Switch Gear Discrimination and Coordination

Cascading or Back-up protection

This is where the upstream breaker has a higher fault break capacity than the down stream breaker. ie for fault levels at or below the capacity of the down stream breaker only the down stream breaker will trip, for fault levels above the capacity of the downstream breaker both breakers will trip - the upstream breaker providing assistance to the downstream breaker.

Discrimination

This is where only the breaker immediately upstream of the fault will trip so the rest of the system is not disturbed.

Just check on the overlapping areas of the I2T characteristics for the breakers.

Try www.schneider.co.uk or www.siemens.co.uk or any circuit breaker maker for details.

BSEN 60947-2 defines it all for you.