do you think the 17th edition has gone too far with RCDs

[Gees]

There's also the factor to consider that by far the largest portion of the body's resistance is in the epidermis. Higher voltages tend to break down the resistance of the outer skin layers, and the resistance of the tissues beneath can be surprisingly low, which is why medical apparatus has to be manufactured to keep leakage currents to an absolute minimum.

Obviously this is an extreme example, but the table about a third of the way down this page shows just how low the internal resistance of the body can be:

http://www.dc.state.fl.us/oth/deathrow/drorder.html

However, this brings up several points about the way the RCD has been promoted to the general public, with its ability to protect being grossly exaggerated. I find a lot of people seem to have been left with the idea that if you have an RCD in use there's no way you could sustain a fatal, or even serious, electric shock, which is nonsense.

A sustained current of less than 30mA through vital areas of the body could quite easily prove fatal to some people, and of course if you get yourself across L & N without there being 30mA flowing to earth simultaneously, the RCD will do nothing to protect you directly.



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Thanks for the interesting link !!

Thats why they shave heads and use salty sponges, I understand that but with an ititial body resistance of lets say 50k ( how low can you get your resistance down ) would there be enough current flow to damage the skin? The answer is almost certainly yes but HOW does the sequence get started? With the chair its the application of thousands of volts. much like climbing a pylon and getting hold of a cable, but with domestic voltages it must take quite a while to get up to 30 ma!!

 

[NoHounds]

i voted yes. i think the 16th edition got it about right.

however i do agree that they should have made it compulsory for showers for showers
and it was a bad idea to permit sockets to go in bathrooms .

 

[timthesparky]

I voted yes. Thought it was a bit overkill.

I would prefer to fit RCBO's if they were cheaper! Know somone from Hager who said it only costs them £3 to make an RCBO! They could sell them a lot cheaper.

 

[martinxxxxxx]

I would prefer to fit RCBO's if they were cheaper! Know somone from Hager who said it only costs them £3 to make an RCBO! They could sell them a lot cheaper.

I agree, RCBO would be the way to go at that price or even at half their current price.

 

[JohnD]

All together now:

RCBOs are great

 

[ban-all-sheds]

Arc fault detectors on the way is my guess.....

 

[Paul_C]

with an ititial body resistance of lets say 50k ( how low can you get your resistance down ) would there be enough current flow to damage the skin? The answer is almost certainly yes but HOW does the sequence get started? With the chair its the application of thousands of volts. much like climbing a pylon and getting hold of a cable, but with domestic voltages it must take quite a while to get up to 30 ma!!

I'm not sure exactly how the physiology of it works, but you can get an idea of how it's the outer skin which is responsible for most of the resistance by grasping the probes of your ohmmeter tightly in each hand and noting the reading, then transferring them both to the same hand and grasping them as near as possible the same way and noting that the reading isn't significantly lower.

however i do agree that they should have made it compulsory for showers and it was a bad idea to permit sockets to go in bathrooms .

This is something I've always found rather bizarre about the IEE's long-standing stance on bathrooms. We mustn't allow a socket (transformer-isolated shaver outlets excepted) in a bathroom even with RCD protection "just in case" somebody does something silly with it, but it's perfectly acceptable to put a box which has 240V behind a flimsy little plastic cover actually inside the shower cubicle?

 

[bernardgreen]

The resistance of the human body varies depending on the method used to measure it.

The area of the contact pads, the pressure applied, the dampness of the skin and the test voltage applied will significantly change the value measured.

Using small probes of a multi-meter with a 9 volt battery will give a very different measurement of resistance to the result from a defibrillators measurement. Defib measurements of the pad to pad resistance will give values between 20 to 200 ohms while a multi-meter probes presses against the skin where the pads would be placed are unlikely to show less than one Meg Ohm

The applied voltage from a defib to STOP the heart from fibrillating has a maximum of 1800 volts with a current of up to 65 amps between the pads.

See pages 18 and 84 of this

http://www.healthcare.philips.com/p...ocuments/resuscitation/frxtechref_4363037.pdf

for more details about pulse duration and energy delivered to the body.

( **caution it is a large 6.5 Meg PDF file )

For some related information about water and electric shock risks look at the turkey test report pages 104 105 and 123

 

[Paul_C]

That's some interesting reading. I'd heard the approximate ranges of total energy (joules) delivered by defibrillators before, but didn't realize that the current pulses were that high, and consequently of that short a duration.

 

[CAWORK]

Good Morning and belated happy new year to everyone!!

Been away from the site for a while. But I am quite glad this topic has come up.

I voted yes, but I am not totally against it. I consider making RCD protection for all sockets and bathroom circuits compulsory a very sensible idea but the 50mm RCD rule is what I think goes too far. I also think allowing sockets in bathrooms is a daft idea as someone else has pointed out.

I looked on the IEE site a while back at a topic asking if the 17th goes too far and about 60% (so over half) thought so.

Regards