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 Post subject: Loads from 208v panel rated 16 cal
PostPosted: Mon May 12, 2014 5:24 pm 

Joined: Mon May 12, 2014 5:09 pm
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A group of guys and myself have been tasked with setting up the electrical safety standards at our facility, arcing is a big issue here because we make carbon fiber parts and the carbon gets into everything and blows things up. we have a handful of 16 cal rated 208 panels that go out to single phase receptacles and small equipment, is there an easy calculation for small single phase loads like that? anything over 8 calories and we are wearing our 40 cal suit to verify that whatever we are working on is dead and it is not feasible to pay to have a study done down to every outlet coming off of these higher rated panels. we have looked into getting our own software to plug in the numbers but the 5k price tag for such software is a bit much.

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PostPosted: Tue May 13, 2014 6:05 am 
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The panel itself may be rated 16 cal/cm2, but the individual feeds off the individual circuit breakers should be much, much smaller than that.

It's very common for lighting panels to have high incident energy, because of the lack of any sort of secondary fusing on the lighting transformer. If you want to lower the energy on the panel, doing something like adding secondary protection between the transformer and the panel will accomplish that.

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PostPosted: Wed May 14, 2014 9:08 am 
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You are gong to have an uphill fight here because it is just too darned easy to say, "well this is what the engineer calculated". Trust me, the engineer is wrong. There is no way short of running a lot of expensive tests to find out the exact arc flash value specifically for under 240 V. Nobody has been studying it very much because it is so difficult to make it arc consistently. I've given you several sources of information below though that are based on both industry consensus safety standards and some of the research that is out there.

There are huge issues with tryng to actually do any test work on arcs below around 250 Volts. First, arcs below 250 V tend to self-extinguish. Second, arcs below 250 V are not very stable and tend to be very weak so the actual incident energy is much less than a prediction at 480 V.

The fact that you even mention a number like 16 cal tells me immediately that whoever did the study did NOT consider this. The engineer probably used the formulas in IEEE 1584 which is based on about 300 tests of STABLE arcs without even considering whether or not the result is even valid. It isn't. There is exactly 1 and only 1 test in that entire set of tests for 208 V. Using IEEE 1584 arc flash calculations for less than 250 Volts is questionable at best. Quoting the actual standard,

[INDENT=1]“It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208 V in a 508 mm × 508 mm × 508 mm box. In all other tests with that box and the 305 mm × 368 mm × 191 mm box, the arc blew itself out as soon as the fuse wire vaporized. An arc was sustained several times at 215 V in a device box (100 mm × 100 mm × 50 mm size). It appeared from the arc flash photos from the 305 mm × 368 mm × 191 mm box that testing arcs usually jumped from the electrodes to the box wall and from another point on the box wall back to another electrode. The magnetic forces created by these arc currents forced them away from each other and into the box wall.”[/INDENT]

Further on the standard also has this to say: “Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.” It does not go on to give you specific guidance but nearly everyone has made the assumption that this means that the incident energy would be less than 1.2 cal/cm^2.

Similar equipment-specific studies have been conducted by EPRI and this has resulted in a rating of 4 cal/cm^2 across the board for circuits that are rated 50 to 250 V under the IEEE C2 standard (National Electrical Safety Code) which is the standard used by almost every electric utility in the U.S. The standard is around $80 but if you just want to see specifically what is in the tables, they are copied into this article here:

However, more recent research has shown that although arcs are possible and can actually exceed 4 cal and sometimes even 8 cal, those conditions are not very common, but they can occur even down to a short circuit current of around 4.5 kA, or roughly a 75 kVA or larger transformer such as [font=Calibri]“Effect of Insulating Barriers in Arc Flash Testing”, IEEE paper #PCIC-2006-6. This technical paper and some additional versions of the same tests is available on Mersen's arc flash web site. Interestingly that paper also shows that some cases, especially when you start adding "barriers" and smaller enclosures can exceed even the calculated value from IEEE 1584. That being said, I am not aware of anyone following the IEEE 1584 calculation and wearing PPE to that standard who has even been injured, despite the fact that the current IEEE 1584 standard itself says that it should fail to provide adequate protection about 5% of the time. So take it with a grain of salt here...lab tests are a lot worse than what has been proven in the field. We are still a long ways off from reconciling what has been done in the lab and what happens out in the real world. Just know that this information exists.[/font]

