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  #1  
Old 02-24-2012, 03:30 AM
lowcrawler lowcrawler is offline
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Default Staying alive - electrical safety.

Though there are tons of topics about safety in a crash, safety for first responders, fuses, etc... there are actually strangely few threads about safety while building an EV.

So what things does one need to do (or not do) in order to not to kill themselves?

I was reminded of the power hidden in our batteries when my bus bar turned 90 degrees while tightening a bold and a shower of sparks and molten copper shot everywhere. I went back to my standard "cover everythign with electical tape" idea after that... I'd clearly become way too comfortable.

Things like:
Can you touch a positive as long as you are also not touching another battery terminal? Do I ever need to worry about touching ONE point in the high voltage loop? What can and can't you touch if you'd like to stay alive? Are the positive and negative terminals just as 'deadly' when it comes to human touch? Etc///
Most 'centralized' BMS wires, as far as I can tell, carry pack voltage -- how do you keep the dozens of wires that all need to be on terminals 2mm from each other from touching? If they did, what's the real risk?
We shoudl 'wrap' our wrenches and tools in electical tape -- but does that really stop 160V+ or is it just for our own 'feel good'? Is standard "automotive" wire acceptable for low amperage, pack-voltage connections?
Etc etc etc...

It just seems this 'basic' safety info is sparse on this forum... might be nice to toss it all in one place.
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  #2  
Old 02-24-2012, 03:50 AM
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Default Re: Staying alive - electrical safety.

The last really big pack I worked on. That is 3p51s of 200Ah cells.


All the spanners and other tools were insulated.
Plywood boards were placed over the pack so that only the area we worked on was exposed.
The pack was tested with many fuse links to split the pack into manageable sections.
As the cells were linked in groups of three parallel cells to get 600Ah we needed to make sure the bus bars were installed correctly. This was always done with two people, one fitting and one checking, to make sure the bars went across the right terminals.

When it was installed in the car the same process was used and each row of cells ended with a fuse link which was only put in afterwards. That meant that during installation there was only the voltage of one row.
Before connection of any cells two of us separately checked all the cells were correctly orientated relative to each other and only when we were both satisfied did we get spanners. Then it was two other people to make sure each connection was still right.
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Old 02-24-2012, 08:39 AM
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Default Re: Staying alive - electrical safety.

I heatshrinked all my tools after my first such incident (just this month, after nearly 2 years of working/driving).

Yes, you can touch a single point in the pack, not two. Don't touch one unless you need to, and when connecting things be sure to touch metal to metal first.

Try to size busbars or other connectors so they can't swing and hit another...btw, how did yours turn? busbars are usually connected to two points.

Put insulation on anything where a short could occur, such as a metal strap latch that rests between batteries.

Use mechanical disconnects, and use them whenever you're working around the pack.

Keep the pack covered, in a box or at least by a tarp or similar to keep things clean and safe.
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Old 02-24-2012, 09:59 AM
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Default Re: Staying alive - electrical safety.

Quote:
Originally Posted by lowcrawler View Post
...
Can you touch a positive as long as you are also not touching another battery terminal? Do I ever need to worry about touching ONE point in the high voltage loop?
At a high enough voltage just touching one terminal can shock you - if not electrocute (that is, kill) you - because it can force enough of a current flow through the atmosphere, your clothes, shoes, etc., for you to feel it. In general, we don't work with voltages in an EV high enough to do more than barely tingle if you were to touch one terminal (and polarity, by the way, is irrelevant).

That said, you still don't want to touch *any* terminal which is at traction pack potential just in case there is already a path to "ground" (ie - the vehicle chassis) somewhere else. The two most pervasive cause of such current leaks are dust from the brushes and condensation/spilled electrolyte on top of the batteries.

Use insulated tools and, where possible, try to follow the old electrician's rule of keeping one hand in your pocket when working on even supposedly dead traction pack circuits.

Quote:
Originally Posted by lowcrawler View Post
...Most 'centralized' BMS wires, as far as I can tell, carry pack voltage -- how do you keep the dozens of wires that all need to be on terminals 2mm from each other from touching? If they did, what's the real risk?
Assuming the wires have an insulation rating appropriate for the total pack voltage, they can all touch each other.

A centralized BMS that only monitors cell voltage should use - in my *opinion* - teflon insulated wire, which can be purchased for a reasonable price as surplus. If the BMS also actively balances the cells with a shunt then you should add a fuse to the cell-terminal end (said fuse needs to be capable of interrupting the full pack voltage, btw, which can be a tall order for a low-amperage fuse combined with a high pack voltage.

