From Electrical Installation Guide
The subject of protection in the electrical power industry is vast: it covers all aspects of safety for personnel, and protection against damage or destruction of property, plant, and equipment.
These different aspects of protection can be broadly classified according to the following objectives:
- Protection of personnel and animals against the dangers of overvoltages and electric shock, fire, explosions, and toxic gases, etc.
- Protection of the plant, equipment and components of a power system against the stresses of short-circuit faults, atmospheric surges (lightning) and power-system instability (loss of synchronism) etc.
- Protection of personnel and plant from the dangers of incorrect power-system operation, by the use of electrical and mechanical interlocking. All classes of switchgear (including, for example, tap-position selector switches on transformers, and so on...) have well- defined operating limits. This means that the order in which the different kinds of switching device can be safely closed or opened is vitally important. Interlocking keys and analogous electrical control circuits are frequently used to ensure strict compliance with correct operating sequences.
It is beyond the scope of a guide to describe in full technical detail the numerous schemes of protection available to power-systems engineers, but it is hoped that the following sections will prove to be useful through a discussion of general principles. While some of the protective devices mentioned are of universal application, descriptions generally will be confined to those in common use on MV and LV systems only, as defined in Sub-clause.