Handbook of Electrical Installation Practice - BeKnowledge

360 Handbook of Electrical Installation Practice
CIRCUIT-BREAKING
An essential element in the operation of any a.c. circuit-breaker is the electric arc,
which permits the current in the circuit to continue flowing after the contacts have
parted, until a suitable current zero occurs. This is illustrated in Fig. 14.1. As will be
described below, the sudden cessation of current flow at any time, other than very
close to a natural current zero, has undesirable consequences in all normal distribution
systems, so the existence of the arc as a natural commutating device is a very
important factor in the operation of a.c. circuit-breakers.
An ideal circuit-breaker is one which acts as a perfect conductor until current
zero is reached, at which point it becomes a perfect insulator.As no practical circuitbreaker
meets this condition, the result is modified to a greater or lesser degree by
the circuit-breaker characteristics. The objective of the circuit-breaker designer is
to create the necessary conditions to sweep away the ionisation products in the
contact gap at current zero and replace them by a medium which will withstand the
application of a very rapidly rising voltage of considerable amplitude, the transient
recovery voltage. This is shown at H in Fig. 14.1.
MEDIUM VOLTAGE SWITCHGEAR
The term medium voltage is usually used to describe switchgear rated from 3.6kV
to 52kV. In the UK it is general practice for m.v. distribution switchgear manufacturers
to supply completely factory-assembled switchgear. This means switchgear
equipment containing busbars and all circuit components up to the cable terminations,
including any switching devices (circuit-breakers, switches, etc.) and their
isolating means. It is, therefore, almost equally general for the switchgear to be
type tested in its completely assembled form. This is not always the case for l.v.
switchgear, since a thriving business exists in this area for the manufacture of l.v.
switchboards incorporating circuit-breakers and other l.v. switching devices made
by a small number of specialist manufacturers. Also, m.v. circuit-breaker switchgear
is designed principally for use with separately protective relays and therefore has a
short time rating, usually equal to the breaking current rating for three seconds,
unless where very heavy fault levels exist, the operating conditions permit a one
second short time rating. The switch or circuit-breaker also has to be able to close
fully against a short-circuit making current peak equal to 2.5 times the rated breaking
current, and to be able to interrupt that breaking current with a degree of asymmetry
which will depend on the opening time of the circuit-breaker being
considered. With some of the modern high speed circuit-breakers using vacuum or
sulphur hexafluoride (SF 6), this time can be quite short and the d.c. component correspondingly
large. A figure of 35% d.c. component is not uncommon under these
circumstances.
All circuit-breakers have to be able to close and open satisfactorily under all conditions
of service. In particular they must interrupt all currents from zero to full
rated breaking current with all possible combinations of power factor, current asymmetry
and recovery voltage that occur. Figure 14.1 illustrates the typical form of the
current and recovery voltage which occurs when a fault current is interrupted. The