Do RCDs actually work?

I ask because holding the probes of an ohmmeter one in each hand, and with wet palms, I couldn't get the reading down to 8 Kohms, the value which on 240V would pass 30 mA. 20K was the lowest I could manage by gripping tightly.
Yet I suspect one could get a very nasty, possible fatal, shock if 240V had been applied instead of the 15V. from an old AVO Mk 8.
How is the 30mA. trip current arrived at? Is it the current which will kill only 50% of the population? In N. America (admittedly a place where there used to be far more ambulance chasers) the usual limit is 5mA.
Are there any statistics showing deaths which occurred despite the presence of an RCD?
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Skin is not an ohmic conductor. It looks somewhat capacitive[1], so the impedance drops as the frequency goes up. It passes less current at DC than it does at 50Hz.
Likewise flesh - you can get a lot more current to flow at 500KHz...
Theo
[1] It's actually more complicated than that, there are various cell effects that kick in as the frequency rises, but the general trend can be thought as behaving a bit like a capacitor.
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On 24/07/14 13:54, Theo Markettos wrote:

Bollocks

its not capacitative. Its almost pure resistive.

so you say.
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On 24/07/2014 13:59, The Natural Philosopher wrote:

Funny how all those capacitance sensing touch screens still work ok though...
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I think you are assuming that resistance of the human body is passive.
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bert

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On Thursday, 24 July 2014 13:55:20 UTC+1, bert wrote:

Women on average have a higher resistance than men which is mostly down to physical size. Havign an intresting time trying to find real examples of people that have electocuted themseleves with low voltage batterise i.e <12V such as a PP3 placed on the tongue. Seems that they are just rumours.
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On Thursday, July 24, 2014 2:12:39 PM UTC+1, whisky-dave wrote:

I hope they are rumours, thats how I test PP3s for charge ! I did wonder if it would affect my taste buds, but I don't do it often these days. Simon.
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On 24/07/14 13:31, Windmill wrote:

I was told 30mA and 40mS were derived from "work" the german WWII scientists did by torturing and progressively electrocuting prisoners.
It's data that is distasteful in how it was arrived at but that you would never really be able to arrive at in any legitimate way.
Not sure if that story is true or not, but "mad scientist tortures people whilst keeping methodical notes" fits many of the WWII era german scientists to a tee.
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On Thursday, 24 July 2014 14:14:47 UTC+1, Tim Watts wrote:

http://rjqelectrical.co.uk/2013/11/rcd-trip-current/
"This RCD trip current level is set at 30mA for household RCD's. The reason why the RCD trip current in your house is 30 mA is because this is taken a s the compromise between the maximum level the human body can take without risking death and the nuisance "tripping" that would occur if it was set an y lower."

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On Thu, 24 Jul 2014 14:14:47 +0100, Tim Watts wrote:

Be careful if you choose to research in depth. Do it on an empty stomach : (
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Wouldn't surprise me. The tables for survival time in cold water are derived from work done that way.
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On 24/07/14 14:45, Huge wrote:

And other useful gems like "what happens if you pour boiling water in the ears of a dwarf?" Yes, really...
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On 24/07/2014 14:45, Huge wrote:

There was some work done like that by volunteer American servicemen which I recall being fairly grim reading.
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[citation needed] https://publications.ki.se/xmlui/bitstream/handle/10616/41328/%20Thesis_Ulrik_Birgersson.pdf?sequence=2 is mine.

So why did I build machines to put 25W into patients at 500KHz then if DC would do?
Theo
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/Theo Markettos

Snip

So why did I build machines to put 25W into patients at 500KHz then if DC would do?
Theo /q
Bless him it's not been the kindest of weeks gaffe-wise, must be the heat?
Jim K
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IIRC the 5mA RCDs in the US are built into the socket and protect appliances plugged into that, not a whole circuit. Better I suppose in terms of overall protection but I suspect rather pricey if all sockets are protected that way; and I'm not sure how the RCD protection would be fitted into a back box which takes our no doubt world-beating[1] BS1363 sockets.
[1] if only for size :)
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On 24/07/14 17:43, Robin wrote:

5mA vs 30mA is not going to make any difference most of the time. If you get between L and E you are likely to pull 30mA or more before either device trips (and get a full on belt while waiting). What's more important is the device breaks the circuit in < 40mS or so to limit the damage.
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http://www.select.org.uk/downloads/BEAMA%20Installation%20RCD%20Handbook%20V2%202010%20updated%20 (2).pdf
Does suggest 20mA is the danger point.
--
Adam


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On 24/07/2014 20:35, ARW wrote:

Worth keeping in mind that RCDs don't limit the shock current - only the duration. So the injury from a 50mA shock will be the same from a 10mA trip RCD as from a 30mA one.
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A 30mA RCD is allowed to take up to 200ms to trip with a 30mA imbalance. The 40ms time limit applies at 5 x 30mA.
--
Adam


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