Mcb trips but tests clear

[Morsey]

Evening all
Let’s see if anyone can solve this one.
Recently wired a new extension. Today connected up ring circuit to 32amp mcb, all relevant dead testing done ir +999mohms and ring continuity.28 .28 .48. So all good turn mcb on and pop it trips off. Rechecked testing removed sockets just to double check for any small nicks etc all fine. Reconnect and turn on all seems fine. Hour or so later same problem. Replaced mcb all fine. Hour or so later same problem. Any ideas????
FYI nothing plugged in yet so no load whatsoever

 

[Pete999]

Hi chaps
Ir testing to its own cpc tests clear. The rcd doesn’t trip either, suggesting a short circuit but again this test is clear?? Not tested with it connected to met.
It’s a bit confusing

What IR tests are you doing L-N, L-E, N-E

 

[Pete999]

Evening all
Let’s see if anyone can solve this one.
Recently wired a new extension. Today connected up ring circuit to 32amp mcb, all relevant dead testing done ir +999mohms and ring continuity.28 .28 .48. So all good turn mcb on and pop it trips off. Rechecked testing removed sockets just to double check for any small nicks etc all fine. Reconnect and turn on all seems fine. Hour or so later same problem. Replaced mcb all fine. Hour or so later same problem. Any ideas????
FYI nothing plugged in yet so no load whatsoever

Did you conduct the figure of eight test on the RFC as a matter of interest?

 

[mhar]

As it is a new circuit with nothing connected I would conduct the ir test at 1000v (cpc connected to met)

 

[davesparks]

As it is a new circuit with nothing connected I would conduct the ir test at 1000v (cpc connected to met)

Why? If the fault is occurring at 240V then the 500V test will identify it.

 

[Morsey]

Ir test done l-n l-e n-e.
I’m going back this morning to try and solve this, my thinking at the minute is what was previously said maybe somewhere line conductor is touching metalwork/pipework through a screw or something just strange I can’t read any sort of fault through the meter (which I know is ok)

 

[mhar]

Why? If the fault is occurring at 240V then the 500V test will identify it.

The op is stating that his test is not identifying the problem. 240v is an rms value and the meter is only performing a low current test. OP is fault finding, a 100v test will be more onerous on the installation.

 

[Deleted member 72666]

The op is stating that his test is not identifying the problem. 240v is an rms value and the meter is only performing a low current test. OP is fault finding, a 100v test will be more onerous on the installation.

I would also bang 1000v into it in this situation.

 

[Fitzy]

Can you split the rfc in to 2 radials, then connect each radial to an mcb to see which leg trips the mcb? This will at least give you a rough idea of which leg of the rfc to inspect further.

 

[Morsey]

So after further investigation and with earths still connected to met, an ir fault between two points is present! This is only through the the fabric of the building and not end to end of the cable. With this link out all test clear again even with earths connected to met. Mcb down rated builders need to make a decision on holes in walls and ceilings down
Hopefully that’s solved
Thanks for your input chaps

 

[Deleted member 72666]

That's why all earths have to be connected to met during IR testing.

 

[davesparks]

The op is stating that his test is not identifying the problem. 240v is an rms value and the meter is only performing a low current test. OP is fault finding, a 100v test will be more onerous on the installation.

The OP had stated that the test has not identified the problem but also that the test had been carried out incorrectly, this is not the same thing as the test not showing the fault. Now he has returned and carried out the test correctly it has revealed a fault.

Yes 240V is the rms value, the peak is around 300V, this is why we test at 500V.
IR testing is a low current test at 500V or 1000V, high current is not necessary for IR testing, it is the voltage which 8s required to stress the insulation, not current.
The insulation of T&E is only rated for up to 500V and shouldn't have a voltage higher than this applied to it.

 

[Morsey]

Just a note original ir tests were carried out with earths connected to met. This was all clear! Since then work has carried on so this is when the fault has arisen. It was only not checked again yesterday that has caused me to only find it this morning.

 

[telectrix]

had similar. nicked L insulation in back of socket. arcing to box when faceplate screwed on. showed clear on IR test @ 500V.

 

[DPG]

Let us know what it ends up being. Photos are good.

 

[sham]

If you have used the same batch of mcb, try and get one from somewhere else, or temporarily connect to an existing mcb as you know that one is ok as it is in use. Otherwise the problem has the symptoms of an overload despite nothing is connected. You did say it went pop first and then later reconnecting it and the it lasted an hour. So therefore it seems a wiring situation, maybe a dodgy accessory e.g socket or spur if used. On your IR test are all socket switches closed, just a thought. If you are confident in your wiring I would disconnect the accessories and then try, I know it's laborious but at least you will know. Hope this helps and please let us know the outcome as It could help me. Thanks.