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 Post subject: PPE level on power plant
PostPosted: Thu Feb 15, 2018 4:10 am 
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Inside a power plant the busbars are located in the ceiling above the walking area. To separate/protect from touching the busbars are protected by grating. But of course there is possible to put small tools etc through the grating. The lowest hight is just 2.4m above walking area.

My question is would it be a correct approach to calculate a PPE level just for walking in the room? Or is the risk for any thing to happen just by walking under the busbar considered so low that it is not relevant? (Note this is only bus bars no active component such as breakers etc.)

(For a calculation the working distance would then be high of busbar- person hight= 0.6m?) And there is a very SC-level so there will actually be a considerable energy level

Anyone have experience with the normal approach?


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 Post subject: Re: PPE level on power plant
PostPosted: Thu Feb 15, 2018 6:02 am 
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Location: Rutland, VT
This is similar to all the discussions about wearing PPE into electrical rooms or near MCC/Switchboard, etc while walking by. Basically, you are not interacting with the equipment in a manner that would cause an arc flash. What would your risk analysis show for this scenario?

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 Post subject: Re: PPE level on power plant
PostPosted: Fri Feb 16, 2018 8:44 am 
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70E is a little strange to interpret in this but the salient point is that if you're not doing something which could cause an arcing fault or that the chance of occurring is unlikely, then there is no arc flash hazard. This is in the definitions section and everything in NFPA documents is meant to be read from front to back. Similarly in your case the limited and restricted approach boundaries do not exist since under normal circumstances the bus bars are not accessible so fail to meet the definition of "exposed".

Yes you could drop a tool through the grating. If it's long enough it might be able to contact both bus bars simultaneously. But this might also be a big stretch. It has happened before in some circumstances but it is comparatively rare. I think I've heard of two cases in two different plants. In one case a mechanic working on a crane accidentally dropped a wrench down into the exposed bus bars inside a dry transformer on board an excavator that didn't have a grating on the top. This happened at a large mine at some point in a 50 year span of time. In the second case the wind blew an aluminized fire jacket off an elevated open deck at a pipe foundry in New Jersey and it shorted out the bus bars in an outdoor overhead style substation. In both cases the magnetic field instantly threw the tool off the bus bars and the overcurrent protection shut everything down. This happened about 10 or 15 years ago and was the first case of it happening in a plant that had been around for 200+ years.

I mentioned the timing just to point out that yes it does happen but the circumstances in which the wrench lands just right is very rare. If you're really worried about it, a lot of plants that have grating above exposed equipment often just lay down rubber mats over the area that might be exposed. It's a cheap and simple solution, especially with coal plants where there is always always an unlimited supply of worn out conveyor belt that can be used for this.


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 Post subject: Re: PPE level on power plant
PostPosted: Mon Feb 19, 2018 11:21 am 
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Typically NFPA 70E does not apply to a powerplant (see scope) but NFPA 70E isn't requiring protection from something happening except when the parts aren't suitably guarded, isolated or insulated. In this case, you would probably consider them properly "guarded".

Obviously IF work is occurring, you would need to establish the AFB and protect, otherwise your risk assessment should guide you as to the probability of an occurrence otherwise. If you have had a fault or if things aren't properly cleaned and maintained, if there are critters in the powerplant that could cause a fault, you might want to do something. I'd doubt you have enough energy at that distance to cause a clothing ignition but calculations and a little understanding of arc physics would help with the assessment.

OSHA 1910.269 does not call out the term AFB BUT says that at 2.0 cal/cm┬▓ exposure level does require PPE. Not as prescriptive as NFPA 70E because workers are typically more qualified. Do your risk assessment with someone qualified to make a real assessment and you should have some guidance.

Hugh Hoagland


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