Fault finding - Blown MCB fuse

BobbyBrown · 23 Jan 2020

Hi,

I've got a separate MCB in the garage that has been working flawlessly for a couple of years. Recently the lighting circuit on it keeps blowing the 5A cartridge fuse (not just tripping it). This circuit supplies the inside light and a number of external downlights. Any thoughts on the fault finding I can reasonably undertake myself?

I had a leaking garden light before Christmas and this used to trip the circuit but something is causing the fuse to go. Any Ideas??

Adam_151 · 23 Jan 2020

What exactly is tripping? your pst is a litte hard to follow, have you got a separate fusecarrier (in a fused spur/fused connection unit possibily) downsteam of the MCB (which is holding in)?

What rating is the mcb?, does the circuit have any rcd protection, and if so does it trip when you have the problems

Pictures might help!

BobbyBrown · 23 Jan 2020

The 5A cartridge fuse in this side keeps blowing. The other circuit (which does the garage door fridge etc, is ok).
Fuse blows without tripping switches.
The outdoor lights are on a timer and it doesn’t blow as soon as they go on, approximately half an hour after.

EFLImpudence · 23 Jan 2020

If a fuse is blowing, you must have a short-circuit (L-N) or Earth fault (L-E) - or overload but that's not likely with lights.

winston1 · 23 Jan 2020

What is the wattage rating of the outside lights?

JohnW2 · 23 Jan 2020

winston1 said:
What is the wattage rating of the outside lights?

It would be pretty amazing if it was anywhere near enough to blow a 5A fuse!

BobbyBrown · 23 Jan 2020

winston1 said:
What is the wattage rating of the outside lights?

All LED's. Less than 50W!

BobbyBrown · 24 Jan 2020

I've managed to isolate the garden lights and the fuse seems to be holding. Before I start opening a ton of junction boxes along the fence and digging up armoured cable would water ingress cause a fuse to blow or just a breaker to trip?

JohnW2 · 24 Jan 2020

BobbyBrown said:
I've managed to isolate the garden lights and the fuse seems to be holding. Before I start opening a ton of junction boxes along the fence and digging up armoured cable would water ingress cause a fuse to blow or just a breaker to trip?

Water ingress is the most common cause of an RCD tripping, but I don't think that we've seen/heard any evidence that there is an RCD. A fuse (or an MCB) is most unlikely to blow/trip as a result of water ingress - that is much more likely to be due to some 'metal-to-metal' fault/'short'.

Kind Regards, John

JohnD · 24 Jan 2020

Dirty water can cause an impressive short

Try dividing the circuit about half way (or at a convenient junction box) and see which half the fault is in. Then again. No need to expose everything at once.

I'd suspect the wet end unless a rat, spade or pruner has attacked the cable.

JohnW2 · 24 Jan 2020

JohnD said:
Dirty water can cause an impressive short

I suppose that depends upon what one regards as 'impressive' - I've never personally seen even the dirtiest of water causing a fuse or MCB (even 5/6A ones) to operate, have you?

To blow a 5A fuse, one would need a resistance of certainly under 46Ω, more realistically not much over 20Ω - and those are lower than I virtually ever see with my TT electrode (deliberately intended to create a low impedance path), even when it is wallowing in muddy water.

However, I agree that little is impossible, even if I've never experienced it - but it certainly would not be very high on my list of possibilities in the OP's case!

Kind Regards, John

JohnD · 24 Jan 2020

Yes, a spectacular one when a burst pipe caused water to run down a chimney.

32A MCB, and cartridge fuse in moulded plug. Good flashbangs.

Other cables in the same property failed under the floor, can't remember why but sections had to be replaced. Might have been previously undetected rodent or installation damage.

Unlike clean water, it didn't clear, even after drying

I've known network engineers who clear wet switchgear by restoring power

JohnW2 · 24 Jan 2020

JohnD said:
Yes, a spectacular one when a burst pipe caused water to run down a chimney. 32A MCB, and cartridge fuse in moulded plug. Good flashbangs.

As I said, little is impossible, but that is certainly well outwith my personal experience - and, as I said, far from the cause I would most expect to find for a 5A fuse blowing!

Kind Regards, John

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Fault finding - Blown MCB fuse

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