Last edit: 03/03/2023
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.
While several devices may respond to the fault, the objective is that the first protective device in the path leading toward the power source triggers and clears the fault. If the next one beyond that one were to open, then other loads not fed by the unfaulted circuit might be turned off unnecessarily.
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.”
GT Engineering has prepared selectivity studies of a Power Distribution systems at sites supplied in both Medium Voltage and High Voltage.
We also include any local generation and analyse in detail the Generator protection settings.