Rented House 5 year Electrical Check Tomorrow

[JohnW2]

going back a bit, RCDs can trip a little bit faster or slower depending where in the AC cycle contact occurs. Current will be zero momentarily, then increase as the cycle climbs (or falls) towards the next peak. So it may be faster than 30ms

Indeed - although there is a minimum trip time imposed by such factors as the maximum speed at which the mechanical tripping mechanism can operate .

I saw this in some tech doc or other, with graphs and timings. It included survivability charts, varying by duration and current, which I don't remember. 30ma for 30ms is mostly survivable. Increase one and unless you decrease the other, survivability falls.

Yep, that's broadly true - but what is your point?

As you will realise, the point I've been making is that a current 'just under 100mA' may, and a current just under 50 mA must, be allowed to flow indefinitely by a 100 mA RCD, whereas a 30 mA RCD will, as it says on the tin, curtail a current >30 mA (and maybe currents as low as ~15 mA) in just a small number of tens of milliseconds (whether 30ms, 40ms or whatever).

So, if body resistance etc. is such that the current through the victim's body is between 30 and 100 mA, a 30 mA RCD must terminate that current very quickly, whereas a 100 mA RCD may not terminate it ever.

Kind Regards, John

 

[JohnD]

but surely a 100mA RCD is not provided to provide protection against personal injury? It will operate in some fault conditions which do not include a person putting their finger on a busbar.

Some of the higher ratings are intended to detect cable damage, or to operate in case of fire.

 

[JohnW2]

but surely a 100mA RCD is not provided to provide protection against personal injury? It will operate in some fault conditions which do not include a person putting their finger on a busbar. Some of the higher ratings are intended to detect cable damage, or to operate in case of fire.

Indeed - and that's surely the whole point of this discussion ...

... in situations in which BS7671 deems it necessary for there to be 'additional protection' in the form of a 30mA, that is at least partially (seemingly, in some people's minds, primarily) to provide direct protection of people from the consequences of electric shocks (rather than clear the fault before they have a chance to get a shock in the first place - which is another value of RCDs). As you've just written yourself, the 30mA IΔn requirement derives from the fact that (as you wrote) "30ma for 30ms is mostly survivable". Were those numbers different, BS7671 would probably asking for something different from a 30 mA RCD for this 'additional protection'.

Kind Regards, John