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 Post subject: ARC FLASH Boundary
PostPosted: Tue Feb 02, 2016 8:13 am 
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We have an 800 AMP 480V freestanding Service Entrance Disconnect in a NEMA 3R enclosure located adjacent the utility transformer. This Main Switch is rated as dangerous. We will be installing a remote operator switch to control this breaker. We would like to install this switch behind the SED. My question is, does the arc flash boundary extend a full 360 degree around the energy source?


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 Post subject: Re: ARC FLASH Boundary
PostPosted: Tue Feb 02, 2016 10:40 am 
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dan collyard wrote:
We have an 800 AMP 480V freestanding Service Entrance Disconnect in a NEMA 3R enclosure located adjacent the utility transformer. This Main Switch is rated as dangerous. We will be installing a remote operator switch to control this breaker. We would like to install this switch behind the SED. My question is, does the arc flash boundary extend a full 360 degree around the energy source?


There is no rating of "dangerous" in the standard. This is a fiction from SKM and other software that is applied to anything that it calculates as 40 cal/cm^2 or higher. There is nothing magic about this particular incident energy level except historically the first "arc flash suits" were only rated as high as 40 ATPV. Now you can buy 100+ ATPV off the shelf so this number is meaningless.

Technically the arc flash extends out in a 360 degree direction but this will cause you some modeling headache. The equipment model assumes that the enclosure more or less survives and that the shape of the enclosure (5 sided box) directs most of the incident energy out from the front. The model does not include considerations for when the enclosure is destroyed at which point this model no longer applies and the open air model would be in effect.

But regardless there's a huge fallacy here. Having a remote operator allows you to do one thing: operate the equipment from outside the arc flash boundary. If the equipment is maintained in good working order and there are no faults with it then this is totally unnecessary because as of 70E-2015 there is no appreciable arc flash hazard and PPE is unnecessary in the first place just to operate it. If however it has faulted or is not maintained properly, then we need to get into the PPE side of things, remote operators, etc. And if its not maintained properly, the incident energy study itself is meaningless.

You can't use a remote operator to substitute for applying temporary protective personnel grounding nor for testing for the absence of voltage. These are both energized work. So the remote operator is only half the answer in the same way that arc resistant gear is only half the answer.

Consider putting in an enclosure with class L fuses or a similar high current/fast acting class between the disconnect and the utility. Coordinate with downstream equipment so that these fuses will only operate in the event of a fault between the fuses and equipment downstream of the disconnect.

With this arrangement and carefully chosen fuse sizes, the incident energy at the disconnect is reasonable so you can just use your disconnect the way it is intended. In addition to operating the disconnect you can also maintain it since the PPE required for grounding and testing for absence of voltage is now reasonable. And this is considerably cheaper than the remote operator.

As for the fuses themselves, at some point they will need to be worked on. If you don't have access to a cutout or similar disconnect from the "utility" side of things especially when you consider coordination with a utility, if you have a pole-mounted feed, add your own cutouts and the cutouts themselves can be fused. If it is not accessible in this manner then you can still buy distribution-grade fused switches that are essentially "cutouts in a box" that are operated with a hot stick. This gives you the remote operation/access feature as well as being able to remotely set grounding clips and test for absence of voltage using shotgun stick mounted tools and equipment.

In the end no matter how you look at it when you run into this common "very high incident energy" problem the answers must always involve access to or modifications to provide access to the upstream side of the equipment precisely because of the requirements of doing an electrical lockout procedure.


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 Post subject: Re: ARC FLASH Boundary
PostPosted: Mon Feb 08, 2016 8:08 am 
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Good Morning,

I would also recommend checking out 130.7(15)(B) informational note 2:
Informational Note No. 2: The collective experience of
the NFPA 70E Technical Committee is that, in most cases,
closed doors do not provide enough protection to eliminate
the need for PPE in situations in which the state of the
equipment is known to readily change (e.g., doors open or
closed, rack in or rack out).

If you are racking a breaker in or out PPE is required when within the arc flash boundary.


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 Post subject: Re: ARC FLASH Boundary
PostPosted: Mon Feb 08, 2016 3:00 pm 
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Carl Johnson wrote:
Good Morning,

I would also recommend checking out 130.7(15)(B) informational note 2:
Informational Note No. 2: The collective experience of
the NFPA 70E Technical Committee is that, in most cases,
closed doors do not provide enough protection to eliminate
the need for PPE in situations in which the state of the
equipment is known to readily change (e.g., doors open or
closed, rack in or rack out).

If you are racking a breaker in or out PPE is required when within the arc flash boundary.


The wording here is kind of confusing because "readily change" is hard to discern. A contactor is a device whose state is known to readily change, but it does so in a controlled manner.

When contacts are designed in such a way that they are designed to control the inevitable arc that forms, the failure rates seem to be relatively low. Equipment which is specifically not really designed to be operated under load, especially when the timing is not very precisely controlled, tends to have very high failure rates. Designs with springs or an overhung cam system or some similar mechanism which creates a system with essentially two stable mechanical states, which create a trigger mechanism where once the motion begins, it moves to completion, and the motion is engineered and controlled, generally have low failure rates such as load break disconnects, contactors, and circuit breakers. Those which do not incorporate control over the motion (not only direction but speed) into the design such as draw out mechanisms or non-load break switches tend to have very high arcing failure rates. So in my mind, "state of the equipment is known to readily change" hardly captures the area of concern in this respect because for instance ABB reported that the failures for drawout switchgear is in the drawout mechanism, not the circuit breaker itself. Similarly careful analysis of studies that include both panelboards and drawout switchgear show similar trends and patterns.

But again this misses the point. The major concern that seems to exist is disconnecting power, whether it is using a circuit breaker, disconnect, or draw-out gear. The problem though is that even if you can accomplish this one subtask remotely, and even if you can apply the grounds through means of a 3-way switching mechanism which some gear includes, the inevitable subtask of testing for the absence of voltage must still be done in the presence of presumed energized equipment. And if "no PPE is available", then no PPE is available...and remote racking doesn't do any good.

I can recite several incidents where I personally found myself in a situation where equipment was in fact energized even after all other LOTO steps had been performed for one reason or another. The last step, testing for absence of voltage, is the only thing that saved me from either hospitalization or a fatality in multiple instances. Sure we corrected the problem that lead up to this situation AFTER THE FACT but if I hadn't have tested in the first place, the correction would have taken place after an injury or in some cases a fatality rather than a near miss.


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 Post subject: Re: ARC FLASH Boundary
PostPosted: Thu Feb 11, 2016 8:15 am 
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You pose a good question and I suppose I'd say "it depends". First, what kind of shape is the gear in? Is the hardware in good shape? I mean, are the nuts, bolts and screws holding the gear together and covers in place bright and shiny? You said it's outside. How has this box been maintained over the years? Does your company keep the box clean and in good shape? Is the box allowed to get really messy inside? What sort of environment is this in? is it really wet, dry, hot, cold, and is the box exposed to air born chemicals? If it is exposed to chemical agents, does the chemical affect the buss, insulators or the integrity of the enclosure? And what about insects and rodents? For example, spiders spin webs. Those webs can easily catch debris. That debris could be slightly conductive and when someone operates the gear, that might be all it takes to cause a fault. All these are things to consider.


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