Wet Location Body Resistance

mapj1:

Some background reading . I suggest not to be read just before bedtime or if you are about to eat and are of a nervous nature.
here Intro to what happens in shock from NIH   


 Model of skin as an RC network.    And test cells of 1cm square sections of post mortem skin tested and getting ~ 10k ohms per sq cm of contact area with salt solution as the electrodes on the 2  sides.

the 1956 work by Dalziel "Effects of electrical shock on man"    Tries to define a time/current curve for electrocution,

Another one by the same author with pictures of folk grimacing holding electrodes.  looks "fun" but I reckon you could not do these tests nowadays.

This one includes figures for conductivity of various internal organs and RC models for things like minced lung. Not sure how relevant that is but the conductivity is of order 100-500 ohm-cm for common organs. So the internal path once the skin is broken through is lower resistance  - the skin is the main resistance.

Grimnes S. Dielectric breakdown of human skin in vivo. Med Biol Eng Comp.  is also worth a read, but no copies seem to have escaped onto the internet, sadly.


edit
work on using pulsed electric current to push drugs through skin to order -Electroporation of mammalian skin:

Mike

Thanks


Reading through I'm getting about 500 ohms wet, 1500 ohms worse case dry.


 

AJJewsbury:

Table 41.1 seems to be based on a 0.8 multipler?

How do you mean?

Any idea how this translates into a disconnection time?

BS 7671, for various wet locations, seems generally to keep 41.1 disconnection times for ADS but add other measures (such as 30mA RCDs or supplementary bonding) - rather than stipulating a shorter ADS disconnection time per se.


  - Andy.

I saw that, and to be honest I don't agree with it from an ethics or code intent perspective. RCD failures are rather common in comparison, and outdoors supplementary bonding is not required.

mapj1:
I think that you meant that the resistance goes up when charring starts. ..

Not really, once the skin has burnt away, the tissue below the surface that is exposed and  is wetter and a better conductor. 

Hm! If the surface is charred, the layer below is already congealed - think hard-boiled egg.


Any road, that's enough conjecture. Do we have my experimental evidence?

mapj1:
Any idea how this translates into a disconnection time? 


As a first bash realise that the disconnections times quoted for  230V TN systems assume a touch voltage of 110-130V, and for TT systems a shock voltage equal to the full mains voltage is assumed. And if you then look at the figures for say 400V and higher then that is comparable to a system at half the voltage in a 'wet' condition.

Mike

From what I'm seeing 100 volts is assumed  vs 115 or the max of 132 volts (110% of 240). Which just by itself is making me question what value to assumed for wet locations.   

ProMbrooke:

What body resistance is assumed in wet locations vs dry locations? I have a feeling Table 41.1 does not take wet locations into account.

What is the reason for the need to find out this information please?


Z.

mapj1:

Of course in reality getting a person wet only alters their surface resistance, the moisture levels of internal organs is not significantly affected. Therefore the effect of being coated in sweat or sea water is more akin to a dry contact over a larger contact area, but as the entry and exit wounds indicate, most of the heat, and so most of the resistance, is where the current path breaks the epidermis.

Once charring starts the resistance drops sharply, by providing cooling and improving the contact, water may reduce this surface burning effect at the penalty of a higher initial current. None of this is good.

Mike, I think that you meant that the resistance goes up when charring starts. However, by that stage, I don't think that it matters anymore. If we are to be really gruesome, in a wet location, e.g. swimming pool, the effect will be more like stewing than barbecuing. Unfortunately, the lethal current is likely to have stopped the heart long before this occurs.

Table 41.1 seems to be based on a 0.8 multipler?

Any idea how this translates into a disconnection time?