Standard DNO Fault Current - incompatible MCB rating

A slight point of order.

" courtesy of annexe ZB of BS EN 61439-3."

Suggests the committee have it in their gift to save your circuit breaker ;-) Actually of course it is the raw physics of fuses melting that get faster as the fault current increases, vs mechanical breakers which kind of run out of steam at very high fault currents and stop getting faster. The upper limit to the let-through energy of the fuse, means that everything beyond it sees any really large fault currents cut off early - the fuse is a 'death or glory' defence against a fairly unlikely silver stake (zero resistance) type fault near the origin. (A true zero resistance fault has no heat,no flash and no bang, real ones do involve some voltage drop, as energy is dissipated throwing hot metal about)

The authors of  the annex simply capture that idea, and translate it into rather dull regs-language, to hide what is really going on.

Mike

A slight point of order.

" courtesy of annexe ZB of BS EN 61439-3."

Suggests the committee have it in their gift to save your circuit breaker ;-) Actually of course it is the raw physics of fuses melting that get faster as the fault current increases, vs mechanical breakers which kind of run out of steam at very high fault currents and stop getting faster. The upper limit to the let-through energy of the fuse, means that everything beyond it sees any really large fault currents cut off early - the fuse is a 'death or glory' defence against a fairly unlikely silver stake (zero resistance) type fault near the origin. (A true zero resistance fault has no heat,no flash and no bang, real ones do involve some voltage drop, as energy is dissipated throwing hot metal about)

The authors of  the annex simply capture that idea, and translate it into rather dull regs-language, to hide what is really going on.

Mike

Where might we see this fault of negligible impedance, other than Mr Blackwell's golden spanner loop tester, on an installation where the source can deliver unlimited current without any voltage depression?

Exactly my point - real faults have resistance, much as real supply transformers and cables do, and they share the volts between them and pass the same amps - so the assumption the PSSC is the full off load  loop voltage divided by the L-N, LL loop resistance or the Zs is not really true, some of the voltage drop is across the fault .   It all conspires to reduce the damage done do circuit breakers etc. But the big damage reduction comes from the early cut-off of the fault current by the disappearance of the fuse into widely spaced blobs of metal.

Mike.