Switchboard Short Circuit Rating
Switchboard Short Circuit Rating
(OP)
I'm doing a study on a relatively old building and have run across something that I'm not sure how to handle.
The building has a main switchboard which has a design kA rating of 63 kA for 1 second. The main ACB circuit breakers have a Icu / Ics / Icw rating of 75 kA. The board is fed by three emergency generators, each of 2250 kVA at 400 volts, 50 hZ.
Te generators have a subtransient reactance of 12.6%, transient reactance of 17.6% and a zero sequence reactance of 3.16%. The feed from the generators to the switchboard is approximately 30 metres. It is slightly more complex than that as the third generator has lower subtransient and zero sequence impedances.
When I do a calculation I find that the symmetrical RMS short circuit current for a three phase fault is approximately 78 kA, which is above the circuit breaker and board rating. This current drops very rapidly, so that after approximately 1 cycle is is down around 55 kA.
When I do the calculation for the single phase short circuit current (L - E)I get a figure of approximately 88 kA. Again this current falling off rapidly.
I'm not a switchboard person, so my question basically is, what should the fault current rating of the switchboard and the circuit breakers be in this situation?
The building has a main switchboard which has a design kA rating of 63 kA for 1 second. The main ACB circuit breakers have a Icu / Ics / Icw rating of 75 kA. The board is fed by three emergency generators, each of 2250 kVA at 400 volts, 50 hZ.
Te generators have a subtransient reactance of 12.6%, transient reactance of 17.6% and a zero sequence reactance of 3.16%. The feed from the generators to the switchboard is approximately 30 metres. It is slightly more complex than that as the third generator has lower subtransient and zero sequence impedances.
When I do a calculation I find that the symmetrical RMS short circuit current for a three phase fault is approximately 78 kA, which is above the circuit breaker and board rating. This current drops very rapidly, so that after approximately 1 cycle is is down around 55 kA.
When I do the calculation for the single phase short circuit current (L - E)I get a figure of approximately 88 kA. Again this current falling off rapidly.
I'm not a switchboard person, so my question basically is, what should the fault current rating of the switchboard and the circuit breakers be in this situation?






RE: Switchboard Short Circuit Rating
RE: Switchboard Short Circuit Rating
I'm 99% sure that this is an issue no matter what code you are using, but I just want some form of assurance that there isn't a 'caveat' hidden somewhere before I raise this as a major defect with the client.
RE: Switchboard Short Circuit Rating
RE: Switchboard Short Circuit Rating
Obviously if the test certificate turns up and they did exactly that (say, 80kA for 1/2 second) then you've got no issues at all, but I suspect that the original certificate won't be able to be found. Beyond that it becomes an exercise in how much risk is held against the consideration of whether a fault occurs when all generators are connected at once, which is often a similar argument to grid connected generators synchronising onto a board and exceeding the board rating for the duration of load transfer, an associated argument I'm not particularly comfortable with either.
RE: Switchboard Short Circuit Rating
I don't know which legislation you're working to but under British law there is no dispensation whatsoever for exceeding the capability of the equipment, even during short-term paralleling.
RE: Switchboard Short Circuit Rating
you didn't write anything about the distribution philosophy. All these generators work in parallel ?
RE: Switchboard Short Circuit Rating
LDKGR, yes the generators operate in parallel in an N+1 arrangement.
I'll probably be able to get around the problem, and not cause the client too much grief, just by changing the generator control scheme and ensuring that only two generators are connected to the bus at any time.
RE: Switchboard Short Circuit Rating
RE: Switchboard Short Circuit Rating
The predecessor standard to AS3439 used to detail the tests required, at least it did if my memory serves correctly, and it was possible to test for greater current at shorter duration. Every time I've been up against this, though, the test certificate has been nowhere to be found.
ScottyUK's correct, its the peak current and bracing, most of the smaller generation I've seen won't maintain a significant fault level for more than about 10 cycles without a significant drop, though I'm aware the scale of equipment ScottyUK deals with does (well, something has to, otherwise there'd be no perception of stiff grid...).
If its a backup system how do they transfer back to grid?
RE: Switchboard Short Circuit Rating
Saying that, I assume I'm stymied anyway, as the single phase fault current certainly exceeds the circuit breaker fault rating by quite a margin.
As to the current drop off. I did the calculation for the three phase fault. At the generator terminals a fault of 28.6 kA at t = 0. At t = 40mS (2 cycles) it has already fallen to 16.5 kA. So there is going to be a large force generated on the busbars with the initial fault, but it falls very rapidly off in strength. The 16 kA fault falls off reasonably slowly after that as we enter the transient reactance period. At t = 1S I still have around 9 kA.
The board is set up with six ATSs, each consisting of a 2 x 4000 amp ACB for the mains and the generator bus. So once mains is available they just do an open transition change on each ATS.
