2 RCD's

[eagleeye]

Hi guys I was just thinking about what someone put about a question I asked a couple of days ago. I asked about putting an RCD proceted fused spur on a ring main to feed an outside light. they said the RCD fused spur will be fine as long as its not protected by another RCD on the cu. My question is, Can anyone tell me what would happen if it was connected up like this using 2 RCD's ? I cant see it being dangerous or anything but maybe im wrong ?

 

[Spudnik]

There would be no discrimination between the 2 if they were both 30mA.

 

[eagleeye]

sorry i know its me being very dumb but what do you mean by "no discrimination between the 2 if they were both 30mA" would they not work ?

 

[Mac]

Hi,i posted it.I said it was ok to use the RCD spur as long as the circuit wasn't RCD protected.If the circuit is RCD protected,just use a normal spur.

sorry i know its me being very dumb but what do you mean by "no discrimination between the 2 if they were both 30mA" would they not work ?

They would work,but both RCD's would trip.

 

[Darkwood]

Discrimination(in this case) is having the rcd downstream (fusespur) tripping under fault condition before a rcd upstream trips(fuseboard )if not this would cause nuisance tripping where other items or circuits are lost but it leads to confusion too.
2 rcds are allowed when discrimination is acheived either by having a larger mA rating upstream device or having a time delay rcd upstream if they were rated the same but reg's on tripping times and mA values still need to be achieved in either of these examples.