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 Post subject: Battery work
PostPosted: Tue Jan 14, 2014 1:18 pm 
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I am working on specifying PPE for work on an Energy Storage System. The Li-ion batteries will be connected inside an enclosure to create a Bus of 1000VDC to supply power to an inverter. Connection of the batteries will be done with the inverter disconnected from the AC Grid so the only power present will be the DC Voltage. I expect the Incident Energy to be less than 8 cal/cm^2 so we will be requiring workers to wear PPE for HRC 2 including a balacava. All work will be done with insulated tools. We also want to protect against accidental contact with the DC Bus.

I have done a search for an insulated apron rated to 1000V online, but I am not finding many choices.

Any suggestions?


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PostPosted: Wed Jan 15, 2014 12:15 pm 
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Not sure what you mean by apron. Are you looking for an actual apron for an individual to wear or an apron to place over the DC bus?
Salibury makes Insulating roll blanket material that can be cut to size and available for up to 7500V ac.

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PostPosted: Wed Jan 15, 2014 1:05 pm 
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I was thinking about an actual apron because the teams of workers will be moving along the DC busbars as they install each individual battery into the system.

The apron I was looking at is a Salisbury APR0 1000V (Class 0) Insulating Apron.


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PostPosted: Mon Jan 20, 2014 10:18 am 
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Larry Stutts wrote:
I expect the Incident Energy to be less than 8 cal/cm^2 so we will be requiring workers to wear PPE for HRC 2 including a balacava.



I could be wrong in a couple of areas, but I don't think ANY of the testing I have heard about, involved having the "person analog" inside a small enclosure with the arc, so I would question if standard "8 cal/cm2" gear is sufficient for that case, even if you would calculate 8cal /cm2 for some one outside of the box.
[Or to use an analogy, the Standards presume you are in the kitchen reaching into the oven, not IN the oven]

How did you decide that you were < 8 cal/cm2? It sounds like you just decided, but I assume you actually did some calculations.

As I see it, based on your description, you could be working on installing the last battery in a fully charged system,
with no overcurrent proteciton (unless each battery gets a fuse), at 1000V and a working distance probably
really less than 18 inches, and you are inside the enclosure with the arc.
[A whole DIFFERENT "inside the box/outside the box" issue]

What limits the IE exposure?
As I see it you have
1: The energy stored in the battery system,
2: Nothing that will interrupt the current other than the arc self extinguishing,
3: You are inside the box with the arc and have no reasonable way of getting away (so no bogus 2 scecond "limit")
4: The limiting impedances are
A: The internal resistance of the battery,
B: The impedance of short sections of DC bus.
5: Very little inductance (finally something positive)


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PostPosted: Mon Jan 20, 2014 1:46 pm 
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JKlessig wrote:
I could be wrong in a couple of areas, but I don't think ANY of the testing I have heard about, involved having the "person analog" inside a small enclosure with the arc, so I would question if standard "8 cal/cm2" gear is sufficient for that case, even if you would calculate 8cal /cm2 for some one outside of the box.
[Or to use an analogy, the Standards presume you are in the kitchen reaching into the oven, not IN the oven]

How did you decide that you were < 8 cal/cm2? It sounds like you just decided, but I assume you actually did some calculations.

As I see it, based on your description, you could be working on installing the last battery in a fully charged system,
with no overcurrent proteciton (unless each battery gets a fuse), at 1000V and a working distance probably
really less than 18 inches, and you are inside the enclosure with the arc.
[A whole DIFFERENT "inside the box/outside the box" issue]

What limits the IE exposure?
As I see it you have
1: The energy stored in the battery system,
2: Nothing that will interrupt the current other than the arc self extinguishing,
3: You are inside the box with the arc and have no reasonable way of getting away (so no bogus 2 scecond "limit")
4: The limiting impedances are
A: The internal resistance of the battery,
B: The impedance of short sections of DC bus.
5: Very little inductance (finally something positive)


