Wet Location Body Resistance

mapj1:

If you go back far enough in old regs, the maximum accessible touch voltage that is normally 50V, used to be set to 25 in damp or conductive locations, suggesting an assumed halving of the body resistance.


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.

Thanks :) 




Any idea how this translates into a disconnection time? 


Table 41.1 seems to be based on a 0.8 multipler?

Legh Richardson:

You can usually half the body resistance, which, in effect corrolates to half the shock voltage when the body is wet, as Mike has stated. These safety measures are usually achieved by supplementary bonding and/or RCDs, reg 415.


RCDs seem to be the preferred method of protection today. See this link below.

https://www.beama.org.uk/resourceLibrary/the-rcd-handbook---guide-to-the-selection-and-application-of-residual-current-devices.html

Legh

Right, though if supplemental bonding is not present, the CPC & ADS should be the first line of defense followed by RCD. RCDs are more likely to fail than a CPC.

Table 41.1 seems to be based on a 0.8 multipler?

Any idea how this translates into a disconnection time?

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.

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:
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.   

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?

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.