What do back-ups and protection have in common?
Back-up is relevant in electrical installations to ensure uninterrupted power supply.
Happy World Back-up Day!
You think back-up only refers to data files? Well, we have some news for you: back-up is very important in electrical installations, providing continuous supply of electrical energy for all devices in your home or building. Here the coordination of different protective devices (upstream and downstream) play an important role (miniature circuit breakers (MCB), molded case circuit breakers (MCCB), selective main circuit breakers (SMCB)). Usually the cascading starts with a selective main circuit breaker or molded case circuit breakers followed by a miniature circuit breaker. The MCB also provides back-up protection to downstream residual current circuit breakers (RCCB) and switch disconnectors.
We talk about back-up protection when the upstream protective device trips in case of a short-circuit current which is higher than the short-circuit capacity of the downstream protective device. In case the short-circuit current is lower than the short-circuit capacity of the protective device, only the downstream device will trip. This is called selectivity.
To build up a reliable protection scheme the coordination between devices cannot be determined only by calculation. Tests should be carried out to select the right coordination type.
By the way: ABB’s selective main circuit breakers S750 and S750 DR can offer selectivity even if the short-circuit capacity of the downsteram MCB is lower than the short-circuit current.
Quite handy, isn’t it?