Safe and unsafe voltages

[brucechase]

Bruce how does the insulation get damaged?

An insulated tool has insulation along the entire shaft of the tool with only the working part exposed. The insulation is there as a barrier from the conductive part of the tool and the person working on the "energized" system.

During the normal course of working, tools slip, get beat up, etc. Any damage to any part of that insulation puts the user at risk. These tools are part of a protective system in which the integrity of the insulation must be maintained. These tools should not be used when just working on a locked out de-energized system due to the risk of damage to the tool.

 

[robpaige]

Okay, so you're just talking normal wear and tear. Thanks, I wanted to make sure I wasn't missing something else.

 

[Johnster]

IIRC the Navy rule was 30v and under was treated differently than 31+v (AC or DC)

I think you could justify a number as "low" as 30v becuase once you get above 30v, you have more than 2 12vdc batteries in series.

Keeping that threshold at 30v also prevents you from running afoul of existing trucks and equipment that work at 24v. I know that US Military trucks have 24v systems (charge at 28vdc...). If you wrote your code to apply to systems less than 30v, you would 'grandfather' in a lot of existing equipment that wasn't so tightly controlled before.

For Q1 - I agree with Brucechase - save the insulated tools for places where Zero energy verification has not been performed.

-John

 

[Lancie1]

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,

Dan,
This is like trying to find one shoe that will fit everyone - not an easy task. The best you can do is train to make sure the workers know how much power each battery can generate. Really it is the Power (volts x amperes) that kills. One without the other is not very dangerous. Power inside the heart muscle is generated by current through the resistance of the body (skin, organs). The higher the voltage, the more POTENTIAL to generate higher currents through the heart muscle. Someone with open bleeding wounds is more likely to have his heart stopped by electric current that someone with with dry intact skin. Therefore it is nearly impossible to come up with ONE voltage level that is safe. It all depends on the circumstances. You might come up with a typical set of conditions for the equipment and tasks that are specific for your project. That is exactly what the NFPA standards encourage for selecting PPE.

Low Voltage, High resistance = low current (not very dangerous)
Higher Voltage, High resistance = (not very dangerous)
Voltage above 50, High resistance = (could be dangerous)

Low Voltage, Low resistance = medium current (could be dangerous)
Higher Voltage, Low resistance = (dangerous)
Voltage above 50, Low resistance = (very dangerous)

The "Resistance" in the above scenarios is the combination of skin and organ resistance AND any PPE (gloves, hoods, other clothing).

 

[leitmotif]

Dan,
This is like trying to find one shoe that will fit everyone - not an easy task. The best you can do is train to make sure the workers know how much power each battery can generate.

That is exactly what the NFPA standards encourage for selecting PPE.

Lancie
You hit the nail on the head. I am looking for the magic bullet one size fits all.
DREAM I had to try.
REALITY Each vehicle will have a different setup battery / cell wise with differing potentials across differing layouts.
I will have to do what I have done for the last 20 years and evaluate each situation as they come up. Thankfully I have been pretty good at this (or lucky maybe?) and had no accidents where I had safety oversight.

A dropped wrench across a single cell can get right exiting - I was a submarine electrician for 4 years. Never saw it never tried it and still do not intend to try it. Isulated tools are a flat out requirement - damn the cost. I will also require rubber mats to cover adjacent cells to reduce the potential.

I have looked at the OSHA stuff but will have to get a copy of the state req's and make sure they are the same.

I am deliberating between 30 and 50 VDC or VAC (RMS) for hot gloves.
We are getting a copy of NFPA 70 E to review. Hopefully that will help.

Thanks to all of you gentlemen who have commented.
I think we can put this thread to rest.

Dan Bentler

 

[Clay B.]

Lancie

A dropped wrench across a single cell can get right exiting - I was a submarine electrician for 4 years. Never saw it never tried it and still do not intend to try it.

Dan Bentler

Know you want to put this thread to bed but the wrench thing made me think of something we used to do when I was just an electrician. We used to wrap copper wire (usually 12 AWG) around the shanks of our screw drivers and short it out across the battery posts to make the shank magnatized. Wire got hot real fast so you had to be quick. I am willing to bet your rules will probably advise against this method for making a magnet.

 

[Tom Jenkins]

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....

;-))

Yeah - I did a double take on that one too when I put the slide togetehr. Tom A. had it right - I guess it's the difference between an open casket and a closed one.

Frequency does make a difference - I've been told tha 60 Hz is just about optimum for causing fatalities - something to do with the heart beat.

And to reinforce the current kills scenario, it is my understanding that when we rub our feet on a carpet and discharge to someone's ear we can be working with a couple hundred thousand volts - but not much current.

 

[Lancie1]

And to reinforce the current kills scenario, it is my understanding that when we rub our feet on a carpet and discharge to someone's ear we can be working with a couple hundred thousand volts - but not much current.

Yes indeed, the skin on the finger and the ear make pretty good high-resistance protection. But if you have 50 volts and 75 milliamperes applied directly to the heart muscle, it will go into fibrillation (uncontrolled contractions).

 

[leitmotif]

Know you want to put this thread to bed but the wrench thing made me think of something we used to do when I was just an electrician. We used to wrap copper wire (usually 12 AWG) around the shanks of our screw drivers and short it out across the battery posts to make the shank magnatized. Wire got hot real fast so you had to be quick. I am willing to bet your rules will probably advise against this method for making a magnet.

