OK, lots of interesting theories here, I have been teaching this for 20 years to 10's of thousands of electricians all across North America, can't believe anyone here didn't attend one of my classes.
There is an IEEE standard, (Std 80-2000) that has all of the real research compiled, Iwire had some nice links where that info was based off IEEE 80.
The resistance of the human body can vary dramatically depending on several factors, sex, weight, diet, etc. But really those don;t make much of a difference. Essentially it is the skin, along with such factors as area of contact, tightness of contact, dryness or wetness of the skin, and cuts, abrasions, or blisters that introduce the variables.
Excluding the skin, human body resistance (Internally) is about 250 Ohms per arm or leg, and 100-500 Ohms for the torso. Unless the skin is punctured, the skin will provide additional resistance. The worst-case scenario is considered to be 500 Ohms hand-to-hand. Some typical values of skin resistance are:
Condition of contact Resistance (Ohms)
Dry Wet
Finger touch 40 k-1 M 4-15 k
Hand holding wire 15-50 k 3-6 k
Finger-thumb grasp 10-30 k 2-5 k
Hand holding pliers 5-10 k 1-3 k
Hand around pipe (or drill) 1-3 k 0.5-1.5 k
Palm touch 3-8 k 1-2 k
Two hands around pipe 0.5-1.5 k 250-750
Hand immersed -------- 200-500
Foot immersed -------- 100-300
Using these typical values, a person can estimate their approximate body resistance. Let?s say a person grabs a wire in a 480/277V panel that they assumed was deenergized while touching the panel door with the other hand. The worker would have about:
250 ohms for the arm +
250 Ohms for the other arm +
32,500 Ohms for the skin
A total resistance of about 33,000 Ohms
277V/33,000 Ohms = 8.4 mA (Mild shock)
Now lets look at the same scenario, but this time with wet or sweaty skin.
250 ohms for the arm +
250 Ohms for the other arm +
4500 Ohms for the skin
A total resistance of about 5000 Ohms
277V/5500 Ohms = 55.4 mA (Respiratory Paralysis, possible fibrillation, may be fatal)
What a difference huh? We can't control when we sweat, or what our skin resistance will be at any given time. Wearing rubber soled shoes (OSHA requirement for all electrical workers) and rubber gloves can make all these variables for skin resistance irrevelent.
Materials Resistance (Ohms)
Rubber gloves or soles More than 20 M
Dry concrete above grade 1-5 M
Dry concrete on grade 0.2-1 M
Leather sole, dry 100-500 k
Leather sole, damp 5-20k
Wet concrete on grade 1-5 k
Now to answer the OP's question. Yes, OSHA used some values to derive the 50V working rule. They assumed a worse case human body resistance of 500 Ohms and the 100mA fibrilation threshhold, 500 Ohms times 0.1A = 50V.
