

 

[Darkwood]

at this point no actual testing done was simply a quick test with voltage/continuity testers to prove that I still had power going to the circuit.

Remember the sequence of tests you were taught, once you established the rcd would not reset even with everything unplugged/switched off then its likely a fault is present and at this stage you shouldn't re-energise any circuit covered by the rcd until you have established the circuits are at least sound through dead testing.

On that note then I hope you disconnected the circuit until you can perform the correct sequence of tests and establish what the issue is here - with those results you may have put your family at risk or even the property, are these results local to the circuit or do we have similar results on the supply to the property and/or other circuits... my worry here is if you have a supply issue and confirm it then you have a dangerous problem and the power should be isolated and DNO emergency called.

 

[TJ Anderson]

I see it like this. I believe the supply is fine based on the readings taken at MCB. It would appear you have a L-E fault out on the circuit AND the earth is floating beyond this fault point (in it's current radial state you have temporarily created by disconnecting one leg at DB). As already said on here, do not leave this circuit energised. You have not cleared the fault at all, you have merely disconnected the leg that was actually connected to earth and left a floating one, hence the RCD not tripping. Find the L-E fault though dead testing and rectify the end-to-end continuity also.

 

[telectrix]

i sincerely hope that the OP is back there at this minute, sorting it out before someone is injured.

 

[Durksy]

i sincerely hope that the OP is back there at this minute, sorting it out before someone is injured.

Ok thanks a lot that makes sense.and don't worry the circuit is currently switched off

 

[davesparks]

Ok thanks a lot that makes sense.and don't worry the circuit is currently switched off

It should be isolated, not switched of, there is a difference!

 

[Durksy]

It should be isolated, not switched of, there is a difference!

i am well aware of this which is why it is locked off.with a locking off kit before you ask.and they are currently managing without

 

[R G]

You would need to isolate the circuit fully as a neutral to earth fault could still affect the RCD , which as you stated covers other circuits also , other than that the correct sequence of testing is whats required with all equipment isolated...

 

[davesparks]

i am well aware of this which is why it is locked off.with a locking off kit before you ask.and they are currently managing without

That's no good if it's a N-E fault, operating and locking off an sp mcb will not have cleared the fault and could leave you with significant diverted neutral currents.