Selection of the protection system of the electrical installation is fundamental both 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 definition of selectivity is given by the IEC 60947-1 Standard “Low voltage equipment – Part 1: General rules for low voltage equipment” Over-current discrimination: co-ordination of the operating characteristics of two or more over-current protective devices such that, on the incidence of over-currents within stated limits, the device intended to operate within these limits does so, while the other(s) does (do) not. Where by overcurrent a current of a higher value than the rated current is intended, due to any cause (overload, short-circuit, etc.). There is therefore selectivity between two circuitbreakers in series when, for an overcurrent which passes through both, the load-side circuit-breaker opens thereby protecting the circuit, whereas the supply-side one remains closed guaranteeing power supply to the rest of the installation.

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