JimHartman
New member
What code number or NFPA number is it that addresses the minimum voltage a person can work on that is not a qualified electrician. Have a concern with persons working on 240 volt 3 phase and above. Please let me know.
minimun voltage for person
- Thread starter JimHartman
- Start date
- Status
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: minimun voltage for person
NFPA70E says 50V and above requires a worker to be qualified.
Of course it is possible to be qualified for some activites (and voltages) and not others. Your company's Electrical Safe Work Practices would clarify these situations and so should be based on NFPA70E as a minimum.
NFPA70E says 50V and above requires a worker to be qualified.
Of course it is possible to be qualified for some activites (and voltages) and not others. Your company's Electrical Safe Work Practices would clarify these situations and so should be based on NFPA70E as a minimum.
bphgravity
Senior Member
- Location
- Florida
Re: minimun voltage for person
Regardless, voltages as low as 30V RMS can be deadly.
Regardless, voltages as low as 30V RMS can be deadly.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: minimun voltage for person
30 volts divided by 1000 Ohms (typical resistance of the average human body) equals 30 miliamps. Enough to reach the maximum let go threshold. After reaching that, the body will probably sweat and therefore decreaseits resistance. With the resistance decreased, the current will increae into the venticular fibrilation area.
30 volts divided by 1000 Ohms (typical resistance of the average human body) equals 30 miliamps. Enough to reach the maximum let go threshold. After reaching that, the body will probably sweat and therefore decreaseits resistance. With the resistance decreased, the current will increae into the venticular fibrilation area.
Re: minimun voltage for person
Ryan - I have to disagree with your comment re 1kohm being "typical" body resistance. There are way to any variables involved. Everything from temperature to body fat to where the measurement is taken. Obviously the resistance between two fingers is going to be significantly different than the resistance from hand to foot.
Ryan - I have to disagree with your comment re 1kohm being "typical" body resistance. There are way to any variables involved. Everything from temperature to body fat to where the measurement is taken. Obviously the resistance between two fingers is going to be significantly different than the resistance from hand to foot.
pierre
Senior Member
- Location
- Westchester County, New York
Re: minimun voltage for person
Ryan did not make up the 1000 ohm value. It is a standard value that has been determined through studies. You are correct in saying there are many variables, but a number has to be used when generalizing, and 1000 is that number.
Ryan did not make up the 1000 ohm value. It is a standard value that has been determined through studies. You are correct in saying there are many variables, but a number has to be used when generalizing, and 1000 is that number.
Re: minimun voltage for person
Copied from the OSHA web site - Wet conditions are common during low-voltage electrocutions. Under dry conditions, human skin is very resistant. Wet skin dramatically drops the body's resistance.
Dry Conditions: Current = Volts/Ohms = 120/100,000 = 1mA
a barely perceptible level of current
Wet conditions: Current = Volts/Ohms = 120/1,000 = 120mA
sufficient current to cause ventricular fibrillation
I have a manual on electrical safety. It cites human body resistance varies from as little as 100 ohms to as much as 500Kohm depending on circumstances. My point was that to state that any body resistance as "typical" is not only misleading it is factually wrong.
Copied from the OSHA web site - Wet conditions are common during low-voltage electrocutions. Under dry conditions, human skin is very resistant. Wet skin dramatically drops the body's resistance.
Dry Conditions: Current = Volts/Ohms = 120/100,000 = 1mA
a barely perceptible level of current
Wet conditions: Current = Volts/Ohms = 120/1,000 = 120mA
sufficient current to cause ventricular fibrillation
I have a manual on electrical safety. It cites human body resistance varies from as little as 100 ohms to as much as 500Kohm depending on circumstances. My point was that to state that any body resistance as "typical" is not only misleading it is factually wrong.
pierre
Senior Member
- Location
- Westchester County, New York
Re: minimun voltage for person
I always thought retired people were more relaxed
I do not know of anyone here who is trying to be factually incorrect. With 25,000 plus potential posters, it usually does not go unnoticed
I always thought retired people were more relaxed
I do not know of anyone here who is trying to be factually incorrect. With 25,000 plus potential posters, it usually does not go unnoticed
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: minimun voltage for person
Bryan's point was factually correct:
Bryan's point was factually correct:
Indeed, yes it can, as Ryan correctly pointed out. That does not mean that it will, every time. But do you want to go grabbing a 30 volt source, just to find out how your body resistance is doing today?
physis
Senior Member
Re: minimun voltage for person
We've argued this before. I think you'd need to cut into the tissue and insert the test probes within a few inches of each other to acheive 1k resistance.
I'm not to sure that the entire body being saturated in saline would get down to 1k.
Worst case data isn't exactly likely to occur.
Then again, I'm not actually familiar with any of these studies, I could be wrong.
We've argued this before. I think you'd need to cut into the tissue and insert the test probes within a few inches of each other to acheive 1k resistance.
I'm not to sure that the entire body being saturated in saline would get down to 1k.
Worst case data isn't exactly likely to occur.
Then again, I'm not actually familiar with any of these studies, I could be wrong.
Re: minimun voltage for person
Pierre - I am so retired and so relaxed...The doctor is astonished how I can survive with a BP of 6 over 5 (factually incorrect its a little higher ). However, when it comes to safety IMO there is little room for opinion or arbitrary standards. This is especially true in the case of electrical safety where so much is known (unfortunately at the expense of many lives) about the dangers.