I will not encourage this kind of thing for sure. Am kinda wondering about using it as a demonstration though.
I firmly believe in demonstrating "dumb stunts" to show the hazard and consequences UNDER PROPER CONTROLLED CONDITIONS. No live subjects of course.

We do hands on as much as we can. I intend to take one or two 12 V batteries and 1/8 weld rod and show how much power you have in a battery. I also intend to demonstrate fuses using aluminum foil.

OK let us resurrect this post and give me more ideas on what to demonstrate. Available power is 460 3 phase 200 amp wye which feed xfmr supplied 200 amp 208 wye.

Am also looking for higher voltage DC equipment (250 or 500)of almost any kind. Motors, disconnects, starters, relays etc. We are starting up and poor as church mice of course. Donations would be accepted quickly to say the least.
We can pay the freight but let us try to keep size down to 50 HP or less to keep freight bill down.

To close on a whine and cry I have a hard time being Chief Pack Rat and Lead Scrounge Artist on a limited budget.
Boss Lady is real careful on money and will not let us go in the red. She will not spend future dollars on present costs.
I really hate it when she is right - bless her heart.

Dan Bentler

 

[Plcwill]

I don't think anybody cover this, since you guys covered everything else. But, these 3 points.

I maintain UPS batteries for Uncle Sam (US Gov) for a super computer building which has 12 battery strings that have 194 cells @2.8 volts each, which comes to 543.2VDC.

1) Some of the PPE that we use is Plastic gloves to protect us from battery acid, Face Shield and Goggles to protect the face from battery acid if a battery happen to explode.

2) We don't work by yourselves, we work in pairs when we do battery maintenance.

3) When it comes the cleaning or replacing any of the links between the cells we try to work 20 cells groups at a time and in these groups, we will have the links taken apart for cleaning or replacement. This way we are not working with high voltages and we also open the disconnect the from the UPS when we are changing links or changing the batteries. The disconnect will also protect us from the UPS connecting to its load and the charging voltage of the UPS.

Remember that batteries have no off switch to turn off and the are floating grounds. Make sure not to touch the battery and earth ground it will get you.

 

[leitmotif]

Bill by links do you mean the bus bars between cells or separately mounted links that let you isolate a group of cells?

Been thinking on breaking battery into sub portions to keep "exposed voltage to minimum.

Battery buildup will start at the pos and neg connections on disconnect and work their way to mid point from each end. Until the final bus piece goes in it is all open circuit.

On submarine battery we never disconnected bus work exept the one time to isolate a cell. Replaced bus with jumper cables. Our chief let the cable slip and dang near shorted a portion of the battery.

Battery grounds - you are one of the few that say anything about them. Submarine battery all DC and AC distribution were ungrounded. We will do this also on traction battery on vehicles.
We had a ground detector with one "probe ?" for the battery itself. Minimal acceptible ground resistance was 50 K.

Two man rule will be easy for us - we have students and one and if needed two instructors for safety in hands on classes.

Thanks for insight

Dan Bentler

 

[Plcwill]

Dan,

What I mean by battery links are short bus bars that go between the posts (terminals) and what I mean by battery is a 2 cells and each cell has links going between them.

The maintenance that we don't use very little PPE is when filling the cells with petrified water.

I forgot to state this in my last post. We also wear a oversized rubber like bib to cover the clothes from getting battery acid on them.

I hope this answers your questions.

 

[nic00]

I will not encourage this kind of thing for sure. Am kinda wondering about using it as a demonstration though.
I firmly believe in demonstrating "dumb stunts" to show the hazard and consequences UNDER PROPER CONTROLLED CONDITIONS. No live subjects of course.

We do hands on as much as we can. I intend to take one or two 12 V batteries and 1/8 weld rod and show how much power you have in a battery. I also intend to demonstrate fuses using aluminum foil.

OK let us resurrect this post and give me more ideas on what to demonstrate. Available power is 460 3 phase 200 amp wye which feed xfmr supplied 200 amp 208 wye.

Am also looking for higher voltage DC equipment (250 or 500)of almost any kind. Motors, disconnects, starters, relays etc. We are starting up and poor as church mice of course. Donations would be accepted quickly to say the least.
We can pay the freight but let us try to keep size down to 50 HP or less to keep freight bill down.

To close on a whine and cry I have a hard time being Chief Pack Rat and Lead Scrounge Artist on a limited budget.
Boss Lady is real careful on money and will not let us go in the red. She will not spend future dollars on present costs.
I really hate it when she is right - bless her heart.

Dan Bentler

How about a 1000kW Mercury arc rectifier

 

[leitmotif]

QUOTE The maintenance that we don't use very little PPE is when filling the cells with petrified water.UNQUOTE

We used distilled deionized water for battery and reactor. Have heard of heavy and light water of course. Not petrified. Live and learn something every day.

Links to us on submarine were just that used on supply side of DC breakers for maintenance had special wrench to open / close em.

Dan Bentler

 

[leitmotif]

How about a 1000kW Mercury arc rectifier

Lessee now
1. Mite oversized
2. Shipping weight and cost?
3. All the green guys around here are screaming about mercury in coal and we should not be shipping coal to China thru Washington ports because of it. I can see my shop with Greenpeace and all them because I have uhhh how many gallons of mercury? I'm gonna kill everyone in the Puget Sound Corridor.

Send photo please?? Would come in handy on rectifier class. Lot lighter too I bet.

Dan Bentler