Selectivity

[Paul Whybrow]

Evening, I have a question if I may. If I have in my garage a 2way DB with a 63 amp rcd fed from main board in house on a 32a mcb but rcd protected. If I perform an auto rcd test would I be right in thinking it would trip the main board 1st as it’s rated at 32amp?

thank you

 

[Andy78]

No. You would be testing the RCD in magnitudes of mA.
The 63A rating is the maximum current rating to be drawn through the device during normal service.

Assuming both RCDs are 30mA either or both could trip when testing the downstream RCD.

 

[Paul Whybrow]

No. You would be testing the RCD in magnitudes of mA.
The 63A rating is the maximum current rating to be drawn through the device during normal service.

Assuming both RCDs are 30mA either or both could trip when testing the downstream RCD.

Thank you for your reply, other than a mechanical check on the 63 amp in the garage dB how else could it be checked without/bypassing the main board one. Or is it not necessary as long as one or the other goes?.

 

[Andy78]

Thank you for your reply, other than a mechanical check on the 63 amp in the garage dB how else could it be checked without/bypassing the main board one. Or is it not necessary as long as one or the other goes?.

Having two RCDs in line with each other like this is bad design and gives no selectivity. The only way to test the garage board RCD properly would be to bypass the supplying RCD, but this would not be recommended if it meant energising a circuit that should have RCD protection if you had removed it.

So short answer is that you can't effectively test the RCD in the current arrangement. Unless you get lucky and only the garage one trips.

 

[Paul Whybrow]

Having two RCDs in line with each other like this is bad design and gives no selectivity. The only way to test the garage board RCD properly would be to bypass the supplying RCD, but this would not be recommended if it meant energising a circuit that should have RCD protection if you had removed it.

So short answer is that you can't effectively test the RCD in the current arrangement. Unless you get lucky and only the garage one trips.

Thank you for taking the time to answer this for me.