FWIW I once taught basic electricity in the military. Part of that instruction included electrical safety. We deliberately emphasised the wide variations in body resistance. The point we were trying to drill into the student was - just because you got away with it yesterday doesn't mean it won't kill you today.
Pierre - I am so retired and so relaxed...The doctor is astonished how I can survive with a BP of 6 over 5 (factually incorrect its a little higher ). However, when it comes to safety IMO there is little room for opinion or arbitrary standards. This is especially true in the case of electrical safety where so much is known (unfortunately at the expense of many lives) about the dangers.
FWIW I once taught basic electricity in the military. Part of that instruction included electrical safety. We deliberately emphasised the wide variations in body resistance. The point we were trying to drill into the student was - just because you got away with it yesterday doesn't mean it won't kill you today.
Re: minimun voltage for person
Physis - I read somewhere that w soaked person standing on concrete in bare feet could have as little body resistance as 100 ohms. However, under "normal" circumstances I'll go with the info below from MIT ...I think they're probably smart enough to get it right.
Copied from the MIT EHS Program
Factors Invovled in Electrical Shock
THE QUANTITY OF CURRENT FLOWING THROUGH THE BODY
Current (amperes) is the killing factor in electrical shock, not the voltage. The voltage only determines how much current will flow through a given body resistance. In general, the body's resistance to electrical shock is minimal (150,000 to 600,000 Ohms.) Even contact with standard 110-volt circuits can be lethal under certain conditions. Refer to the chart below.
Physis - I read somewhere that w soaked person standing on concrete in bare feet could have as little body resistance as 100 ohms. However, under "normal" circumstances I'll go with the info below from MIT ...I think they're probably smart enough to get it right.
Copied from the MIT EHS Program
Factors Invovled in Electrical Shock
THE QUANTITY OF CURRENT FLOWING THROUGH THE BODY
Current (amperes) is the killing factor in electrical shock, not the voltage. The voltage only determines how much current will flow through a given body resistance. In general, the body's resistance to electrical shock is minimal (150,000 to 600,000 Ohms.) Even contact with standard 110-volt circuits can be lethal under certain conditions. Refer to the chart below.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: minimun voltage for person
The point is to impart a healthy respect for the unhealthy side-effects of touching electrifried stuff. The difference between a group of 1000 people who believe it is safe to touch a 12 volt car battery (hey, I've done it before ? it stung a bit but I was fine) and a second group of 1000 people who believe it is not safe to touch a 12 volt car battery is that some member of the first group is going to get killed by touching a car battery. See Ryan's comment about not being able to let go and the body resistance getting smaller as the sweat and strain take their toll.
The purpose of using "worst case" data in a training environment is to have fewer people in the first group and more people in the second group.
True, but irrelevant.
The point is to impart a healthy respect for the unhealthy side-effects of touching electrifried stuff. The difference between a group of 1000 people who believe it is safe to touch a 12 volt car battery (hey, I've done it before ? it stung a bit but I was fine) and a second group of 1000 people who believe it is not safe to touch a 12 volt car battery is that some member of the first group is going to get killed by touching a car battery. See Ryan's comment about not being able to let go and the body resistance getting smaller as the sweat and strain take their toll.
The purpose of using "worst case" data in a training environment is to have fewer people in the first group and more people in the second group.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: minimun voltage for person
You are right, of course. But I for one am going to continue using them, when I do any electrical training presentations. I prefer my audience to be dramatically affected.
macmikeman
Senior Member
- Location
- Planet macmikeman
Re: minimun voltage for person
Over 600 volts and I am not qualified. And I plan to stay that way.
Over 600 volts and I am not qualified. And I plan to stay that way.
physis
Senior Member
Re: minimun voltage for person
I wont disagree with you Charlie.
But the real clinical numbers seem pretty hard to come by.
I feel like it's some kind of marshmallow world we have to live in when the reality of a thing is compromised for something else.
Edit: Error A
[ December 01, 2005, 08:11 PM: Message edited by: physis ]
I wont disagree with you Charlie.
But the real clinical numbers seem pretty hard to come by.
I feel like it's some kind of marshmallow world we have to live in when the reality of a thing is compromised for something else.
Edit: Error A
[ December 01, 2005, 08:11 PM: Message edited by: physis ]
B
bthielen
Guest
Re: minimun voltage for person
I don't know how one could generalize the danger. All I know for sure is that a 12v car battery hurts when you provide the right conditions. I was installing a new tractor battery and tightening the post clamps when I learned that it is best to install the (-) cable last. My batteries are located under the floor in the cab and I was standing outside reaching in. It was a hot day, I was wearing no shirt, I was sweaty, and when I contacted the (+) cable I provided a current path from the batter post through my arm to the tractor cab frame via my armpit. The shock caused my arm to react quite suddenly. Luckily I didn't bang my elbow on the cab otherwise I think I could have been hurt seriously.
Bob
I don't know how one could generalize the danger. All I know for sure is that a 12v car battery hurts when you provide the right conditions. I was installing a new tractor battery and tightening the post clamps when I learned that it is best to install the (-) cable last. My batteries are located under the floor in the cab and I was standing outside reaching in. It was a hot day, I was wearing no shirt, I was sweaty, and when I contacted the (+) cable I provided a current path from the batter post through my arm to the tractor cab frame via my armpit. The shock caused my arm to react quite suddenly. Luckily I didn't bang my elbow on the cab otherwise I think I could have been hurt seriously.
Bob
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: minimun voltage for person
Someday, Sam, you are going to have to explain that "Error A, Error B" stuff.
- Status