I'm sure they could, but the reality is not like that - since they will not reset in the absence of an adequate earth (maybe, as flameport suggests, any earth at all), which has nothing to do with the 'test button electronics'.
Kind Regards, John
Is a 15mA socket rcd available?
- Thread starter dave_104
- Start date
Kind Regards, John
Sponsored Links
Well you could ask a manufacturer to make you a xx mA RCD but it would cost. Manufacturers make as range of standard ratings. Commonly 10,30,100,300 500 the a choice of non delayed or delayed. Not all of this common range is always available or easily available. 30 or below is considered personal protection. you should achieve discrimination if your mA rating are at least a factor of 3 and also if the upstream one is time delayed. Therefore the usual set up is a 30mA RCD fed by a 100mA delayed RCD
I'm not so sure that that rule-of-thumb necessarily works with RCDs, since, to the best of my knowledge, there is no restriction as regards how rapidly a (fairly high IΔn) RCD may reach 'the point of no return' (in terms of tripping), particularly in the face of a suddenly - appearing large residual current.
Indeed, but in that case it's the time delay, rather than the difference in IΔns, that provides the discrimination.
Kind Regards, John
Bit of a combination of both as far as the rationale behind it I think John. Obviously an RCD will not limit imbalance but purely trip once the threshold is reached, if that imbalance is rising before the trip occurs it may well reach the trip current of the upstream device so it will hopefully cascade in correct order. at least that`s the theory behind it. I would expect that there will be many successes but I don`t think it is guaranteed to happen 100% of times
Sponsored Links
That’s only true if you have an earth fault that creeps up on you, rising through the mA until it hits the lowest supplied RCD.
But the real world isn’t like that. Mr Jones chops through a mains wire in his shed. The earth fault current is several amps.
All the RCDs in the chain will probably trip. Maybe the one with the fast trip characteristic will trip, leaving the others in play.
The only reliable way to discriminate RCDs is on the basis of time.
TTC has really said it all but, as I said, the greatest issue arises when there is a "suddenly - appearing large residual current". If the residual current only increases gradually then yes, of course, the one with the lower IΔn will trip before the other one even thinks about tripping.
I suppose that, in terms of common causes of RCD trips, water ingress might often present he 'gradually increasing residual current' which will, indeed, facilitate discrimination between devices with different IΔns. However, in the case of a fairly rapidly developing fault of relatively low impedance, I think all bets are probably off, regardless of respective IΔns.
Kind Regards, John
Agreed John, you put it better than I did and ditto TTC
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
