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

 

[Strima]

IR testing with all earth paths in place?

 

[suffolkspark]

Have you checked insulation resistance to the MET or just to its own cpc?

 

[DefyG]

As you haven't mentioned RCD tripping (assuming there is one as its a ring circuit) I presume that is ok so looking for a L-N fault!
If not showing up on the meter, its a matter of elimination!

 

[Morsey]

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

 

[westward10]

The IR test should be done with all protective conductors in place.

 

[mattg4321]

Assuming you are testing with no appliances connected, are you reconnecting them before it 'goes pop'?

 

[DefyG]

Just to clarify, you are testing the circuit with all disconnected from the 'mains'.
You connect to the mains, including cpc's to MET and the RCD does not operate but the mcb does! without any appliances connected (ie No Load).
Have you checked Ze? If within spec and if L-E fault RCD should operate. Fault current defaulting to N!
Matter of elimination!

 

[westward10]

A high current line to earth fault may well not trip the rcd.

 

[Wilko]

Hi, my thoughts : Confirm ir is still good. Clamp the L and confirm there really is no I with circuit energised. Finally relocate circuit into an other working mcb and see if it still trips. My suspicion is there's a trapped conductor to be found.

 

[buzzlightyear]

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????

had this before, faulty mcb .

 

[Morsey]

Yeah that’s right no load at all( kitchen not even fitted yet)
All tests done disconnected then reconnected and the mcb trips not straightaway the last time but within a hour or two. Ze reading of 0.12 on tns. It may be possible the rcd went the last time as I was not there.

 

[buzzlightyear]

Yeah that’s right no load at all( kitchen not even fitted yet)
All tests done disconnected then reconnected and the mcb trips not straightaway the last time but within a hour or two. Ze reading of 0.12 on tns. It may be possible the rcd went the last time as I was not there.

change the mcb first .had one a couple of months a go .customer kept say the electric off .waited 2 hour watching the cu ,and low and behold it trip right in front of my eyes .and all circuits where covered by rcd .c**P BG, screwed fix mcb

 

[Morsey]

Thanks for that buzzlightyear but I have already changed it, trying to rule it out I guess. It’s an mk dual rcd board.

 

[Richard Burns]

Somewhere there is a line conductor touching a piece of (partially) earthed metal that is not part of the circuit cpc.
With a trip time of one hour you are looking at a current of about 40A.
Because it was instant trip initially the fault has burned this contact partially clear which is why it is now taking one hour.
So I would be looking for a point where there is metal pipework near the circuit or a metallic structural part that may have been screwed in and the screw hit the line. Then testing IR line to MET as you break down the circuit.

 

[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.

 

[Deleted member 9648]

The 1000v IR test will sometimes find a fault when nothing shows up at 500v, especially if you zap it for 10 seconds or so. Regardless of whether it's right or wrong it's done the business for me on numerous occasions over the years.

 

[Deleted member 9648]

Not creative at all Rpa07, fact.

 

[Toneyz]

Sounds like a cable between both points has been nailed/screwed etc catching the live and going into the fabric of the building what work has been carried out between the two points in time?