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Research says YES ( It is only expected to save 95% of the population). According to B & L (I can`t spell  Biegelmeier and Lee)

Humm, I've been pondering that. Isn't the 'only necessarily good for 95% of the population' bit come from the tables of body resistances?


When we do the background calculations for ADS we start with the line voltage (230V say), work out from that what the touch voltage is likely to be (e.g. half that for TN, all for TT) from that (using an assumed body resistance figure - typically something around 1000 Ohms) work out how much current could flow though a victim (115mA say) and then look that up on the electric shock graph (the one with AC-1, AC-2, AC-3 etc regions on it) which then gives us a maximum disconnection time if we're to keep out of the AC-4 (possible death) area - say 0.4s.


The problem with that approach is that if the victim happens to have a lower body resistance than our assumption, they'll suffer a larger shock current, so the same disconnection time might be quite inadequate for them.


So basically it's the calculation converting voltage to current that's flawed.


But when using 30mA RCDs for additional protection, we're already responding to current - not voltage - there's no (well not much*) assumption about body resistance - so I'm not sure the '5% gap' we have with ADS does exist when using 30mA RCDs - if it opens within 40ms (for higher currents) then is everyone safe regardless of their body resistance?


* OK, there's still a bit of assumption - in that body resistance will limit the shock current below about 500mA - otherwise we're into the AC-4 region regardless of how fast we can disconnect, but for now I'll assume no-one has a body resistance that low.


(notwithstanding, of course, that any RCD is only going to protect 93% if the population if it's going to fail to trip 7% of the time when required)


   - Andy.

Research says YES ( It is only expected to save 95% of the population). According to B & L (I can`t spell  Biegelmeier and Lee)

Humm, I've been pondering that. Isn't the 'only necessarily good for 95% of the population' bit come from the tables of body resistances?


When we do the background calculations for ADS we start with the line voltage (230V say), work out from that what the touch voltage is likely to be (e.g. half that for TN, all for TT) from that (using an assumed body resistance figure - typically something around 1000 Ohms) work out how much current could flow though a victim (115mA say) and then look that up on the electric shock graph (the one with AC-1, AC-2, AC-3 etc regions on it) which then gives us a maximum disconnection time if we're to keep out of the AC-4 (possible death) area - say 0.4s.


The problem with that approach is that if the victim happens to have a lower body resistance than our assumption, they'll suffer a larger shock current, so the same disconnection time might be quite inadequate for them.


So basically it's the calculation converting voltage to current that's flawed.


But when using 30mA RCDs for additional protection, we're already responding to current - not voltage - there's no (well not much*) assumption about body resistance - so I'm not sure the '5% gap' we have with ADS does exist when using 30mA RCDs - if it opens within 40ms (for higher currents) then is everyone safe regardless of their body resistance?


* OK, there's still a bit of assumption - in that body resistance will limit the shock current below about 500mA - otherwise we're into the AC-4 region regardless of how fast we can disconnect, but for now I'll assume no-one has a body resistance that low.


(notwithstanding, of course, that any RCD is only going to protect 93% if the population if it's going to fail to trip 7% of the time when required)


   - Andy.