You may also want to see how some others have addressed the "<250 V" question. I think this particular document does a great job of summarizing it and coming up with a different calculation to try to "fix" some of the issues with IEEE 1584. It also summarizes a lot of what I already said, I just don't subscribe to "fixing" IEEE 1584:

You may also want to look through this presentation which has a lot of suggestions on how to fix arc flash issues even where the rating really is above 4 or 8 cal:

I have personally reviewed every case of electrical injuries from 2002 to current in the OSHA investigations of accidents or just about 10 years of data. There is one and only one case of an actual arc flash on a 240/120 V temporary construction panel where two yahoos dressed in flip flops and shorts decided to start disassembling a live panel before a Georgia Power lineman arrived to open the cutouts which resulted in a fatality for one and hospitalization for the other. The only other cases I know of are rare cases of plugs exploding while someone had their hand on it which current arc flash standards are not meant to prevent. So let me just say that an arc flash at 208 V can be fatal. There is no denying it because it happened. What we can't say is under what circumstances.

So in conclusion:
1. IEEE !584 is wrong, and it is wrong by a very wide margin. I don't think you can use the results directly from that calculation method for 240 V or lower voltages, even if you "fix" it as Nehring suggests. IEEE 1584 grossly overestimates the actual hazard.
2. Since there is no valid calculation, then what? In this case the only thing we can go on is to go with what the standards recommend for <250 V. 70E has a table for 240 VAC or lower voltage panelboards that boils down to using 1.2 or 4 cal PPE depending on the task and is based on the 70E Technical Committee's recommendations. NESC has a table I referred to above which boils down to either 4 or 8 cal depending on the equipment design for all tasks. IEEE 1584 has an "exception" as explained above which boils down to a partial "1.2 cal rule". The exception is under review. Papers I referred to above are being looked at by the IEEE 1584 Committee to decide what the "right" cutoff should be and it will be lower. At least right now though we just don't have any good way of "calculating" 240 V or less arcs and we may never have a way to do it. Instead of calculating it, I'm advocating using tables from various industry consensus safety standards.
3. Either way the basic reason why you are probably seeing extremely high ratings is because you have a "classic" setup. The transformer is protected upstream by either fuses or a breaker. Then the secondary connection goes straight to the top of a main breaker in a panelboard. The issue here is that inside the panel board, the real danger is if an arc forms (or propagates) to the primary side of the breaker. Since the only protection is the fuse/breaker upstream of the transformer, it will arc for a very long time before tripping. This is a serious problem for all equipment. It does not matter what the voltage is. Potential solutions are:
A. De-energize and test upstream first where frequently the arc flash rating is a LOT lower. This is simpler but recognize that unless it is dead clear that there are no back feeds or criss-crossed conduits or any other possibility of misidentifying the
B. If it is a fancy enough breaker on the primary side that it can shunt trip, install CT's inside the transformer enclosure on the secondary side and another relay to trip, or CT's in the panelboard, or an "arc flash sensor" in the panelboard to cause an instantaneous trip and drastically reduce your hazard. Unless it is a new installation though typically electronic breakers with shunt trip capability are just not all that common. It will become more common in the coming years though as high end protection technology that was previously only used in large "utility" substations is migrating its way into more and more low end applications.
C. Install a second breaker or fused disconnect between the transformer and the panelboard. If the instantaneous trip is set below the arcing current then this will trip in the event of an arc and drops the incident energy inside the panelboard to a very low value. This is basically the same as option B but slightly more expensive to install. In practice one never actually operates this breaker or disconnect except if there is a trip (use the disconnect on the primary side of the transformer) or for maintenance purposes (testing). It just sits there passively providing arc flash protection.
D. Arc "termination" or "quenching" devices. This is almost certainly not going to be practical for a 208 V panelboard but these are high speed breakers installed just downstream of the main breaker with a load that is nothing but shorted bus bars. The high speed function kicks in very quickly (often in 1/4-1/2 cycle) and essentially causes an intentional short circuit across all 3 phases. Since the short is a lower resistance than the arc, it instantly extinguishes the arc. Although the energy dissipated inside the device is considerable, the high speed breaker only has to survive long enough for the main breaker or fuses to open.

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