I. What is Selectivity?
Selection of the protection system of the electrical installation is fundamental to guarantee correct economical and functional service of the whole installation and to reduce the problems caused by abnormal service conditions or actual faults to a minimum.
The coordination between the various protectivie devices dedicated to protection of sections of an electrical installation is required in order to:
- guarantee safety of the people and equipment;
- identify and exclude the problem area without indiscriminate trips in areas not involved in the fault;
- reduce the effects of the fault;
- reduce the stress and damage on components;
- guarantee service continuity and quality;
- guarantee adequate protection in case of protection malfunction;
- provide the information needed to restore service rapidly;
- achieve a good compromise between reliability, simplicity and cost-effectiveness.
II. Selectivity Categories
The selectivity category of a circuit-breaker shall be stated with reference to whether or not it is specifically intended for selectivity by means of an intentional time delay with respect to other circuit-breakers in series on the load side, under short-circuit conditions.
| Selectivity Category | Application with respect to selectivity |
|---|---|
| Category A | Circuit breakers not specifically intended for selectivity under short-circuit conditions with respect to other short-circuit protective devices in series on the load side, i.e. without an intentional short-time delay provided for selectivity under short-circuit conditions, and therefore without a short-time withstand current rating.
Type of circuit-breaker without rated short-time withstand current Icw (indicated for the energy selectivity) |
| Category B | Circuit-breakers specifically intended for selectivity under short-circuit conditions with respect to other short-circuit protective devices in series on the load side, i.e. with an intentional short-time delay (which may be adjustable), provided for selectivity under short-circuit conditions. Such circuit-breakers have a short-time withstand current rating.
Type of circuit-breaker with rated short-time withstand currentIcw (indicated for the time selectivity) |
References
- IEC 60947-2 - Circuit Breakers
- Low voltage selectivity with ABB circuit-breakers