Insulation resistance test readings.

[Jm1980]

Had an interesting one today, ring circuit blown the Mcb in a kitchen apparently a few times over last day or so. It has blown once when nothing was really in use in kitchen so kind of ruling out overload.
When turned up and turned it back on it held for maybe 5 minutes before blowing again…. Unplugged everything and tested IR which was down on L-N,L-E & N-E.
Rcd never tripped & also tests fine.

Quickly traced this down to one leg of the cable between the CU & socket outlet…. Readings all 0.01M ohm so thought maybe still something connected.
Switched the meter to continuity and got
L-N 6kohm
L-E 80k ohm
N-E 70k ohm

Tested the other side of the ring and all fine so powered this up with a 20A MCB now as cpd. This 'questionable leg left out of circuit.

Plugged items in individually, tried them and circuit held fine.
Had another hour looking at the cable leg id left out to see if I’d missed anything plugged in or neons ect and couldn't see anything. Powred up half of circuit held all this time with equipment plugged in.

Anyone had similar readings as they look like something connected to me but the fact the mcb has held & appliances seem ok means cable damage can’t be ruled out on this questionable leg?

Cheers.

 

[Pretty Mouth]

I'd say cable damage to the dodgy leg, but it is unusual that it's not causing the RCD to trip. Perhaps it's snagging on something sharp that's causing the L to short to N, but not to E.

 

[Jm1980]

Continuity of the suspect cable appears fine all cores, from this I could work out the length of it and it really doesn’t appear looking at the install that it could go anywhere else to be connected to appliance so definitely leaning towards faulty cable.
Just seemed a funny one with the working rcd not operating but with the L-N resistance lower maybe this is why the mcb goes first….
Wouldn’t be the first time I’d seen a fried rat across a cable!