Safe and unsafe voltages

I am starting a new thread since I feel this is too large a tangent from another thread entitled "UL safe voltage"
http://www.plctalk.net/qanda/showthread.php?p=447296&posted=1#post447296

BACKGROUD I am in a group that is starting out training the future electric driven land or sea mobile apparatus - anything between garden tractor or railroad locomotive or ship - whether by land or sea if it is propelled with an electric motor that is our area.

One of the hats I wear is Safety. As you may remember I have a MS and 20 years field experience in that and as many years in electrical. So I get to write the electrical safety class notes and I also get to write the safety manual.

BACKGROUND
The DC voltage source is batteries - you CANNOT turn them off - thus always hot work. We will be working anywhere between one cell ie 3 or 4 volt in case of lithium up to 325 or 350 VDC for a complete battery. Yes we will have to teach students how to safely make all connections starting with cell one and ending up at cell 120 or so. The high voltage traction battery is not bonded to vehicle frame and is therefore floated ie not "grounded" (using common vernacular).

CHALLENGE
find one number that sets the "die or live" point for DC voltage.
Or maybe more accurately at what point do I say
below XX it is OK to work bare handed,
over XX work with hot gloves,


I think the reality is
there is NO single magic number and
if push comes to shove I may end up setting history by finding it and putting to an end all arguments on this matter.

In a way that will be disappointing since there will no longer be a "start point" for some really neat to watch dog fights among safety guys over this issue.

In YOUR minds
1. should I always require properly insulated tools. I may have my answer but I want to hear yours.
2. What do you think my magic number is?

Thanks for the insight

Dan Bentler

Good question. I personally have no clue. I always remeber 30 VDC as the limit but with what your talking about wouldn't the potential be the real kicker.

Myself, I would say any system where the total voltage is below 30 is safe to work without gloves. Above 30 gloves required.

It ain't the volts its the amperage.

I worked with cathodic protection once, 12 volts and 200 amps. With DC and batteries it's easy to get very high amperage.

Dan, I've developed many training classes for electrical safety (including at 2 VPP sites). I've based most of my training on the OSHA guidelines (1910.331 - 1910.335) along with several classes that I've been to.

OSHA does not mandate any type of PPE or guarding at voltages below 50 Volts. I have always taught that anything below 50 volts (1910.333.a.1), and your body has enough resistance to prevent the lethal currents.

Using a 1000 ohm body resistance (generally accepted as that of soft, wet skin), a 50 volt potential difference generates 5 mA of current. That is enough to be felt, but not enough to send your heart into ventricular fibrillation (accepted at 75 mA).

Yes, current is what will kill you, but you need a potential difference across your body (ie heart) for that current to flow.

My answers:

1. No, insulated tools should be required for opening cabinets and anything that is "Live" above 50 volts. Once the system is de-energized, Locked out, and tested for the absense of voltage, then the use of insualted tools is not only unnecessary, but opens up the potential for damage to the insulation thus rendering the tools dangerous.

2. 50.

you might try somthing like nfpa 70e table 130.7 and article 320.9

At least in the case of a car battery, it isn't so much the current that is flowing through you that is the problem. If you accidently short the battery out with something conductive the amperage through that item can liquify metal in a heartbeat, especially aluminum or a small gauge wire. So it can be an explosion/burn/splatter hazard.

DamianInRochester said:

At least in the case of a car battery, it isn't so much the current that is flowing through you that is the problem. If you accidently short the battery out with something conductive the amperage through that item can liquify metal in a heartbeat, especially aluminum or a small gauge wire. So it can be an explosion/burn/splatter hazard.

Click to expand...

Or a wrist watch with a metal band. I saw a guy at Auto Zone do this once while changing a battery. He got the watch between the frame and the positive post.The band got so hot it burned his skin.

I have pretty much decided that all battery work will be done with properly insulated tools.

One of my demos will be arc welding using a single 12 V battery (maybe two to keep the arc voltage in the correct range). To demonstrate fuses intend to us aluminum foil (cheap) and simulate a fuse shorted across either 12 or 24 volt DC.
Both demos properly shielded and all that for emitted light, UV etc and metal splatter.

Our lithium cells will have a capacity around 80 to 120 (plus?) ampere hours.

Dan Bentler

See W. L. Mostia's post half way down this very long thread:

http://www.control.com/thread/1026220674#1316391390

for URL's to various sites. Topic is slightly different, danger of AC vs DC, but likely to shed some light on the topic.

Of course it's the amperage that kills, but voltage can increase the amperage for a given set of conditions. The resistance of the skin varies tremendously, so there isn't any "safe" voltage. How the current passes through the body matters too - a path through the heart is worse than a path through a leg only. I know a guy that took 4160 and lived, but others have died at 120. I use the attached in my classes.

I believe that UL-508 classifies some batteries and power supplies as a self limiting current source, but I suspect that in your case the limit is well over the 100 mA threshold for a fatal current.

Tom, not wanting to make light of what is undoubtedly a serious subject but it tickled me that your diagram shows "Death" between 0.1 and 0.2 Amps. But if you were to ramp that current up to 1A then you would get away with severe burns....

;-))

off subject a bit but I once heard(but never confrimed) that frequency had somthing to do with it too. and 60hz was more dangerous than 50hz due to the human hart rate. this sounds like a good one for myth busters. LOL
a tid bit:
http://en.wikipedia.org/wiki/Electric_shock

uptown47 said:

Tom, not wanting to make light of what is undoubtedly a serious subject but it tickled me that your diagram shows "Death" between 0.1 and 0.2 Amps. But if you were to ramp that current up to 1A then you would get away with severe burns....

;-))

Click to expand...

That chart is part of the "old school" of current damage to the body. Anything above the "Death" part is a secondary reaction. Besides being dead, you would have severe burns. OR - the "Death" section assumes that there is a perfect, conductive path across your heart. The seemingly low current can and does kill. I've known people who have had severe shocks, resulting in burns and other damage. I guarantee it's more current, but it didn't pass through their heart.

tomalbright said:

That chart is part of the "old school" of current damage to the body....

Click to expand...

Yep, totally agree with you. Just looked funny is all.

brucechase said:

the use of insulated tools is not only unnecessary, but opens up the potential for damage to the insulation thus rendering the tools dangerous.

Click to expand...

Bruce how does the insulation get damaged?