Handbook of Electrical Installation Practice - BeKnowledge

CHAPTER 22
Protective Systems
B. Dakers, BSc, CEng, MIEE
D. Robertson, CEng, MIEE
Revised by P.R. Rosen, BSc(Eng)
(Electrical Consultant)
The term ‘protective systems’ refers to the electrical protection of plant associated
with industrial systems and power systems such as alternators, motors, transformers
and cables. Although this chapter is predominantly concerned with protective
systems applicable to industrial systems and distribution systems up to 11kV, the
basic principles and requirements apply equally to power systems operating at
higher voltages.
A power system is designed to deliver electrical energy without interruption to
the points where it is utilised. By definition, therefore, a fundamental requirement
in the design of a system is flexibility. If one part of the system becomes faulty it
should be disconnected quickly and ideally without affecting other parts of the
system. Switchgear and protective gear provide this isolation flexibility and thus can
be considered in the context of an insurance against loss of supply. The switchgear
must be designed to be capable of interrupting the current resulting from a fault
while the protective gear must be capable of recognising a fault condition and
initiating the switchgear to disconnect the faulty part of the power system with
minimum disturbance to the rest of the system.
PROTECTIVE SYSTEM REQUIREMENTS
Protective gear is the collective name given to all the components necessary
for recognising,locating and initiating the removal of a faulty part of the power system.
The most important qualities of a protection scheme are reliability and
selectivity, with other important qualities being speed, stability and sensitivity,
none of which,however,is acceptable unless they are provided in such a way that reliability
and selectivity are maintained, and the equipment is within reasonable cost.
Although total reliability is impossible, duplicate schemes, alarm supervision
circuits and regular maintenance have resulted in the number of incorrectly cleared
faults being less than 5%.
Selectivity, or discrimination as it is more commonly known, may be defined
as the quality of a protective system whereby it detects and responds to a fault
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