No - no work is done with the technician inside the box with the batteries.
I did the AC and DC calculations for the enclosure. The AC ratings should never come into play since we are not going to allow access to the batteries when the AC portion of the enclosure is energized. We decided on 8 cal/cm^2 as daily wear due to other tasks. Maybe I could have conveyed that a bit better in my previous post.
There are multiple fused strings of batteries in the system. Each string has a contactor to isolate it from the main bus


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 Post subject: Re: Battery work
PostPosted: Sat Jul 12, 2014 9:56 pm 
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Larry,

I have the same task on 25kW/25kWh Energy Storage System. The difference is 670V DC output instead of 1000V. The battery comes from Korea and no work will be performed but return the entire unit to the manufacture for warranty replacement. There are two fuses between the battery and the inverter; one is inside the battery enclosure and the other is on the control unit. Those Busman and SIBA fuses are supper fast to clear the Arc of 8kA less than an eighth of a cycle (16kA bolted fault). I use NFPA 70E - 2012 to calculate the DC Arcflash (maximum power delivery) and the AFBP turns out to be less than 3" at the connection point between the two fuses. Event with working distance 6" or 12" the Ei is so small; therefore, daily AF clothing should be fine. The only concern I have is the glove requirement since the technician's finger can be within an inch where the Ei near to 7 cal/cm2. Most of the information I have about the glove is relate to the voltage level not thermal rating.

By the way, can you explain why the balacava is required?

Thanks


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 Post subject: Re: Battery work
PostPosted: Sun Jul 13, 2014 9:20 am 
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"Inside the box" is usually the same as open air.

Balaclava is required at over 4 cal because the back of the head falls inside the arc flash boundary.

DC arcs are different from AC for a few reasons. First, no zero crossing so no self extinguishment. Second, lower voltage cutoff will increase to 100 volts in 2015 edition because effectively there is no fibrillation hazard...it becomes a pain threshold. Finally typically they are modelled as maximum power transfer, so Iarc = Ibf / 2. Not a lot of empirical testing so its all theoretical equations. There is a well known minimum arcing current and voltage though that has been well studied for over a century, unlike AC where there is little information predicting lower arcing limits.

My results with batteries have been similar. By the time you take internal series resistance into account, its nearly impossible to get above 1.2 cal/cm2 except with the 70E tables that use very unrealistic bolted fault currents compared to most industrial DC systems. I have a large, 15 MVA peak MG set. Generator maximum output is about 6 kA at 90 Volts. I still don't get to 1.2 cal but you'd be surprised looking at the aftermath of a flashover.


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 Post subject: Re: Battery work
PostPosted: Sun Jul 13, 2014 9:26 am 
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The apron requirement for batteries is meant for exposure to acids in lead acid cells. I'd be looking at task/equipment design before using an apron as shock protection. Linemen are about as exposed as you can get and they don't use one. A lineman would apply coverup (insulated covering of some sort) to the equipment first rather than trying to wrap his/her belly in rubber. Sleeves are required only for the awkward position of reaching over.


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 Post subject: Re: Battery work
PostPosted: Sun Jul 13, 2014 8:33 pm 
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Paul,

The AFBP is couple inches from the Arc tip in the battery case, that is why I wonder the use of Balaclava. As you mentioned no zero crossing, no self extinguishment in DC and the aftermath of the flash-over; I wonder if HRC-1 AR shirt suitable in this case?

I also tried to calculate Ei for the AC-secondary between the 100A breaker and the inverter as well as AC load. However, the 37 kVA single phase transformer is far away from the breaker which I believe it falls under the exception of IEEE where 240V and <125kVA transformer. I also looked into a combination mode where AC and DC active sources feed the fault but couldn't believe it will ever occur.
=============================


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 Post subject: Re: Battery work
PostPosted: Mon Jul 14, 2014 10:19 am 
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PaulEngr wrote:
The apron requirement for batteries is meant for exposure to acids in lead acid cells. I'd be looking at task/equipment design before using an apron as shock protection. Linemen are about as exposed as you can get and they don't use one. A lineman would apply coverup (insulated covering of some sort) to the equipment first rather than trying to wrap his/her belly in rubber. Sleeves are required only for the awkward position of reaching over.


