Main switch short circuit capacity.

Ah well then post the numbers you have, and we'll do it on here - that way our dodgy maths gets corrected by however many people read this thread and spot the mistakes..

joking aside lets start.

20m of one core of 240mmsq

at 0.33milliohms per metre there and back is

is about 6.6 milliohms so if the transformer had infinite capacity then an L-N short would be 230V/ 3milliohms ~ 35kA Rather more for a phase to phase short.

Realise that a couple of metres of 25mmsq is the same resistance as 20m of 240mmsq, so you can halve the PSSC by having a few m of 'meter tails' between the 100A fuses and the REC 4


But the transformer is 800kVA, and what follows  is a bit 'ready reckoner' and may be out by  root of 3 or similar.

Divide by 250 to estimate individual phase currents ( I assume its 250 phase to star point.) 3200 amps between all phases, so 1.05KA per phase.

lets assume a 5% drop at full rated load.  Or if you have the right figures to hand  for the TX use that.

if we do assume 5%, then  immediately short circuit current is 20 times full load current, say 21kA any one phase to ground, or a touch under twice that, 35-40kA perhaps, for a fault between any 2 phases.

Of course the upstream source impedance means that these figures are not accurate, and really the PFSC will be lower than this suggests, as the MV side will droop a bit on high loads too. ABB do a rather wordy transformer handbook,  with all this in, and their technical folk are normally happy to advise, or at least used to be.


- the 23kA  is probably correct for the TX on an unyielding HV supply, assuming an L-N or L-E fault before you add in the effect of the cables. But when you do the impedances are added.


If we treat them as resistances, which is not quite right, but is normally close enough, the  combined PSSC is 1/ (1/PSSC1 +1/PSSC2) so even assuming no extra length of 25mmsq,the fault current comes down to  more like about 16kA.for a fault at the load end of the 240mmsq cables.

Or the 23kA  may be about right for a phase to phase fault at the load end.

Add 2m  of 25mmsq between the 100A fuses and  the REC4 and the 16kA  becomes more like 10kA....  

In any case if you can get us armed with the correct figures, and could indicate where in thee circuit the 23kA was quoted for ( i.e. direct on the transformer terminals, or the load end of the 240mmsq, or after the 100A fuses)

Or if  we know what the impedance of the transformer is,  then indeed all we need to do is to eyeball the fuse curves.


So  off we go to the makers data here and  go to page 3, helpfully numbered 2 in by PDF reader and look at cut-off current. So

if it was originally 16kA,then the cut off current is indeed about 9kA, and ordinary 10kA MCBs are fine.

Actually the effective cut off current changes reasonably slowly at fault currents above the kink point, only rising about 20% for a doubling in PSSC above 10kA, so you do not need to agonise about the last kilo-amp of PSSC - the effect on the cut-off current, and therefore the increase in the thermal damage, is small.

So, can a REC4 survive events equivalent to an occasional 10kA equivalent let-through? - I suspect it can, but the Wylex data sheet does not say so, so an Email or call is in order.



Edit  We have discussed something similar on the old forum https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=58230  not sure if that helps provide another way of looking at this that may be more familer. This is not my bread-and butter, and my way of explaining may not be the clearest.

Ah well then post the numbers you have, and we'll do it on here - that way our dodgy maths gets corrected by however many people read this thread and spot the mistakes..

joking aside lets start.

20m of one core of 240mmsq

at 0.33milliohms per metre there and back is

is about 6.6 milliohms so if the transformer had infinite capacity then an L-N short would be 230V/ 3milliohms ~ 35kA Rather more for a phase to phase short.

Realise that a couple of metres of 25mmsq is the same resistance as 20m of 240mmsq, so you can halve the PSSC by having a few m of 'meter tails' between the 100A fuses and the REC 4


But the transformer is 800kVA, and what follows  is a bit 'ready reckoner' and may be out by  root of 3 or similar.

Divide by 250 to estimate individual phase currents ( I assume its 250 phase to star point.) 3200 amps between all phases, so 1.05KA per phase.

lets assume a 5% drop at full rated load.  Or if you have the right figures to hand  for the TX use that.

if we do assume 5%, then  immediately short circuit current is 20 times full load current, say 21kA any one phase to ground, or a touch under twice that, 35-40kA perhaps, for a fault between any 2 phases.

Of course the upstream source impedance means that these figures are not accurate, and really the PFSC will be lower than this suggests, as the MV side will droop a bit on high loads too. ABB do a rather wordy transformer handbook,  with all this in, and their technical folk are normally happy to advise, or at least used to be.


- the 23kA  is probably correct for the TX on an unyielding HV supply, assuming an L-N or L-E fault before you add in the effect of the cables. But when you do the impedances are added.


If we treat them as resistances, which is not quite right, but is normally close enough, the  combined PSSC is 1/ (1/PSSC1 +1/PSSC2) so even assuming no extra length of 25mmsq,the fault current comes down to  more like about 16kA.for a fault at the load end of the 240mmsq cables.

Or the 23kA  may be about right for a phase to phase fault at the load end.

Add 2m  of 25mmsq between the 100A fuses and  the REC4 and the 16kA  becomes more like 10kA....  

In any case if you can get us armed with the correct figures, and could indicate where in thee circuit the 23kA was quoted for ( i.e. direct on the transformer terminals, or the load end of the 240mmsq, or after the 100A fuses)

Or if  we know what the impedance of the transformer is,  then indeed all we need to do is to eyeball the fuse curves.


So  off we go to the makers data here and  go to page 3, helpfully numbered 2 in by PDF reader and look at cut-off current. So

if it was originally 16kA,then the cut off current is indeed about 9kA, and ordinary 10kA MCBs are fine.

Actually the effective cut off current changes reasonably slowly at fault currents above the kink point, only rising about 20% for a doubling in PSSC above 10kA, so you do not need to agonise about the last kilo-amp of PSSC - the effect on the cut-off current, and therefore the increase in the thermal damage, is small.

So, can a REC4 survive events equivalent to an occasional 10kA equivalent let-through? - I suspect it can, but the Wylex data sheet does not say so, so an Email or call is in order.



Edit  We have discussed something similar on the old forum https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=58230  not sure if that helps provide another way of looking at this that may be more familer. This is not my bread-and butter, and my way of explaining may not be the clearest.