Caravan Park

Yep ref cascading like-minded RCDs.

Given the (theoretically possible) 7% RCD failure rate and the theoretical 95% successfull life saving ability of RCDs the I prefer the (ideally but not reality very theoretical) improved rate two cascaded RCDs might achieve - especially in differing locations (possibly differing "Stiction" risks) . Might make us feel a bit safer.

RCDs are wonderfull things. How did we do without them? Well we did, just a fusewire between us and eternity but we survived it.

The problem is that many folks now take more risks thereby eroding the safety enhancements.


I do prefer to err towards RCDs cascaded  sometimes though.

Cascading the supply to each caravan does have merits, I.e. an RCD at  the source of each and an RCD in each caravan.

But a 30mA RCD covering the whole site is just adding to risk of injury as folk stumble around in the dark to reset it.

an installation without them used to be deemed safe, adding an RCD improves the safety but adding another cannot be making much difference and is starting to add other risks.

Harry, the point I was making that the 7% failure rate combined with the 95% success rate suggests possible worse case of nearly 12% (11.65%) mortality. Cascading might in theory help reduce this figure a little bit and if you end up being the one relying on it you might appreciate the nuisance trips possibiliity.


Of course this calculation is made on a purely arithmetical basis without other regards factored in

I am trailing around houses where the electrical installations have not been touched for years and if there is a RCD it certainly has not been tested regularly.


The initial RCD failure rate is probably in excess of 50%, though many respond to a kiss of life taking the complete failure rate down.


Andy B.

Just looking back through my notes at inspection on a park last month.


49 static caravan hook up points, 9 RCD's changed due to various reasons, no test button trip, no trip, etc.


Touring field, 22 hook up points, no unit over three years old, 6 RCD's changed again various faults.


10.8% failure rate.


As you say luckily we have a 100mA S type three phase at origin.

RCD's in each hookup point, RCD's in static vans and touring.


George.

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

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.

Yes A.ndy, the whole lot is based on assumptions cross relating figures gained etc etc and not wholly based on experimental data because there would no doubt be a bit of an outcry. The figures might be skewed high and probably the 5% is more of a theorectical max risk Actually I understand that the Nazis experimented on Jews etc during world war 2 and proved it was possible to kill with 10 or even 5mA, perhaps less in the right (wrong) circumstances. Whether such terribly gained info was actually takem into account the who knows?. 5% should be a figure we should be aware of even if it`s an absolute max it still shows RCDs are not totally risk free,  they are risk reducers and even those fully in spec probably do not remove all risk and we should be thinking "Well punk, do you feel lucky?" as Clint would say. Sobering thoughts!

What evidence is there of Nazi experimentation? If there is any doubt, (shaved) pigs would probably be a good model for the effects of electricity upon humans.


Is there any evidence that the mortality rate has been reduced by the fitment of RCDs? (It may be so small that it is difficult to measure any benefit reliably.)


And let's not forget that a belt across a finger tip which has been inserted into an empty lamp holder is not at all the same as grasping a line conductor whilst standing bare-footed on an earthy surface.