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.