What is the impedance of the human body and why does it vary?

[Nathi ORea]

TL;DR Summary

What is the electrical resistance across the human body

I was researching what the total electrical resistance of the human body was and got some quite far ranging values. It seems to range from 1, OOO to 1OO,OOO ohms. Some quite elaborate ways of testing it too.

Then I thought I'd just get the old multimeter out. I have two and both measured about the same. Depending on the part of the body, it was about 2 Mohm to about 15 Mohm. I tried my dog too, but it kept reading over limit.

This seems like I would be able to grab mains power quite easy and not even feel it? I = 24O/2,OOO,OOO = .12 mA?

Is this right? What am I missing?

 

[Guineafowl]

The value varies because there are so many factors involved.

The main component is the resistance of the dead skin layer, stratum corneum (visible skin). Thickness will make a difference, as will sweat/dampness. Try wetting your skin with salt water, then measuring.

If the test voltage is high enough, then the stratum corneum can be breached, at which point your resistance will drop dramatically - 1500 ohm is the figure I have, hand to hand. This can happen during a shock, where resistive heating burns away the stratum.

Your meter uses about 6V dc to measure resistance, but a mains shock involves much higher voltages, and ac. As well as the effects above, you can pass current capacitively, charging and discharging on each half of the mains cycle.

So your effective resistance (impedance) depends not only on your own factors, but also on the characteristics of the voltage source.

Research, and play with your meter, by all means, but don’t go touching the mains.

 

[Nathi ORea]

Thanks so much for that. So you’re saying that resistance lowers under high ac? I think now I remember seeing a YouTube vid on this I’ll have to try and find. Something about the effects of ac vs dc on the body.

you don’t have to worry about me playing with mains;). I barely trust myself with an Arduino.

Thanks again

 

[Guineafowl]

This is a good introduction:
https://en.m.wikipedia.org/wiki/Electrical_injury

 

[berkeman]

Summary:: What is the electrical resistance across the human body

Then I thought I'd just get the old multimeter out. I have two and both measured about the same. Depending on the part of the body, it was about 2 Mohm to about 15 Mohm. I tried my dog too, but it kept reading over limit.

I'm pretty sure you won't be able to use your dog as a multimeter...

 

[Nathi ORea]

I'm pretty sure you won't be able to use your dog as a multimeter...

I was told each mm of pupil dilation was a kohm .

You just need to give your dog a teaspoon of brine and have the lights medium bright.

 

[Babadag]

For above 1 kV substation IEEE 80 considered an average body resistance but BS EN 50522 states a relation between touch voltage and body impedance:
IEEE 80/2013 IEEE Guide for Safety in AC Substation Grounding

7.1 Resistance of the human body
For dc and 50 Hz or 60 Hz ac currents, the human body can be approximated by a resistance. The current path typically considered is from one hand to both feet, or from one foot to the other one. The internal resistance of the body is approximately 300 Ω, whereas values of body resistance including skin range from 500 Ω to 3000 Ω, as suggested in Daziel [B27], Geddes and Baker [B75], Gieiges [B76], Kiselev [B97], and Osypka [B121]. The human body resistance is decreased by damage or puncture of the skin at the point of contact.

A value of 1000 Ω in Equation (10), which represents the resistance of a human body from hand-to-feet and also from hand-to-hand, or from one foot to the other foot, will be used throughout this guide.

BS EN 50522/2012 Earthing of power installations exceeding 1 kV a.c.

 

[DaveE]

The question of what is the impedance of the human body can get pretty complex (get it? LOL). It has been investigated for safety as well as medical diagnostic reasons. For example, here is a short paper (chosen mostly at random). There's lots of other material on the web.