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 Post subject: Telecommunication Battery Arc Flash Concerns
PostPosted: Thu Apr 21, 2011 12:50 pm 

Joined: Fri Aug 21, 2009 10:20 am
Posts: 6
I am an Electrical Engineer working for a large federal power administration doing telecommunications work. We have been struggling with Arc Flash concerns surrounding our nominal 48 volt battery installations. We recently have completed some testing in our test lab using copper plates with a small piece of fuse wire between them and energizing them with a 48 volt battery to see if there is any ability to sustain an arc as or after the fuse wire blows. Our testing showed at this voltage it is virtually impossible to sustain an arc, which is what we (or I) had thought would be the case.

48 volts also does not pose an electrocution hazard so the only issues we have for properly fitting our employees with Personal Protective Equipment is for possible short circuit issues that can create arcing with thermal burn issues or splashing around some molten metal. The only other concern is dealing with the electrolyte which may require the use of acid proof aprons, gloves, and faceshields.

The issue that I'm struggling with currently, however, is the requirement by the NEC as well as the IEEE that the battery rack must be grounded. Telecommunication batteries are normally grounded on the positive side of the battery bank in order to reduce noise in the telecom equipment. This is different then a normal substation control battery (130 VDC in our case) where this battery is left floating. The result for telecommunication batteries is that if you accidentally ground the negative leg of the battery (or any post of any battery in the string), you have applied a dead short on the battery. The substation control battery doesn't have in issue with this unless there is an existing partial or full ground of the battery which can have the same outcome.

I have read most everything that I can get my hands on and all of what I have read states that the battery rack is required to be grounded. I am suspicious that this statement was made not considering a grounded telecommunications battery but was assuming floating DC battery plants.

My question is if anyone knows the assumptions that were made by the IEEE when writing their Recommended Practice for Installation Design paper or have any good reason why a rack of a 48 VDC telecom battery should be required to be grounded. I can think of many reasons why not to ground it. I can't think of any good reasons that would require it. Your insight would be greatly appreciated.

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PostPosted: Thu Apr 21, 2011 1:35 pm 
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Joined: Mon Mar 29, 2010 5:02 am
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You should be able to ground the battery rack and the positive terminal with no ill effects. In the oilfield most of the facilities I have been at have the negative grounded, charger and racks bonded. When you work on replacing the cells, or cleaning connections, if both batteries connections are unhooked, you will break the earth conenction (assuming that there is a charger on the system to supply your 48 vdc). That way the risk of creating a short is gone with the batteries isolated.

Not sure if that helps...

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PostPosted: Fri Apr 22, 2011 9:14 am 
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Joined: Tue Jan 13, 2009 5:00 pm
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IEEE 484 allows "adequately insulated supports" instead. Seems like your rack could be mounted on insulators. This might reduce the seismic capability if that is a concern. 450 requires that workers use insulated tools to reduce the risk of accidental short circuit, and to neutralize static prior to work by touching a grounded surface. So retraining might be needed if workers are accustomed to discharging on the rack, and it is now insulated.

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