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 Post subject: Batteries & Arc Flash for DC Less than 50V
PostPosted: Fri Aug 30, 2019 2:18 pm 

Joined: Fri Aug 30, 2019 1:50 pm
Posts: 1
I have a client who is installing 48V battery banks in manufactured telecom enclosures. We've previously provided AFHS for another of their facilities, but we've now been tasked with analyzing this particular process. They had a recent incident where a string was shorted and the terminals of one battery melted and were expelled. I feel like I've researched this topic to death (50V and less DC work) and am still without a final conclusion. There are serious hazards with working with batteries and I feel confident in the hazards we've identified and recommendations we are making EXCEPT whether Arc Flash is a serious issue. Could anyone help with the following questions?

Are DC arcs truly sustaining under 50V? If so with what parameters and what are some reasonable arcing times? I've used the DOE spreadsheet in the past, but with the sustainability of an arc at these voltages highly suspect, I hesitate to utilize this tool.

Any industry sources or code that states clearly to ignore A-flash at these voltages? NFPA is very frustrating on this.

Any help is much appreciated. I've read through a number of posts on this forum from folks that seem to have more experience in the realm so hopefully they can chime in.


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 Post subject: Re: Batteries & Arc Flash for DC Less than 50V
PostPosted: Mon Sep 09, 2019 9:22 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
When I've looked at 48 V and 125 V batteries with no protective devices, I've assumed the arc will persist for 2 s until the worker can get away. In all cases the IE was so small that chemical PPE was a much bigger concern for someone clad in 8 cal/cm^2 clothing.


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 Post subject: Re: Batteries & Arc Flash for DC Less than 50V
PostPosted: Mon Sep 09, 2019 2:19 pm 
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Joined: Tue Oct 26, 2010 5:00 pm
Posts: 188
Location: Maple Valley, WA.
Our calculations have also shown that the AF energy is very low for most of these systems. However, I have been told that the DOD sometimes uses many parallel banks of 48 V batteries at their radar sites. These means the short circuit current can be much higher than just a single string of batteries.

If you only have a single string of batteries making up a 48 V bank, then I would not be too concerned. However, if you have multiple strings that are paralleled, then it would be worth while trying to determine the amount of energy that could be available.

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


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 Post subject: Re: Batteries & Arc Flash for DC Less than 50V
PostPosted: Mon Sep 16, 2019 6:40 am 

Joined: Mon Jan 08, 2018 8:43 am
Posts: 10
A 2017 study conducted by Bonneville Power Administration proved that the maximum arc time for nominal 125VDC systems is 0.715 seconds. They went to a lot of work to maintain an arc for that length of time as the normal arc time was considerably less. They did not test at 48VDC.

I use an arc time of 0.8 seconds for 125VDC and less systems to be conservative.
Don’t forget to include cable resistance when calculating arc flash energy for a battery that has inter-tier or inter-row cables. The cable resistance will significantly reduce the arc flash energy at the battery terminals.
Example:
• 60-cell EnerSys 4DX-11 battery. Cell Short Circuit Current: 11,100 amps.
• One intertier cable with resistance of: 150 micro-ohms.
• Average intercell connection resistance: 7 micro-ohms.
• Short Circuit Current at the Battery Terminals: 5941.3 Amps.
• Arc Energy at the Battery Terminals at 45.7 centimeters: 1.536 cal/cm2
Cables in a battery can make a significant reduction in the arc energy at the battery terminals


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