Extreme non-discrimination or faulty MCB?

[JohnW2]

On Boxing Day, we had visitors sleeping in a room in a rarely-used corner of the house. When they switched on the light, there was a loud bang (‘explosion’’) and it transpired that the entire glass envelope of a 25W incandescent candle bulb/lamp had fallen, blackened but physically intact, onto the bed below. The power to that part of the house immediately ‘died’.

The B6 MCB in the local CU protecting the lighting circuit did not operate. However, the 60A BS1361 serving the sub-main in question did 'blow'.

This could possibly be a fairly extreme case of non-discrimination. Alternatively, the B6 may have become faulty and hence failed to do what it says on the tin. Given that the MCB functionality cannot be tested (it still works OK with manual operation of its lever), my inclination is to replace it, on the grounds that it may have become faulty/non-functional and, even if it hasn’t, it may possibly have suffered as a result of what must have been a pretty hefty fault current. What would others do?

The RCD in the local CU (which was protecting the lighting circuit in question) still tests fine - so probably hasn’t suffered from the experience.

Kind Regards, John

 

[phatboy]

For the cost, certainly worth doing.

 

[JohnW2]

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The MCB certainly sounds faulty. Even with a high source loop impedance, a lighting circuit should have enough circuit impedance for a fault current of significant enough duration to cause the MCB to magnetically trip.

I don't really understand what you are saying here. The 'source loop impedance' and the impedance of the final circuit are obviously both components of the overall fault loop impedance, through which any fault current has to flow. Whatever, the Zs, as measured at the offending light fitting, was well below that required for magnetic tripping of a B6 - not to mention the fact that we know enough current flowed to blow a 60A fuse. I'm certainly very suspicious about the MCB, but would not totally discount the possibility that (obviously contrary to expectations) the fuse blew before the MCB had 'enough time' to operate.

Kind Regards, John

 

[333rocky333]

what make model is the mcb

 

[JohnW2]

what make model is the mcb

Wylex NHXB06

Kind Regards, John

 

[ericmark]

This PDF seems the say there is a known problem and a recall has been issued April 2009 to February 2010 are the dates given but clearly you have no idea of the dates so no option but to change them.

 

[RF Lighting]

It might be a lack of discrimination combined with the existing load on the submains, but it seems very unlikely that this fault would take out a big 1361 and leave the 60898 unoperated.

Personally for the sake of a couple of quid, I'd replace the circuit breaker for peace of mind.

At least the installation was well designed and the submains provided backup protection for a fault.

One thing to double check is that the submains has not some how been connected reverse polarity, although this normally only becomes apparent on a live (neutral) to earth fault.

 

[JohnW2]

This PDF seems the say there is a known problem and a recall has been issued April 2009 to February 2010 are the dates given....

AIUI, that problem/recall related only to NSB06 MCBs, not NHXB06 ones. In any event, that problem related to possible overheating and fire risk, not a failure to operate under fault conditions.

Kind Regards, John

 

[JohnW2]

It might be a lack of discrimination combined with the existing load on the submains, but it seems very unlikely that this fault would take out a big 1361 and leave the 60898 unoperated.

Indeed - particularly given that, at the time, that submain will have been minimally loaded.

Personally for the sake of a couple of quid, I'd replace the circuit breaker for peace of mind.

Indeed. As I implied, that's what I'm going to do, given that there is no safe way of testing even a B6.

At least the installation was well designed and the submains provided backup protection for a fault.

Sort-of, I suppose - although the 60A fuse is obviously there to protect the distribution cable (16mm² IIRC, maybe 25mm²) and would not afford very meaningful 'backup protection' to the 1mm² cable in the lighting final circuit!

One thing to double check is that the submains has not some how been connected reverse polarity, although this normally only becomes apparent on a live (neutral) to earth fault.

That was actually one of the first things I checked. However, it would seem very unlikely that it was an 'L'-E fault - and, of course, no RCD operated.

Kind Regards, John

 

[ericmark]

Is it one of these

it would seem there are counterfeit MCB's around.

 

[mfarrow]

I don't really understand what you are saying here.

Sorry I meant 'low' source impedance. As in external loop impedance.

Although I'm questioning now whether I said anything at all!

 

[bernardgreen]

Is it one of these

it would seem there are counterfeit MCB's around.

That is a cheap fake, the good quality fakes have a lump of scrap metal to make them feel as heavy as the real thing,

 

[JohnW2]

Is it one of these <pic of fake MCB> it would seem there are counterfeit MCB's around.

That is a cheap fake, the good quality fakes have a lump of scrap metal to make them feel as heavy as the real thing,

It certainly was not one of those - since, as I said, manual operation of the lever still makes the MCB work as a switch (and, as you imply, I would almost certainly have noted the lack of weight when it was installed). Although nothing is impossible, I very much doubt that it is a fake of any sort, since it would have been obtained from a reputable mainstream supplier. Whatever, once I've replaced it, I will perform the autopsy and let you know!

Kind Regards, John

 

[JohnW2]

The MCB certainly sounds faulty. Even with a high source loop impedance, a lighting circuit should have enough circuit impedance for a fault current of significant enough duration to cause the MCB to magnetically trip.

I don't really understand what you are saying here....

Sorry I meant 'low' source impedance. As in external loop impedance. ... Although I'm questioning now whether I said anything at all!

Quite. I still struggle to understand what you were trying to say Did you perhaps also mean "...should have low enough circuit impedance..."?? However, as I said, what matters is terms of magnetic operation of the MCB is the overall loop impedance ('external' + distribution circuit + final circuit) - and that was plenty low enough. As I said, I measured the 'overall' Zs - which itself was plenty low enough. What I didn't add was that this was almost certainly a L-N fault -in which case the FLI would be expected to be event lower than the Zs. In any event, as I also said, enough current flowed to blow a 60A fuse - so (unless the fuse itself was very seriously 'faulty', which is very unlikely) obviously enough to result in magnetic operation of a B6 MCB (provided it flowed for long enough).

Kind Regards, John

 

[Adam_151]

As others have said, time to replace it if suspect, NHX are the current range and are readily available and at a low price

That said, I'd be tempted to try and find a way to test the old one with a high current low voltage AC source before slinging it into the dustbin.

I've actually seen a fault (caused by a crushed cable) on a 1.5mm blow itself clear before a breaker tripped, but that was on a federal stablok type 4 Zs was out of spec at end of circuit, however probbaly wasn't at the point the fault occurred. Backup protection was BS88 100A which also stayed intact.