In this instance there is no practical way to use a cover over the busbar. There are three tiers of batteries on each side, and there was some concern over the technicians leaning over the lower tiers to connect the batteries. But there is no belly-wrapping involved - Just an apron draped over the workers front like one uses when working with lead-acid batteries.


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 Post subject: Re: Battery work
PostPosted: Mon Jul 14, 2014 11:22 am 
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Larry Stutts wrote:
n this instance there is no practical way to use a cover over the busbar. There are three tiers of batteries on each side, and there was some concern over the technicians leaning over the lower tiers to connect the batteries. But there is no belly-wrapping involved - Just an apron draped over the workers front like one uses when working with lead-acid


I have seen a clear type plastic half tube used over the intercell connectors. This was bought from the battery manufacturer/supplier and sized accordingly.


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 Post subject: Re: Battery work
PostPosted: Mon Jul 14, 2014 11:31 am 
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wbd wrote:
Larry Stutts wrote:
n this instance there is no practical way to use a cover over the busbar. There are three tiers of batteries on each side, and there was some concern over the technicians leaning over the lower tiers to connect the batteries. But there is no belly-wrapping involved - Just an apron draped over the workers front like one uses when working with lead-acid


I have seen a clear type plastic half tube used over the intercell connectors. This was bought from the battery manufacturer/supplier and sized accordingly.


Yes, there is a cover that goes over the connection once they have the cells connected


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 Post subject: Re: Battery work
PostPosted: Mon Jul 14, 2014 1:40 pm 
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I have seen insulating roll material that is available as Class 00, Class 0 and Class 1 that comes in a roll and you cut what you need for your specific use. Perhaps something like this that can be cut and placed over the battery terminals would work. It is a Salisbury product, p/n RLB00 and I have seen it in the Grainger catalog, p/n 3KWU9


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 Post subject: Re: Battery work
PostPosted: Mon Jul 14, 2014 4:27 pm 
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Are you really exposed to 1000V? Close bus work? Grounded battery bank plus grounded racks?


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 Post subject: Re: Battery work
PostPosted: Mon Jul 14, 2014 8:00 pm 
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7604 wrote:
Paul,

The AFBP is couple inches from the Arc tip in the battery case, that is why I wonder the use of Balaclava. As you mentioned no zero crossing, no self extinguishment in DC and the aftermath of the flash-over; I wonder if HRC-1 AR shirt suitable in this case?

I also tried to calculate Ei for the AC-secondary between the 100A breaker and the inverter as well as AC load. However, the 37 kVA single phase transformer is far away from the breaker which I believe it falls under the exception of IEEE where 240V and <125kVA transformer. I also looked into a combination mode where AC and DC active sources feed the fault but couldn't believe it will ever occur.
=============================


1. There is some data publicly known which indicates that the 125 kVA" rule is too high. Its more like <=4.5 kA for 3 phase at 208 V, which works out to roughly 45-75 kVA. I have not seen any data for 120 V. There was a fatality logged by OSHA in Georgia in 2009 involving 240/120 with an unknown transformer size so it is at least possible. IEEE C2-2012 gives a rating of 4 cal for essentially any size system below 300 Volts. They don't list anything below 4 cal though. The result is supposed to be based on data from EPRI, PG&E, and one other unnamed source. So there is some room for argument about AC for whether it would be a 1.2 cal or 4 cal cutoff for 37 kVA, 120 volts.
2. It is almost impossible to get over 1.2 cal with a 100 A breaker even at 480 V because mccb's of that size typically trip in 1 cycle. You need really long feeders to make the arcing fault current very low.
3. Because ther is no zero crossing, there is less of a case for the "2 second" rule but since part of that rule is the idea that victims might be literally blown away or crawl away, most practitioners still use a 2 second cutoff. There is little substantiation for 2 seconds other than it is specified (weakly) in IEEE 1584.


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