Do RCDs actually work?

[Windmill]

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?

[Theo Markettos]

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.

[bert]

In message , Windmill writes

I think you are assuming that resistance of the human body is passive.

[The Natural Philosopher]

Bollocks

its not capacitative. Its almost pure resistive.

so you say.

[whisky-dave]

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

[Tim Watts]

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.

[Jethro_uk]

Be careful if you choose to research in depth. Do it on an empty stomach : (

[Huge]

Wouldn't surprise me. The tables for survival time in cold water are derived from work done that way.

[whisky-dave]

"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."

[Tim Watts]

And other useful gems like "what happens if you pour boiling water in the ears of a dwarf?" Yes, really...

[Theo Markettos]

[citation needed] is mine.

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

Theo

[Robin]

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 :)

[John Rumm]

The flaw in this test is that the meter will only be driving a small DC voltage across the probes. You would find should you repeat the test with an insulation resistance test meter rather than an ohm meter, you will get significantly different (and more painful) results.

Indeed.

Have a look at:

I have not seen any - and would not expect to see any for this country in cases where the presence of the RCD is relevant. The reasoning being that deaths are very rare in the first place, so looking for a much smaller sub set of the originally very small set is going to be difficult.

Getting stats for those suffering no or reduced injury is also likely to be hard to isolate, but more doable. You may be able to extract something from the projected reduction in injuries if there were a greater uptake of properties fitted with RCDs at the CU.

[John Rumm]

Funny how all those capacitance sensing touch screens still work ok though...

[Tim Watts]

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.

[Dave Plowman (News)]

You can get a single socket with RCD which fits a standard two gang (deep) box.

[spuorgelgoog]

Twin socket in a twin gang box

Single socket in a single gang box

Of course quite a lot of American 'receptacles' aren't earthed anyway, even if they're 3-pin.

Owain

[harryagain]

Most of the resistance of the human body is in the skin because it is oily. If you wash your hands in detergent, you will see a difference.

10ma through the average heart is considered sufficient to cause fibulation (stopping the heart) However if you get an electric shock, the assumption is that only part of the current goes through the heart (if any) The current follows the blood vessels. Blood is a good conductor. 220V is only the RMS voltage, (a sort of average) the peak is 320V.

The RCD works by measuring the difference between incoming and outgoing currents with a current transformer. In virtually all circumstances, power is cut off before a lethal current can flow to earth (or through you). Getting a shock while immersed in water is one exception.

Few people are killed by electric shock these days, usually they get the shock and fall off a ladder or something.

[harryagain]

Years ago, the lowest voltage anyone was killed by was 40V. Pretty exceptional though.

[newshound]

There was some work done like that by volunteer American servicemen which I recall being fairly grim reading.