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 Post subject: PPE when working on oil-filled battery?
PostPosted: Wed Oct 10, 2018 7:42 am 

Joined: Tue Apr 26, 2016 3:47 pm
Posts: 2
I need help! One of our techs (who specializes in reverse engineering) will be opening up an oil-filled battery (with partial charge, however not tampering with the cells) to retrieve a circuit card for repair. This is a Li-ion battery pack 300V, 200A. Typically, we've had this work contracted, however, the shop is being instructed to do a feasibility study if we are able to do this in house (hence, the task-at-hand).

My job is to figure out if there is a safe way to do this. This is something new and I can't even wrap my head around this and I honestly don't like it... but I will do my due diligence for research. :shock: So, my questions...

Has anyone here have any experience or knowledge about this sort of thing?
what controls can I use to mitigate risk?
Also, since there is still a partial charge (I don't know the details yet as the % but it is in an almost drained status), flash and shock are still hazards present (among hazards dealing with Li-ion). Oil degrades our PPE materials, how can I protect the PPE, such as the rubber insulating gloves (there will be a point that the gloves will be in contact with mineral oil)?

I appreciate it. :geek:


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 Post subject: Re: PPE when working on oil-filled battery?
PostPosted: Sat Oct 13, 2018 5:18 am 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
In terms of shock, the "limit" for DC is 100 VDC. This is based on Dalziel's work and is NOT a fibrillation hazard. Since there's no AC to interfere with heart rhythm that's not really an issue. It's a pain threshold. There might be some sparking depending on what you are doing but I've seen sparking with high current 24 VDC power supplies. It looks exciting but it's not dangerous. Human resistance is usually considered 1 Kohms when soaked and the skin is broken (stabbed with a wire). It does go lower to 500 ohms as the skin quickly breaks down at high voltages but even at 300 V you are nowhere near that.

In terms of arc flash worst case would be at the outputs (the terminals on the ends). You need to know or measure (with a DLRO) the series ohmic resistance of the battery. This limits current output in the event of a short circuit. You could use Ammerman's much more complicated model but for DC purposes the much more simple model is the DC equivalent of Lee's model which is in Annex D.8 of NFPA 70E attributed I believe to Neal. Maximum power transfer occurs when arc resistance equals system resistance. Knowing the battery resistance then we can calculate the maximum arc power using the battery series ressitance to calculate maximum short circuit.


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 Post subject: Re: PPE when working on oil-filled battery?
PostPosted: Mon Oct 22, 2018 6:29 pm 

Joined: Mon Jan 08, 2018 8:43 am
Posts: 10
PaulEngr wrote:
... You need to know or measure (with a DLRO) the series ohmic resistance of the battery...


It appears that you are suggesting a DLRO be used to measure the internal resistance of the battery? Ohm meters and DLRO's usually don't tolerate being connected to an energized circuit. The battery manufacturer can probably provide the theoretical internal resistance of the battery or you may be able to measure the internal impedance of the battery with an impedance meter such as the Megger BITE or other similar meter.

A study conducted by Bonneville Power Administration in 2017 essentially proved that 125VDC batteries have very low arc flash energies. The longest arc time for 125VDC batteries was less than 0.8 seconds.


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 Post subject: Re: PPE when working on oil-filled battery?
PostPosted: Tue Oct 23, 2018 6:32 pm 
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MikeO'Brien wrote:
PaulEngr wrote:
... You need to know or measure (with a DLRO) the series ohmic resistance of the battery...


It appears that you are suggesting a DLRO be used to measure the internal resistance of the battery? Ohm meters and DLRO's usually don't tolerate being connected to an energized circuit. The battery manufacturer can probably provide the theoretical internal resistance of the battery or you may be able to measure the internal impedance of the battery with an impedance meter such as the Megger BITE or other similar meter.


Standard ohm meters don't like it agreed. The test works with AC as opposed to most testers use DC.

Quote:
A study conducted by Bonneville Power Administration in 2017 essentially proved that 125VDC batteries have very low arc flash energies. The longest arc time for 125VDC batteries was less than 0.8 seconds.


I believe you missed a decimal. I have never seen the actual report first hand but my notes culled from secondary sources stated 0.08 seconds. If it was 0.8 seconds, it is definitely not a "low" hazard. If you use their numbers and estimate incident energy at 18", it comes out to right around 1.2 cal/cm2. For utilities (30 CFR 1910.269) the distance shrinks to 15" but the incident energy cutoff increases to 2.0 cal/cm2 and once again it squeaks by. The same data set for 250 VDC batteries showed that these are a problem from an incident energy point of view.


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