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Rcd discrimination

NGUACE
Any Information greatly appreciated.


Each outdoor socket has to be RCD protected, so if you have a junction box supplying more than one outdoor 30ma socket, how do you achieve RCD discrimination all the way back. I have an older C/U and the rcd on it is also 30ma?


A spur from internal plug to fused switch outside, which then feeds multiple outdoor sockets?


this is just a scenario I am trying to figure out.


thanks for anyone’s time.


Andy
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    you cannot say with any confidence which RCD will operate, or indeed if both will - they may have both started moving before the contacts in either one remove the power.

    However, what does happen is that the chance of a dangerous no-trip due to an RCD failure and then a fault (about 5-10% chance according to some historical data) is now reduced to the chance of 2 independent failures, so  more like 1%, assuming as part of the reset process you then test both after clearing the fault, and replace any that do not trip.

    As per OMS, if you need discrimination, then you need to delay and to increase the threshold - typically by about a factor of 3 to be reliable.

    so for example, 30mA instant on a 32A final circuit,  fed by 100a supply and 100mA 1/10 second, fed by 300A genset at 300ma 300ms etc. by the time you get to a few seconds you are looking at seriously expensive earth fault relays 

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  • 0
    you cannot say with any confidence which RCD will operate, or indeed if both will - they may have both started moving before the contacts in either one remove the power.

    However, what does happen is that the chance of a dangerous no-trip due to an RCD failure and then a fault (about 5-10% chance according to some historical data) is now reduced to the chance of 2 independent failures, so  more like 1%, assuming as part of the reset process you then test both after clearing the fault, and replace any that do not trip.

    As per OMS, if you need discrimination, then you need to delay and to increase the threshold - typically by about a factor of 3 to be reliable.

    so for example, 30mA instant on a 32A final circuit,  fed by 100a supply and 100mA 1/10 second, fed by 300A genset at 300ma 300ms etc. by the time you get to a few seconds you are looking at seriously expensive earth fault relays 

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