Quote:
Originally Posted by lowcrawler View Post
We shoudl 'wrap' our wrenches and tools in electical tape -- but does that really stop 160V+ or is it just for our own 'feel good'? Is standard "automotive" wire acceptable for low amperage, pack-voltage connections?
Any tape that says it is for electrical applications must actually pass some dielectric strength and flammability tests so, yes, it will stop 160V+ (a typical rating for vinyl electrical tape is 600V per single layer).

Standard "automotive" wire is generally *not* acceptable for carrying pack voltage. Anything rated for "AC wiring" use is going to have much tougher insulation, such as THHN, THWN, MTW, etc. Alternatively, the teflon insulated wire above is my number one choice.
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Last edited by Tesseract; 02-24-2012 at 11:25 AM. Reason: removed duplicate link
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Old 02-24-2012, 12:07 PM
lowcrawler lowcrawler is offline
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Default Re: Staying alive - electrical safety.

Good ideas, all. Any others?

Quote:
Originally Posted by Ziggythewiz View Post
Try to size busbars or other connectors so they can't swing and hit another...btw, how did yours turn? busbars are usually connected to two points.
I put the (first) busbar spanning to the next cell , started screwing in the bolt on cell and the bus bar must have caught as I was turning. It turned with the screw 90 degrees before I realized "hey, that's not a good idea" and it started a little spark show. Given that's just the potential from ONE cell, and I've got 48 it really drove home how much deadly power we are messing around with here.

I've since gone through about 3 rolls of electrical tape.


Quote:
Originally Posted by Ziggythewiz View Post
Yes, you can touch a single point in the pack, not two. Don't touch one unless you need to, and when connecting things be sure to touch metal to metal first.
Quote:
Originally Posted by Tesseract View Post
At a high enough voltage just touching one terminal can shock you - if not electrocute (that is, kill) you - because it can force enough of a current flow through the atmosphere, your clothes, shoes, etc., for you to feel it. In general, we don't work with voltages in an EV high enough to do more than barely tingle if you were to touch one terminal (and polarity, by the way, is irrelevant).

That said, you still don't want to touch *any* terminal which is at traction pack potential just in case there is already a path to "ground" (ie - the vehicle chassis) somewhere else.
So that brings up the question that kind of was the impetus for this question. We aren't supposed to touch the terminals or anything at traction pack voltages (for me, 150V)... yet we do, in fact, need to assemle these things. How are you connecting the last few batteries if you aren't touching them? Ya know?
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Old 02-24-2012, 12:17 PM
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Default Re: Staying alive - electrical safety.

Quote:
Originally Posted by lowcrawler View Post
...So that brings up the question that kind of was the impetus for this question. We aren't supposed to touch the terminals or anything at traction pack voltages (for me, 150V)... yet we do, in fact, need to assemle these things. How are you connecting the last few batteries if you aren't touching them? Ya know?
By wearing gloves?

Seriously. These gloves are good for working on the pack as they are thin enough to allow for good dexterity yet the rubber coating is a pretty dependable insulator.
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Old 02-24-2012, 01:12 PM
lowcrawler lowcrawler is offline
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Default Re: Staying alive - electrical safety.

Interesting. I've been doing research on this for a year and I've never heard of people advocating the use of electically-insulating gloves before.

Nice.

Are they really required, or are you probably safe at 150V using the 1-hand trick?
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Old 02-24-2012, 01:24 PM
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Default Re: Staying alive - electrical safety.

Nothing is required. Just recommended. I don't have any gloves, but don't get shocked too frequently. Also, don't know how anyone does anything one-handed. Almost all my shocks were one-handed.
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Old 02-24-2012, 01:29 PM
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Default Re: Staying alive - electrical safety.

Quote:
Originally Posted by lowcrawler View Post
...Are they really required, or are you probably safe at 150V using the 1-hand trick?
Depends. Seb shocks himself all the time working on the battery pack in the Porsche 911 because those new Helwig brushes seem to be throwing off a huge amount of carbon dust. He neither wears gloves nor puts one hand in his pocket and suffers the consequences. Maybe one day he'll depolarize his atrioventricular node and that will be the end of tinkering on that infernal Headway pack for Seb.
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Old 02-24-2012, 04:16 PM
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Default Re: Staying alive - electrical safety.

Quote:
Originally Posted by Tesseract View Post
By wearing gloves?

Seriously. These gloves are good for working on the pack as they are thin enough to allow for good dexterity yet the rubber coating is a pretty dependable insulator.
I use very similar gloves, mostly to keep my hands clean, but also good for working on the pack.

Funny thing though, I work on live 240V ac bare hands and have never had a shock.
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