Main switch short circuit capacity.

Although the PSSC is high, the fuses will limit the duration of that fault and therefore the damage, as if the fault current were lower - you need to look at the spec of the REC 4 versus the effective fault current the fuses give you  (so called 'current limiting' with fuses, is really I2t limiting)


I'd be surprised if protection by the fuses could not be arranged to be enough, as this use of a 'death or glory' last ditch fuse to allow  downstream kit to be more modestly sized is common - and avoids needing a light switch rated for a hundred kA fault in the substation room, and avoids also the cable to be tied to the wall like the book of monstors needs to be tied shut in Harry Potter...

So some perusal of fuse datasheets and maybe an email or a  call to the maker of the isolator to be sure you are OK , but yes it ought to be possible, or something in that style anyway.

Thanks for that Mike, but this is where I start to struggle.

I take it that I need to look at cut-off current curves which for a 23kA fault gives a approx cut of 9kA peak or have I missed.

Missed tech dept for Rec 4 today so will try tomorrow.

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.

Thanks Mike for a very informative description for what is a complex subject. My values are simply calculated using the IET Electrical Installation design guide to give me a general idea as to what I'm dealing with. I think as you stated, it falls with manufacturer being able to confirm one way or the other.

As a matter of interest and looking BS7671 Characteristics for the fuse, that for a 0.1 disconnection time that the fault clears at 1450 A which I take it is true up to 33kA. It's just the switch being able to with stand the energy let through during this period.


Many thanks again.

Actually, the nice thing about fuses is that as the fault current rises they get faster, more or less as a constant energy device - the energy to evaporate so many grams of fuse wire is almost fixed, once it is all so fast that there is no time for it to cool down -  this drives the I 2t is a constant approximation - I squared R is wattage, and I squared  R times time is energy .

Given that resistance of the fuse wire is always the same for all fuses of a given construction and rating, so the energy to blow that fuse is also fixed. Then it follows that I squared times time is also pretty much constant  - and indeed if you look at fuse curves, in the fast blow side of the chart, you see this is pretty much true.

In extreme (well beyond the ratings faults), the fuse does not interrupt safely, you may get a running arc inside the fuse between the opening ends, and all the broken bits of ceramic have to be swept up, and the welded and discoloured end caps may need to be levered out of the holder, but even then fuses do not generally fail to open, pretty much ever. (The sand fill supresses this arcing tendancy)


Circuit breakers do not have this nice (almost) constant energy feature - yes initially the contacts move faster with more fault current, up to a point, but there is a limit to how fast contacts can be separated. For example to open a 3mm gap in 10msecs is 300mm/sec, which sounds quite slow. However, given the inertia of the contacts, and the fact the magnetic core of the sense coil saturates at high current, very few normal circuit breakers ever open in less than a few milliseconds.

Then there is a potential problem, as the let through energy rises at higher fault currents, - for a high enough fault current, a breaker may absorb enough energy and weld shut or blow up before the contacts manage to open.

So we always want at least one fuse in the system for the day it all goes wrong. It won't blow in half a century of normal use, but it will be ready when needed. Unlike a mechanical thing that may stick.

The REC4 will not be opening or closing onto a fault, so there is no arcing expected, just thermal heating of the contacts. I do not expect a problem.

Let us know what they say

As a matter of interest and looking BS7671 Characteristics for the fuse, that for a 0.1 disconnection time that the fault clears at 1450 A which I take it is true up to 33kA.

The fuse will certainly be much, much, faster than 0.1s at higher fault currents - if anything the energy let-though (I²t) of fuses declines (or at worst remains flat) with increasing fault currents (unlike circuit breakers). For some reason BS 7671 chopped off the data below 0.1s a few editions ago (possibly a hint not to use the adiabatic formula directly but compare manufacturer's energy let-though data - as per the 2nd paragraph of 434.5.2).


   - Andy.

Ah, yet again Mike's quicker than me!

 

The REC4 will not be opening or closing onto a fault

Erm, I'm not sure if that's a safe assumption - e.g. a fault introduced during maintenance work, if the REC4 is then used to 'switch on' it'll have to close onto the fault. The 18th seems to have introduced a lot of words on this sort of subject - e.g. reg 536.4.2.3 (a full page no less).


   - Andy.

Actually that is a point - there could be some wiring error or damage to the cables between the REC4 and the load so closing onto a fault is possible. mea culpa.


But the contacts will not be required to interrupt a multi kA fault current, which is certainly the most damaging act with MCBs if they are repeatedly closed and instantly re-trip due to a fault. This sometimes  happens as frustrated gorillas try to make the fault disappear, and sometimes succeeds in vaporising enough of the screw/nail/cable to get the power back on, leaving a damaged installation and no evidence. If it is the CPC that has burned back first this can be quite dangerous.

Having contacted technical regarding the REC4 who informed me the switch is tested up to 16k and were unable to confirm operation above this. Without confirmation as to suitability it looks as though I should consider another switch.


Thanks for the feedback

I think you are mixing up peak and rms values


The transformer could deliver a maximum RMS fault current of 24kA


Applying that to a 100A Fuse, suggests to me that you could not possibly exceed 9kA RMS


You have a switch capable of 16kA (making and breaking)


Not seeing the problem


If it gives some comfort, a typical DNO distribution TX is normally 800kVA - if that 100A fuse was at the cut out of a house, would you think twice about the REC isolator at 16kA


Regards


OMS