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#147296 - 07/02/02 06:20 PM Unusually high skin resistance?  
AlphaOperator  Offline
Member
Joined: Jul 2002
Posts: 20
Erie, PA, USA
Hey there. I am a student interested in electrical safety. I have read in books that dry skin has (typically) 1 million+ ohms of electrical resistance in it, but wet skin has dangerously low resistance, less then 1,000 ohms.

I decided to try and use a digital multimete to check my own skin's resistance. I tried several routes (hand-hand, hand-foot) and found the following:

MINIMUM Conductivity:
LHand-RHand Dry: 400,000 ohms
LHand-RHand Wet: 40,000 ohms

These were the "lowest average" levels the voltmeter gave me when I pinched the leads as hard as I possibly could. The 400,000 dry looks right, but I was surprised at the 40,000 for when my hand was wet! I have repeated the test a few times, getting similar results. I also checked the multimeter with some of my dad's "standards" (big blocks for testing) and the multimeter proved to read very accurately from it's lowest level (100 ohms) to 1M Ohms.

So, is it possible for someone to have this high electrical resistance? I've been told that some people do have high resistance- I've heard a story about an electrician back when they used screw-in fuses who would stick his finger in the socket and "feel" if it was live or not. I'm not planning on going out and grabbing power cables, of course, however I could (NOTE: NOT SAFE!) handle live 120 when dry with minimal shock hazard.


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#147297 - 07/02/02 08:01 PM Re: Unusually high skin resistance?  
ElectricAL  Offline
Member
Joined: Oct 2001
Posts: 597
Minneapolis, MN USA
Here's a suggestion. Take two conductive metal plates about 4" x 12" and lay them on a sturdy table about 20" apart. Set the VoltOhmMeter on the table and connect one lead from the VOM to each plate. Do some physical exercise and get a good sweat going. Lay your forearms on the plates, the long way, and put some weight on them. I'd suggest setting the scale at 10K Ohms to start.


Al Hildenbrand

#147298 - 07/02/02 10:37 PM Re: Unusually high skin resistance?  
AlphaOperator  Offline
Member
Joined: Jul 2002
Posts: 20
Erie, PA, USA
Ah, I see where you are going on this one- a larger contact area. This will probably produce a far lower resistance reading. The probes worked for me because, in a sence, they "simulated" contact with something like a bare wire or socket plug- about the same size and area of contact. The thing you described would probably simulate contact with an entire energized surface. I'm going to have to try this! (With the multimeter, of course.)


#147299 - 07/03/02 02:53 PM Re: Unusually high skin resistance?  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Hello Alpha & welcome!

I think it's very hard to accurately determine anything like an average resistance of the human body.

So much depends upon the contact area, the individual's skin type, whether wet or dry, and so on that the resistance value can vary tremendously. Using normal meter probes grasped between thumb and first finger of each hand I can usually go from about 2 meg when dry and lightly gripped down to about 50K when damp and tightly held.

Increasing the contact area to something like that described by Al would certainly make the resistance much lower.

Most of the resistance is in the outer skin of the body, and direct contact with an open wound will lower the resistance considerably.

Some studies I've seen also suggest that the application of higher voltages results in a breakdown of the skin's resistance, and once that happens current flow increases dramatically. The couple of volts from an ohmmeter doesn't break down the barrier. In the extreme, readings from the electric chair indicate that with firmly attached electrodes the head to ankle resistance can sometimes be under 200 ohms once the high voltage has broken down the skin resistance.


#147300 - 07/03/02 03:28 PM Re: Unusually high skin resistance?  
ElectricAL  Offline
Member
Joined: Oct 2001
Posts: 597
Minneapolis, MN USA
Here's the kicker.

A. Albert Biss, P.E., in his 2/28/92 CPSC Technical Report on GFCI states that the average adult male cannot let go when current reaches 16 mA. Currents above 18 mA, when flowing cross-body, cause the chest muscles to contract resulting in suffocation.

Applying Ohm's Law: 120 V รท 0.016 A = 7500 Ohms . . . that's 7.5 K.


Al Hildenbrand

#147301 - 07/18/02 08:02 PM Re: Unusually high skin resistance?  
Anonymous
Unregistered

Al,
I presume that the sweat would offer better conductivity because of the salt content, wouldn't it? This would be closer to a real life scenario since I would imagine, like myself, most electricians sweat like a pig.
Brian



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