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 Post subject: Incident Energy Calcuation an Amplifier Based Power SourcePosted: Fri Aug 07, 2015 4:25 pm

Joined: Fri Aug 07, 2015 12:31 pm
Posts: 3
Hello:

I have been asked to determine the arc flash potential from a test source we constructed and use in our facility. It uses a 250W amplifier coupled on one channel to 250VA voltage transformer which can generate up to 400 Vrms on the other channel a 360VA current transformer which can generate up to 35 Arms. Because the voltage generation is above 250V, it is necessary to do an incident energy calculation. I would appreciate any insight and/or direction to verify accurately our configuration. Thanks

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Mon Aug 10, 2015 4:32 pm

Joined: Sat Feb 23, 2013 9:25 am
Posts: 20
So the amount of power available from one channel is about equal to two 100W light bulbs?

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Tue Aug 11, 2015 10:48 am

Joined: Fri Aug 07, 2015 12:31 pm
Posts: 3
Yes. It sounds silly but when you are asked to prove that an arc flash will not occur based on a calculation or directive; I cannot find the proof I need to show the auditor. Again, if you can advise me on how to prove compliance; I would greatly appreciate it. Thanks

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Tue Aug 11, 2015 11:27 am
 Sparks Level

Joined: Sat May 17, 2008 5:00 pm
Posts: 88
Jimbobc wrote:
Hello:

I have been asked to determine the arc flash potential from a test source we constructed and use in our facility.

Whoever did the asking might be thinking way to much about this. The incident energy is based on short circuit current and duration. The short circuit current will be quite low and I doubt if it will amount to much in the way of an arc flash - maybe similar to car battery sparks? The duration is typically defined by a protective device operation. Two issues here - First, can you find what that device would be an how it would operate and second, I can't imagine the arc would sustain regardless of what the predicted time would be.

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Tue Aug 11, 2015 12:26 pm

Joined: Fri Aug 07, 2015 12:31 pm
Posts: 3
Thanks for the reply. Based on test performed by the amplifier manufacturer, the amount of current generated would be 75A single phase for 85ms before the amplifier's internal protection would take effect. Again, the current and is low as compared to what usually is for arc flash calculation but how does one prove that an arc flash with not occur. I apologize for being so persistent.

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Tue Aug 11, 2015 4:10 pm

Joined: Sat Feb 23, 2013 9:25 am
Posts: 20
For example a 250W amp rated into 4 ohm would have roughly a +/-40V supply. 225W into 4 ohm is about 7.5A. With your transformer that would bring the voltage to 400V (peak, I am being lazy) and the current would be down to 0.75A. This is constant power not peak. The 75 A peak current you were given for the time period of 85 mS is roughly 12 Hz. Unless your transformers are flat to 12 Hz you are not going to get the peak current. And while the amp's protection circuit may kick in at that current we do not know if the voltage (roughly 30 VAC) will be available from the amp.

Lot of half baked assumptions but it can give you an idea of the magnitude we are dealing with.

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Wed Aug 12, 2015 2:08 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1637
Location: Scottsdale, Arizona
It looks like you have some pretty good input here already. I'll add a little bit more. I think the problem is no one (that I know of) would be willing to say an arc flash does not exist for your case.

The real issue is to be able to say is the resulting arc flash dangerous. The car battery analogy was a good one, there is an arc flash but it is just a short duration spray of sparks.

At such a low current, I don't see how you can have any significant incident energy but the opinion is not based on any testing or calculations. The IEEE 1584 equations don't apply to single phase and the current you have is well below the lower limit minimum of 700 Amps for the IEEE equations. I agree that someone is likely thinking too much about this one.

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Wed Aug 12, 2015 2:28 pm
 Sparks Level

Joined: Tue Nov 12, 2013 6:31 am
Posts: 238
Location: Port Huron, Michigan
So, what everyone is dancing around:

There is no need, in this case, to use arc rated PPE or to take any special precautions regarding a possible arc flash.

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Wed Aug 12, 2015 4:52 pm

Joined: Sat Feb 23, 2013 9:25 am
Posts: 20
Not dancing around, just not going to spend the time figuring out how to prove it. Interesting exorcise but have too many other things on my plate. Voltage is large enough to create an arc current if you accidentally short the two terminals of the transformer together and separate them but you have very little current available to heat the air with. At most you will get a spark. The transformer can only supply 250 VA, a far cry from the kVA of a distribution transformer.

http://etap.com/support/articles/electr ... lang=en-US

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 Post subject: Re: Incident Energy Calcuation an Amplifier Based Power SourPosted: Sun Aug 16, 2015 3:08 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
The same logic applies to drives. In that case first you can simply look first at the amount of stored energy in capacitors and then use this as the starting point for calculating incident energy. Second, you can look at the maximum current through the system (semiconductors). Since semiconductors have a very short life span if their current limits are exceeded (milliseconds), the maximum short circuit through the "drive" (amplifier in this case) is what should be considered. This is the required inputs to plug into the empirical calculation or Lee equation.

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