MCB off - can't locate fault

[PrenticeBoyofDerry]

I would suggest proving the installation was dead via AC voltage setting, before carrying out any tests on resistance.

 

[Taylortwocities]

I switched the mains off, and whilst the mains was completely off, the offending MCB still would't go up to the ON position

There's not point doing any of this testing malarky. If the MCB lever won't go up with the MAIN SWITCH OFF then the MCB is scrogged, knackered and needs replacing.

There may be another fault too, but lets just start with the basics?

 

[AndyPRK]

I think the OP has ordered a replacement MCB.

Just telling him what to do while I had the time on how to test it.
And while waiting for MCB to arrive

 

[PrenticeBoyofDerry]

There's not point doing any of this testing malarky. If the MCB lever won't go up with the MAIN SWITCH OFF then the MCB is scrogged, knackered and needs replacing.

There may be another fault too, but lets just start with the basics?

But as you say and has been mentioned by others, including me, there may be a fault on the circuit and I am of the opinion that any potentially faulty circuit should be investigated and not be made live, or it may just knacker the replacement MCB again.

 

[kai]

There is a possibility that a very high fault current was interrupted by the Breaker, in excess of its "M-Number" or breaking Capacity (Often M6 KA).
If that happens, a component within the breaker itself is designed to permanently shut down the breaker, preventing its re-use after such a fault.
(I have encountered such a situation previously in a Wylex unit like yours).
The safe bet is to buy a replacement breaker, and get a sparky to examine the faulty circuit, and install the new breaker once repairs are complete).

 

[JohnW2]

There is a possibility that a very high fault current was interrupted by the Breaker, in excess of its "M-Number" or breaking Capacity (Often M6 KA).

I've asked this before, since I genuinely don't know the answer ... does it really happen (at least, outside of London - and the OP appears to be in Sussex) that one sees domestic installations with Ze so low that a fault, even if very close to the CU, will result in a fault current greater than 6kA, let alone 10 kA?

Kind Regards, John

 

[Risteard]

There is a possibility that a very high fault current was interrupted by the Breaker, in excess of its "M-Number" or breaking Capacity (Often M6 KA).

I've asked this before, since I genuinely don't know the answer ... does it really happen (at least, outside of London - and the OP appears to be in Sussex) that one sees domestic installations with Ze so low that a fault, even if very close to the CU, will result in a fault current greater than 6kA, let alone 10 kA?

Kind Regards, John

Outside London the prospective fault current for a domestic installation will generally be quoted as being up to 16kA (higher in London).

 

[JohnW2]

Outside London the prospective fault current for a domestic installation will generally be quoted as being up to 16kA (higher in London).

Yes, but that's just a 'standard', hence essentially meaningless, answer -and presumably widely ignored, since (AFAIAA) most domestic MCBs have rated breaking currents <16kA. What I was asking was asking was whether, in practice, it really happened (outside of London) that PFCs were anything like as high as 6kA, let alone 10kA or 16kA. My experience is very limited but, FWIW I can't recall ever having seen a Ze below about 0.1&#937; (i.e. a PFC of around 2.3kVA) - and that low only very rarely. What sort of domestic Zes are the electricians around here used to seeing?

A 16kA PFC implies a Ze of around 0.014&#937;, which is very low - to put it in context, about 5m of a pair of 16mm² conductors or 8m of 25mm² ones - those distances would not even get one to the boundaries of the property in many cases.

Kind Regards, John

 

[ekmdgrf]

I switched the mains off, and whilst the mains was completely off, the offending MCB still would't go up to the ON position

If the MCB lever won't go up with the MAIN SWITCH OFF then the MCB is scrogged, knackered and needs replacing.

'E's not pinin'! 'E's passed on! This MCB is no more! He has ceased to be! 'E's expired and gone to meet 'is maker! 'E's a stiff! Bereft of life, 'e rests in peace! If you hadn't screwed 'im to the busbar 'e'd be pushing up the daisies!

 

[Risteard]

(AFAIAA) most domestic MCBs have rated breaking currents <16kA.

Couple of points: the up to 16kA rating allows for changes in the network. (It may be upgraded at a later date despite what the current prospective fault current is measured at.)

Secondly, MCBs despite having a 6kA rating (or 10kA for some) will be safe at higher fault currents when backed up by a BS 1361/88-3 fuse of a rating up to 100A (Annex ZA - which is a UK-only derogation) in a fully type-tested assembly. This is because they will not be exposed to the higher fault currents as the service fuse will safely interrupt the supply.

As such the assembly will be awarded a conditional rating of 16kA.

 

[JohnW2]

Couple of points: the up to 16kA rating allows for changes in the network. (It may be upgraded at a later date despite what the current prospective fault current is measured at.)

Quite possibly - but, as we often discuss in all sorts of contexts, attempting to design on the basis of 'what may change in the future' can lead to crazy and impractical situations. DNOs are, IMO, simply giving a 'safe and unhelpful' answer!

Secondly, MCBs despite having a 6kA rating (or 10kA for some) will be safe at higher fault currents when backed up by a BS 1361/88-3 fuse of a rating up to 100A (Annex ZA - which is a UK-only derogation) in a fully type-tested assembly. This is because they will not be exposed to the higher fault currents as the service fuse will safely interrupt the supply.... As such the assembly will be awarded a conditional rating of 16kA.

Fair enough - but all this leaves my question unanswered. In effect, I'm wondering/asking how hypothetical these considerations actually are - i.e. how common (if at all) is it to see domestic installations (perhaps outside of London) with a PFC (currently, not 'possible in the future'!) anything like as high as even 6kA? I have to ask, since I have far too little experience to know - as I said, I personally haven't seen a Ze below about 0.1&#937;.

Kind Regards, John

 

[Simon321456]

I switched the mains off, and whilst the mains was completely off, the offending MCB still would't go up to the ON position

If the MCB lever won't go up with the MAIN SWITCH OFF then the MCB is scrogged, knackered and needs replacing.

'E's not pinin'! 'E's passed on! This MCB is no more! He has ceased to be! 'E's expired and gone to meet 'is maker! 'E's a stiff! Bereft of life, 'e rests in peace! If you hadn't screwed 'im to the busbar 'e'd be pushing up the daisies!

I chuckled

Sparky supposed to be due any time soon with the replacement MCB.
Figured whilst it looks easy enough to change, I still need to figure out WHY I have an Ex MCB anyway, no point just replacing and hoping for the best