15a breaker randomly tripping..

stevel98 · Aug 28, 2022

HI..
I'm in Scotland, UK..
The 15a Wylex push button breaker that supplies my garage with power (30m from the house mains panel) randomly trips.
I threw the isolator switch in the garage, so there was nothing loading the circuit but it still trips out after a few hours.
Changed the breaker to see if it was developing a hair trigger, but to no avail.
I disconnected the cable at the panel and also at the garage's isolator and measured the resistance values in ohms between the E, L & N at each end of the supply cable which are:
House..
E-L 11K, E-N 10K, L-N 5K
Garage..
E-L 5K, E-N 1K, L-N 3K
Could anyone tell me what these values should be for the 30m cable supply to my garage before I retreat to the 1m crawl space underneath my house and start tracing it out (no idea where the cable routes under my back garden, but I can probably trace it under the house).
Cheers.

littlespark · Aug 28, 2022

When you tested the cable, was it totally disconnected from everything? The mcb, the neutral and earth bar?

What did you use to test? Just a multimeter won’t be accurate enough.

The cable, with nothing connected either end should have insulation resistance values in the Mega Ohm scale, not kilo ohm

Could it be rodent damage to the cable? Any other signs of the little critters?

Those push button circuit breakers are getting old now… and it might be worth your while having the consumer unit changed…. Which will introduce all the new equipment needed to satisfy the regulations including RCDs and an SPD.

Lucien Nunes · Aug 28, 2022

@littlespark is spot on with the fact that although a multimeter won't give you an accurate picture of the condition of the cable, the readings are on the order of a thousand times too low. We probably don't need an actual insulation tester to tell us there's damage somewhere. The readings you give won't in themselves trip an MCB, but they hint at the kind of fault that will arc over or break down under mains voltage.

What type of cable is it? Do you know whether there are joints?