RE: Switchboard Short Circuit Rating
BS EN 60947-2 requires that for circuit breakers of breaking capacity greater than 50kA the test conditions be as follows: short-circuit power factor = 0.2; time constant = 15ms.
For breakers with a breaking capacity between 20kA and 50kA the power factor increases to 0.25 with a time constant of 15ms.
RE: Switchboard Short Circuit Rating
RE: Switchboard Short Circuit Rating
A cheap fix may be to lengthen the cables from the generators.
I have also seen this solution employed by other designers.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Switchboard Short Circuit Rating
Scotty, yep, that seems to be the standard in the IEC world, AS3439.1 gives the same figure, it's equivalent to an X/R ratio of 5. Plug that into the appropriate formula and it means that the peak asymmetrical current is 2.2 times the RMS rating of the circuit breaker. So my 75 kA circuit breakers have an asymmetrical peak rating of 165 kA, of course assuming that 25 year old Masterpact breakers were tested to the same limits. The new one's certainly are...but. I'd better call Schneider on that one.
Now I need assurance that the same thing can be applied to the switchboard busbar and bracing system. I talked to one switchboard manufacturer who told me the type tests for his 100 kA boards state that they have a peak withstand of 220 kA. So that seems to be exactly the same. Now if I could be assured that the 25 year old 63 kA switchboard that I have had the same test certificates I'd sort of know where I stand.
Saying that, with the three generators supplying the main switchboard I'm going to exceed the symmetrical RMS rating and the peak asymmetrical ratings anyway, albeit for a very short period.
The client actually wants to put another generator onto this system, one of the reasons I've been kicking tyres on the site. I suspect I may have to suggest an intermediary generator switchboard, with higher kA rated circuit breakers, higher bus rating, and using the extra cable length (as per waross), and use the current limiting aspect of the new circuit breakers to sort out the issues.
RE: Switchboard Short Circuit Rating
I this board split into busbar sections? If not, how are you managing to run 3 x 2250kVA generators on the same bar? In IEC land, the largest thermal rating of LV switchboard is 6,300A, but your 3 x 2250kVA gens = 9,740A. You could only run 2 gens at a push on a 6,300A rated board, so why would you want to add another gen? If the bar is split then you shouldn't have the fault level problem.
BTW, 4000A ACBs are not normally current limiting.
RE: Switchboard Short Circuit Rating
In IEC land, the largest thermal rating of LV switchboard is 6,300A
I've had a read through IEC60439 part 1 and cannot find anywhere that states that the maximum bus rating is 6300 amps. Can you please expand on that.
Cheers.
RE: Switchboard Short Circuit Rating
There are MV breakers specifically for generator applications which thermal ratings in the 20,000A range but they are a completely different type of device and are both massive and expensive.
RE: Switchboard Short Circuit Rating
RE: Switchboard Short Circuit Rating
The breakers in this board have to satisfy 2 different ratings. The breakers from individual generator feeds will not see the 88kA calculated total, they will only see their share of that (1/3) or 30kA rms. Breakers feeding other loads from this combined bus do need that 88kA+ rating.
If it is a open transition n+1 scheme, doesn't that mean the bus will only ever see 2/3 of 88 (approx 60kA). If that is the case, would that come under the 63kA board rating?
In answer to your other question regarding 3 phase faults being larger that L-N faults. It is common for generator peak currents to be L-N due to the relative magnitude of the zero sequence impedance compared to transient quantities. Zero sequence impedance is a component of L-N faults, but not L-L-L faults. I suspect you could consider a neutral earthing resistance/reactance on the generator to increase zero sequence impedance and bring down peak fault current, although I am not an expert on this.
I had a quick calc with assumed cables and got about 92kA peak if 3 generators can parallel and 62kA if an open transition interlock with only 2 generators in parallel used. If it is the later, it looks ok to me.
One thing I noticed is that if you are going to try to retrospectively satisfy the arc fault requirements as per AS3000 (which would not have been in effect in 1991), or even just to ensure you pick up the minimum fault current (about 13kA assuming 4x500mm2 cables) you will need to wind the short time setting on the breaker back a long way and check the knock on effects from this downstream.
RE: Switchboard Short Circuit Rating
My question over the L-N or L-E fault currents being the highest wasn't so much about the generators per se. I've known for a fairly long time that for generator systems it is quite common for this to be the case. It was more a question on whether there was buried, somewhere in the IEC land standards, a difference to be handled between three phase and single phase fault. I was 99% sure there wasn't, but wanted to cross it off my list.
A letter was sent to the client today laying out the issue, and hopefully this will lead to another job being raised. I only discovered this whilst performing an earthing / ground fault investigation on the building, and whilst I have a duty of care to inform the client, and perhaps raise future work, I wasn't going to go into to much depth on the solution side until they come up with some filthy lucre.