Can we use RCBO as sub-main breaker to protect outgoing breakers instead of RCCB /ELCB plus Miniature circuit breaker MCB?

Yes you can use RCBO as a replacement to MCB and RCB. In general, RCBO is basically circuit breaker in addition to residual current device. It provides over current, short circuit as well as earth leakage protection.

Released for:Schneider Electric Saudi Arabia