Short circuit but no bang or flash

[stuartR1963]

Hi all I ended up cutting a live cable to the upstairs ring to extend which I thought was dead but obviously wasn't, the thing is tho there was no flash or bang, It just tripped the 32 amp breaker, the cutters were not even damaged.
What happened here? I thought a short circuit causes a loud bang/flash and blows a big hole in your cutters. It was definately a live to earth/neutral short as it did trip the breaker, I am just surprised there wasn't any fireworks. Why was there no bang?
Thanks

 

[jelliottelectrical]

Too many variables to give a definite answer but depends on your L - N impedance and L - E impedance

What is your earthing type?

 

[Harry Bloomfield]

Too many variables to give a definite answer but depends on your L - N impedance and L - E impedance

What is your earthing type?

..and possibly the point in the waveform when the cable was shorted.

 

[jelliottelectrical]

..and possibly the point in the waveform when the cable was shorted.

Very true, too many variables, you could do the same again right now and create a huge bang

 

[JohnW2]

..and possibly the point in the waveform when the cable was shorted.

True, but if a B32 tripped 'immediately' (i.e. magnetically), then 96A-160A presumably must have flowed for at least a while.

Kind Regards, John

 

[jelliottelectrical]

True, but if a B32 tripped 'immediately' (i.e. magnetically), then 96A-160A presumably must have flowed for at least a while.

Kind Regards, John

But for all we know, TT system with an RCBO, could have tripped on the L - E rather than short circuit

 

[EFLImpudence]

Or N - E. Much quieter.

 

[Harry Bloomfield]

True, but if a B32 tripped 'immediately' (i.e. magnetically), then 96A-160A presumably must have flowed for at least a while.

Kind Regards, John

The damage is mostly the result of the arc, when there is no metal to metal contact. Rapid, good, metal to metal contact, at the right point in the cycle will reduce the blast damage.

Contactors regularly make and break 160amp and much larger currents without damage.

 

[Harry Bloomfield]

Or N - E. Much quieter.

OP - was it an RCD/RCBO which tripped or an MCB?

 

[JohnW2]

But for all we know, TT system with an RCBO, could have tripped on the L - E rather than short circuit

True - or, come to that, even a TN system with an RCBO (and, as EFLI has said, in either case maybe just an N-E fault, which would certainly explain the paucity of flashes and bangs)!

However, my first guess would be that it was most likely an L-E or L-N fault tripping an MCB in a TN installation, and that what was experienced was just a 'chance thing' - such that, as you said, if the exercise had been repeated immediately after there might well have been lots of flashing and banging the second time!

Kind Regards, John

 

[jelliottelectrical]

True - or, come to that, even a TN system with an RCBO (and, as EFLI has said, in either case maybe just an N-E fault, which would certainly explain the paucity of flashes and bangs)!

However, my first guess would be that it was most likely an L-E or L-N fault tripping an MCB in a TN installation, and that what was experienced was just a 'chance thing' - such that, as you said, if the exercise had been repeated immediately after there might well have been lots of flashing and banging the second time!

Kind Regards, John

Agreed, most likely that one time when it didn’t go bang, would never happen again if you tried to make it happen!

 

[stuartR1963]

OP - was it an RCD/RCBO which tripped or an MCB?

It was just 32a mcb that tripped not the 63a 30ma rcd. The system is tnc-s. It was definitely not a neutral to earth short as that would not be enough to trip a 32a mcb.

 

[JohnW2]

It was just 32a mcb that tripped not the 63a 30ma rcd. The system is tnc-s.

Thanks for clarifying/confirming (it's nice when my guesses are right, once in a while ).

It was definitely not a neutral to earth short as that would not be enough to trip a 32a mcb.

Indeed, but EFLI's point was that had it been a 32A RCBO, then a N-E short could well have tripped it (probably with no flashing and banging).

Kind Regards, John

 

[stuartR1963]

Thanks all, I will be much careful in the future and not go cutting cables unless absolutely certain it is dead. It's sounds like I was lucky on this occasion and was a one off. I have heard short circuit currents can reach the thousands, no wonder it capable of blowing large holes in hardened steel wire cutters. Thanks

 

[JohnW2]

...It's sounds like I was lucky on this occasion and was a one off. I have heard short circuit currents can reach the thousands ...

Indeed they can, albeit in most domestic installations probably usually 'hundreds' of amps.

... no wonder it capable of blowing large holes in hardened steel wire cutters.

Indeed - it can, and sometimes does! The vaporised and/or fragmented steel can also do some nasty things to nearby human beings.

Kind Regards, John