Arc Flash Hazard? Yes or No?
Arc Flash Hazard? Yes or No?
(OP)
According to our arc flash study, we have a maximum of 47,000 A available 3-phase fault current coming into a 600 V cabinet via some really big copper buswork and wires. It connects to a main circuit breaker which is rated at 50,000 A. However, there are also three 4 AWG wires that tap off the line-side of each phase (no fuses) and go to an adjacent cabinet to a much smaller circuit breaker which is rated at only 35,000 A. The arc flash study is treating this all as a single electrical node, hence, the 47,000 A fault current exceeds the maximum current of the circuit breaker in the event of a bolted fault. Case closed. Or is it?
Question - Can #4 AWG wire carry 47,000 A of electricity for any meaningful period of time? If not, what would be the limit for this size wire?
Seems to me, the #4 wire would serve as a fuse link and simply vaporize with this kind of current, but this is way outside my training (my background is microelectronics). Any help is greatly appreciated.
Thanks,
FEinTX
Question - Can #4 AWG wire carry 47,000 A of electricity for any meaningful period of time? If not, what would be the limit for this size wire?
Seems to me, the #4 wire would serve as a fuse link and simply vaporize with this kind of current, but this is way outside my training (my background is microelectronics). Any help is greatly appreciated.
Thanks,
FEinTX






RE: Arc Flash Hazard? Yes or No?
47 kA will definitely melt the wire in time. This in itself is a major arc-flash hazard. But the breaker will probably open its contacts in a cycle or less. That's another problem if the breaker can see more current than it is rated for.
It's important not to confuse arc-flash hazards with short circuit rating issues. They are two different (but somewhat related)issues. The fact that all breakers are adequately rated for the expected short circuit current does not mean there is no arc-flash hazard in the equipment. The arc-flash hazard is based on an arcing fault inside the equipment. In fact, the arc-flash calculations assume that all breakers are adequately rated and operate as expected.
RE: Arc Flash Hazard? Yes or No?
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Arc Flash Hazard? Yes or No?
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RE: Arc Flash Hazard? Yes or No?
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: Arc Flash Hazard? Yes or No?
As noted by others, none of this says anything about arc flash hazards.
RE: Arc Flash Hazard? Yes or No?
Yes, there are actually 2 issues that I'm looking for help with:
(1) Will the smaller CB actually see 47 KA coming across the #4 wire? If so, what happens? Will it not open? Will it explode?
(2) The arc flash incident energy for the line-side of the main CB is 48.5 cal/cm2 at 18". This makes energized access to this cabinet off-limits even with Class IV (40 cal) flash suit. This is OK because there is nothing needed in this cabinet from a service point of view.
The adjacent cabinet is a different story, however. There are many service items and troubleshooting points. Things such as small CB's for 110 V circuits (receptacles, lights), sensors, and more. If we also have 48.5 cal/cm2 in this cabinet, then we have a very difficult situation for operations teams in that we must de-power the pad-mount transformer for many routine tasks. If, however, we have < 40 cal/cm2, then we can at least open the door to this cabinet with a 40 cal flash suit and operate breakers and do some limited troubleshooting using switching sticks while maintaining a safe distance.
Note - we are posing these questions back to the company that performed the original arc flash study, but I'm very interested in what I can learn from those of you at this site.
Thanks!
FEinTX
RE: Arc Flash Hazard? Yes or No?
The only way to know what the fault current can be at the smaller breaker is to calculate it. A certain length of #4 wire has a certain impedance. It will reduce the fault current. Impossible to say how much without more data.
The breaker does not "know" (in general) how much current it can interrupt, so if it sees current exceeding its trip setting it will trip and the contacts will open. What happens next depends on a lot of factors, but if the available fault current exceeds the breaker rating, the breaker can fail catastrophically, causing fires, bodily injury and other exciting things. It's also an NEC violation. However, this is probably still preferable to having the #4 wire melt, which very likely going to create a major arcing fault.
RE: Arc Flash Hazard? Yes or No?
RE: Arc Flash Hazard? Yes or No?
Will the breaker explode? - Again, not likely, but it could. It will likely take several more cycles to clear the fault, and get damaged in the process. Or it could just melt down. You never know. In the world of arc flash, you need to calculate for the worst case.
2) Perhaps the simplest solution to this problem would be to install a set of fuses. 60kA rated fuses are not that expensive. You could install them on the 4 AWG tap wires before the breaker.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Arc Flash Hazard? Yes or No?
But at 35 kA the #4 awg is gone in less than 1 cycle. The fault current has to be below 20 kA to have a chance of saving the #4 awg wire. (See the Okonite nomograph.)
