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 Post subject: Distribution Feeder Arc Flash CalculationPosted: Fri May 12, 2017 1:10 pm

Joined: Fri May 12, 2017 1:04 pm
Posts: 2
OSHA States as a note in 1910.269 the following:

Note 2 to paragraph (l)(8)(ii): This paragraph does not require the employer to estimate the incident heat energy exposure for every job task performed by each employee. The employer may make broad estimates that cover multiple system areas provided the employer uses reasonable assumptions about the energy-exposure distribution throughout the system and provided the estimates represent the maximum employee exposure for those areas. For example, the employer could estimate the heat energy just outside a substation feeding a radial distribution system and use that estimate for all jobs performed on that radial system

It seems to me like you would have a higher incident energy further away from the substation because the further you go out the less fault current there is and therefore a longer trip time / arcing time. All the studies I have done have shown that to be true. It seems very odd that OSHA posted that note to me. Thoughts?

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 Post subject: Re: Distribution Feeder Arc Flash CalculationPosted: Sun May 14, 2017 9:46 am
 Plasma Level

Joined: Wed May 07, 2008 5:00 pm
Posts: 873
Location: Rutland, VT
I do not know OSHA's reasoning on that as studies I have done show that the incident energy can be higher the farther you get from the substation protective device. So I will look at some of the feeders to determine the IE on the farthest point from the sub protective device.

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Barry Donovan, P.E.
www.workplacesafetysolutions.com

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 Post subject: Re: Distribution Feeder Arc Flash CalculationPosted: Mon May 15, 2017 9:49 am
 Sparks Level

Joined: Tue Oct 26, 2010 5:00 pm
Posts: 201
Location: Maple Valley, WA.
Yes, you are correct, the arc flash energy can be higher the farther away you are from he substation. For fuse protected circuits, I would say that this is true. However, using relays, you may be able to set it so that the tripping time would be constant over the entire length of the feeder.

You can do this by using a 50 Element with a delay of 0.5 to 0.2 seconds. This is similar to the Short Time function of a low voltage circuit breaker. The pick up should be set 1.1 X times the minimum arcing (not bolted) fault current of the entire line.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies

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 Post subject: Re: Distribution Feeder Arc Flash CalculationPosted: Mon May 15, 2017 10:44 am
 Plasma Level

Joined: Wed May 07, 2008 5:00 pm
Posts: 873
Location: Rutland, VT
Also more modern relays have a Hot Line Tag feature which can be used to reduce the incident energy values also

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Barry Donovan, P.E.
www.workplacesafetysolutions.com

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 Post subject: Re: Distribution Feeder Arc Flash CalculationPosted: Mon May 15, 2017 7:13 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2173
Location: North Carolina
In a constant time region as impedance increases (such as with longer cables), incident energy decreases as expected. Constant trip times for instance would be "insrantaneous" (51) regions,or at the two second cutoff.

In an inverse time region depending settings it can go up, down, or stay the same. In practice most of the time ".

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 Post subject: Re: Distribution Feeder Arc Flash CalculationPosted: Fri May 18, 2018 4:59 am

Joined: Mon Aug 04, 2014 5:19 pm
Posts: 15
On aspect that one need to consider is the utility perspective in that most utility practices require something like ..." where the feeder bolted fault current along the feeder drops below X, a protective device shall be installed at that point" Thus a lower rated device with resulting shorter clearing time will be serving the section of feeder below this protective device. Jim

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