S-Type RCD Selection

[JXYM89]

I was wondering if someone might be able to help either confirm or change my understanding on the use of S-Type RCDs. I believe that they can be used to aid with selectivity and avoid potential nuisance tripping, providing the ratio of the rated residual operating current of the upstream S-Type RCD to that of the downstream RCD is at least 3:1. This said I have recently seen a setup where there's been a 30mA RCD within a lodge CU, with a 30mA C32 RCBO at the supply side DB - external to the lodge. When the RCD within the lodge is under test it would almost inevitably cause the 32A RCBO upstream to trip. Is this situation one where an S-Type RCD, or 100mA RCBO, would be better suited in the DB?

 

[loz2754]

If the main DB RCBO is there to provide additional protection (as it most probably is), then no, as S type RCDs are not suitable for providing additional protection. This is because S type RCDs introduce a time delay to help achieve selectivity, which makes it too slow to meet the requirements for additional protection.