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If all of the assumptions are accurate, you may not have a problem with the breaker being overrated. A bolted fault on the downstream side of the breaker may not be greater than the 35kA breaker rating, if the melting of the wires doesn't affect the breaker's ability to clear the fault.
A fault on the breaker's line side terminals has to be cleared by the upstream protection; probably the transformer high voltage windings' protection fuse or breaker. Clearing time will be a lot longer than the wire's rating.
I would guess that your breaker is OK and your arc flash danger is off the map.
RE: Arc Flash Hazard? Yes or No?
However, from this forum there is a good soluion to the problem. TubineGen said:
"Perhaps the simplest solution to this problem would be to install a set of fuses. 60kA rated fuses are not that expensive. You could install them on the 4 AWG tap wires before the breaker. "
I agree and give a star for that suggestion.
RE: Arc Flash Hazard? Yes or No?
RE: Arc Flash Hazard? Yes or No?
I would replace the #4. It might have met and still meet the tap rule but needs replacing. You have not mentioned the breaker sizes so we do not know yet.
If the fault duty is above the rating of the breaker, then you either need to replace it or limit the current, as has been noted.
Once I see the fault duty exceeding 80% of the breaker rating, I start to get nervous. I sure can't handle the duty that I once could, but I think it is age and poor maintenance!
"Will the smaller CB actually see 47 KA coming across the #4 wire? If so, what happens? Will it not open? Will it explode?"
If you cannot calculate the necessary currents and implement the appropriate mitigation, then please find someone to help you who can. With all due respect, you said your background is in micro-electronics and I tnink you have realized that this isn't an IC (no offense intended).
Alan
RE: Arc Flash Hazard? Yes or No?
This is very questionable in my opinion and I would agree that it can only be counted on if the devices are tested for that capacity. I would say UL tested at a minumum.
However, I do not think that this rule would apply when the upstream device is, by itself, capable on interrupting the fault current. A CL fuse operates in less than one cycle and is, by definition, used to reduce the fault current contribution to downstream devices. At least that is what I believe. Is this wrong?
RE: Arc Flash Hazard? Yes or No?
RE: Arc Flash Hazard? Yes or No?
I would install current limiting fuses at the bussbar. These fuses should almost be installed on the buss - in such a way that the bussbar to fuse seperation and wiring gives little chance the wires between the bussbar and fuses could short. These fuse will protect against a short circuit becoming a huge failure that blows up the wiring and the bussbar. With this protection, the arc flash rating of the tap cabinet should also be lowered to a level where work can be performed.
I would increase the wire size. I'd think about 1/0 wire is suitable for the application once current limiting fuses are installed. This wire also should have some decent bracing so it doesn't jump around during a fault - look at wire clamps or other such methods.
I would also change the circuit breaker in the tap cabinet to one rated for at least 65kA. You can not calculate the reduced current due to the fuses and assume a breaker of that current can be installed. You could install a long enough wire run that the current is reduced to match a lower rated breaker but once you upsize the wire this likely isn't a very practical solution.
RE: Arc Flash Hazard? Yes or No?
t=(k*A/I)^2
k is determined by conductor or insulation. Bare copper is 0.0845, PVC is 0.03465.
A is the cross sectional area in circular mils. 4/0 is 211600.
Assuming bare copper you have 0.145 s before it fuses. Any type of insulation covering will fail much sooner.
Unless your conductor is supported like bare copper so that melting insulation will not cause a second fault; no.
RE: Arc Flash Hazard? Yes or No?
RE: Arc Flash Hazard? Yes or No?
How long is the #4 tap?
RE: Arc Flash Hazard? Yes or No?
Each leg of the #4 tap (this is 3-phase) is just 28" in length. See attached drawing. (If there are 2, use the 2nd one).
FEinTX
RE: Arc Flash Hazard? Yes or No?
RE: Arc Flash Hazard? Yes or No?
RE: Arc Flash Hazard? Yes or No?
I'm just a little skeptical about the 47 kA if the source is a local low-voltage generator.
RE: Arc Flash Hazard? Yes or No?
FEinTX
RE: Arc Flash Hazard? Yes or No?
In this case the main breaker can not reliably clear a fault on the tap circuit regardless of the tap conductor size.
RE: Arc Flash Hazard? Yes or No?
See my last post;
- install fuses right at the bussbars
- install larger size tap wire
- install new tap breaker with higher fault rating
This is the only way to address the fault rating problem and it should also lower the arc flash rating of the tap panel to a workable level.
RE: Arc Flash Hazard? Yes or No?
RE: Arc Flash Hazard? Yes or No?
ABB S4L 3 Pole Unit