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[Pete Erskine]

On page seven there's a picture of skeleton dude.  He appears to be wearing rubber soled shoes and not standing in a puddle.  He also appears to be touching his stereo knob with only one hand.  How exactly is he being "shocked"?

Even in that situation if the chassis has 110vac you would feel it-probably not die but definitely get shocked.

[John Roberts {JR}]

On page seven there's a picture of skeleton dude.  He appears to be wearing rubber soled shoes and not standing in a puddle.  He also appears to be touching his stereo knob with only one hand.  How exactly is he being "shocked"?

We all express impedance to the environment around us, and the human body is like a lumped capacitor/resistor that can build up a charge and pass current. In fact the body isn't passive since it generate voltage internally for sundry uses.

For a voltage in the range of 120VAC, with non-conductive shoes the shock hazard from a one hand touch is minimal. There will be a tiny current as that AC voltage drives our internal lumped capacitance, but that current is probably too low to be felt. I have used this small current with a sensitive VOM to identify hot wire from neutral in house wiring.    Literally holding the ground lead of the VOM with one hand, and touch the wires with red lead. DO NOT MEASURE CURRENT this way, especially if not wearing rubber soled shoes. 

Higher voltages may be felt. A high enough DC voltage like static can draw a spark, the larger source of injury is from conduction though our internal resistance, so in hand out foot, or in one hand out the other.

JR   

[John Halliburton]

We all express impedance to the environment around us, and the human body is like a lumped capacitor/resistor that can build up a charge and pass current. In fact the body isn't passive since it generate voltage internally for sundry uses.

For a voltage in the range of 120VAC, with non-conductive shoes the shock hazard from a one hand touch is minimal. There will be a tiny current as that AC voltage drives our internal lumped capacitance, but that current is probably too low to be felt. I have used this small current with a sensitive VOM to identify hot wire from neutral in house wiring.    Literally holding the ground lead of the VOM with one hand, and touch the wires with red lead. DO NOT MEASURE CURRENT this way, especially if not wearing rubber soled shoes. 

Higher voltages may be felt. A high enough DC voltage like static can draw a spark, the larger source of injury is from conduction though our internal resistance, so in hand out foot, or in one hand out the other.

JR   

As I recall from my 'lectrical days, a dry human body has about 10mohm resistance.  Since this is rarely the case, it goes downhill from there.  It does not take much current at all to cause problems, typically 5-6ma can produce locked muscles.  It is worse if the current travels across your chest, hence the old journeyman lesson of keeping one hand in your pocket when you reach into an open electrical panel.  Around 100ma is the point where you're heart can be disrupted violently, and death is almost certain.

Best regards,

John

[John Roberts {JR}]

As I recall from my 'lectrical days, a dry human body has about 10mohm resistance.

Not to be overly pedantic 10mohm is 10 milli-ohm or .01 ohms ( a very low resistance), I suspect you meant Mohms (Meg-ohms or millions of ohms). Then there are MHOS units of conductance equal to 1/ohms.

Skin resistance varies hugely with moisture content/presence. I have a bathroom scale that presumes to measure body fat by measuring resistance between two foot pads. The results are not very reliable and vary widely when I am sweaty right after a run, or walking around barefoot with dry skin.

A stupid old electrician trick is to touch hot electrical wires with two fingers of the same hand to gauge the voltage. Do not do this at home. Dry callused fingers are higher resistance than our soft and maybe sweaty fingers. People do get killed by 120V while not generally across two fingers of the same hand. 

Since this is rarely the case, it goes downhill from there.  It does not take much current at all to cause problems, typically 5-6ma can produce locked muscles.  It is worse if the current travels across your chest, hence the old journeyman lesson of keeping one hand in your pocket when you reach into an open electrical panel.  Around 100ma is the point where you're heart can be disrupted violently, and death is almost certain.

Best regards,

John

It is difficult to come up with simple criteria for human risk from shocks because contact resistance varies hugely with things as subtle as how tight you grip a wire, sweaty hands or not. Standing in a pool of water is never a good idea around electricity.   

It takes very little current to interrupt the heart signaling but getting that current to flow  directly through the heart muscles is not always the shortest or direct path so don't read too much into "how much current it takes".  The medical community has come up with numbers for this and I recall one exercise where we calculated a capacitor size that could be put in series with a guitar ground lead to protect the players heart from stoppage while still providing enough of a ground impedance at 60 Hz to shield against hum. Of course it will probably still hum while he touches hot 120VAC. 

JR

[duane massey]

He will probably hum because the lyrics fell from his music stand....

[John Halliburton]

Not to be overly pedantic 10mohm is 10 milli-ohm or .01 ohms ( a very low resistance), I suspect you meant Mohms (Meg-ohms or millions of ohms). Then there are MHOS units of conductance equal to 1/ohms.

JR,

Yes, that was a typo on my part, I meant mega.

And I agree, there are a lot of variables in the physical realm when touching live wires, and it is always best to kill the circuit in question when working with any service, residential on up to industrial.



John

[Geoff Doane]

Not to be overly pedantic 10mohm is 10 milli-ohm or .01 ohms ( a very low resistance), I suspect you meant Mohms (Meg-ohms or millions of ohms). Then there are MHOS units of conductance equal to 1/ohms.

Since we're being pedantic, isn't the unit of conductance "mho", or "ohm" spelled backwards?

GTD

[John Roberts {JR}]

Since we're being pedantic, isn't the unit of conductance "mho", or "ohm" spelled backwards?

GTD

Actually its properly called a "Siemens"...   Not sure how spelling Ohm backwards signifies the reciprocal, but it does make it easier to remember that it equals 1/Ohms. 

I wonder if there a set of Mho's laws that applies to conductance. 

JR

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