Selectivity Study_Electrical Power Protection IEC60947-364

What is “selectivity”?

Complete selectivity means that the protective devices will minimize effect of a short circuit or other undesirable event on the power system. The amount of the power system that must be shut down in response to the event is kept to the absolute minimum.

The definitions of total selectivity and partial selectivity are given in Part 2 of the Standard IEC 60947-2 “Low voltage Equipment – Part 2: Circuit-breakers”.

“Total selectivity (2.17.2): Overcurrent selectivity where, in the presence of two protection devices against overcurrent in series, the loadside protection device carries out the protection without making the other device trip.”

“Partial selectivity (2.17.3): Overcurrent selectivity where, in the presence of two protection devices against overcurrent in series, the loadside protection device carries out the protection up to a given level of overcurrent, without making the other device trip.”

Overload zone

In which the thermal protection for thermal-magnetic trip units and protection L for electronic trip units are normally called upon to trip.

Short-circuit zone

In which the magnetic protection for thermal-magnetic trip units or protections S, D and I for solid-state trip units are normally called upon to trip.

The different selectivity techniques, also from the operational point of view