Do the contacts in the MCB show any signs of prolonged arcing or vaporisation of the contacts. An arc or plasma inside the MCB could have lasted long enough for the current to take out the fuse.
Extreme non-discrimination or faulty MCB?
- Thread starter JohnW2
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Nope - as I said, everything inside looked absolutely pristine, and the contacts were obviously something I looked at particularly carefully. Anyway, don't forget that the MCB did not operate - so there's really no reason why there should have been any arcing, prolonged or otherwise.
Kind Regards, John
Fuses can get 'tired' but I doubt this particular submains was ever loaded enough to result in this occurring.
The chance of an MCB not having enough time to trip would he slim on paper. Looking at the Wylex spec sheet, from a 0 degrees start the device takes 20ms to clear the fault. What they don't do is stipulate what happens with a fault at 90 degrees.
They also state how discrimination is achieved with upstream devices by use of a low inertia mechanism. Maybe it just wasn't low inertia enough in this situation?
Hire yourself an injection test set and do some experimenting.
The chance of an MCB not having enough time to trip would he slim on paper. Looking at the Wylex spec sheet, from a 0 degrees start the device takes 20ms to clear the fault. What they don't do is stipulate what happens with a fault at 90 degrees.
They also state how discrimination is achieved with upstream devices by use of a low inertia mechanism. Maybe it just wasn't low inertia enough in this situation?
Hire yourself an injection test set and do some experimenting.
On a massive overload a fuse will always operate faster than a circuit breaker.
A circuit breaker is a mechanical device and as such it takes time for the parts to move and open the device, where as a fuse only needs to melt a bit of wire. Of course we are only taking milli seconds here, but it could be enough.
You'll notice on your fuse operating time graphs in bs7671 that fuses have a perfect curve to them, where circuit breakers start with a straight line and then a curve.
Having said all that, I'd have expected both devices to operate, not just the fuse.
A circuit breaker is a mechanical device and as such it takes time for the parts to move and open the device, where as a fuse only needs to melt a bit of wire. Of course we are only taking milli seconds here, but it could be enough.
You'll notice on your fuse operating time graphs in bs7671 that fuses have a perfect curve to them, where circuit breakers start with a straight line and then a curve.
Having said all that, I'd have expected both devices to operate, not just the fuse.
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Indeed. As you suggest, this fuse will never have been very heavily loaded and, in fact, only very rarely loaded at all. It has, however, been in service for a good few years.
Don't forget that it didn't trip. Although it may take 20ms (or whatever) to clear a fault, it will presumably reach 'the point of (mechanical) no return' much quicker than that - after which the tripping will inevitably complete, even if something else (like the fuse) clears the fault in the meantime.
Kind Regards, John
That's all true, but I rather doubt that we are talking about a "massive overload". For a start, the PSCC would only have been about 250A, which is not really all that massive in relation to a 60A fuse. Also, FWIW, there was no evidence of a massive overload. The capsule of the lamp/bulb remained intact, there was no suggestion of any blackening etc. around either bulb base or lampholder, and the contacts (and all other parts) of both the base and the lampholder looked absolutely normal.
It's obviously a bit of a mystery but I think I'm most inclined to the theory that the MCB correctly did not trip immediately in response to a fault current which was actually less than 5In but that an ailing (or congenitally abnormal) fuse for some reason blew with that modest current.
If you mean the 'straight line' at high current (5In for a Type B), that 'two-part' curve is obviously a reflection of the fact that the MCB has both magnetic and thermal modes of action. The thermal part of the curve is not dissimilar to that for a fuse.
If the current really was high enough to blow a (correctly functioning) 60A fuse then, yes, I would have expected the MCB to also operate. However, as above, I'm not convinced that the current was necessarily that high.
Kind Regards, John
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