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short-circuit currents. An overload
current is one that exceeds the
normal operating parameters of
the conductors, but is confined to
the electrical distribution system,
whereas a short-circuit current flows
outside these normal conducting
paths.
A temporary overload, frequently
between one and six times the
normal current level, is usually
caused by a harmless electrical
surge that occurs when motors
start up or equipment is energised.
Brief in duration, any conductor
temperature rise is trivial, with no
harmful effect, and it is imperative
that protection devices should
not react to them. Continuous
overloading though can be caused
by defective motors, worn bearings,
equipment working beyond its
normal operating parameters or too
many loads connected to one circuit.
These overloads are destructive
and must be removed by protection
equipment in a timely manner to
prevent damage.
Unlike overload currents, a short-
circuit current can be many hundred
times larger than normal operating
current levels, rising to in excess
of 50,000 A. If not isolated within
a few milliseconds, damage and
destruction can become rampant,
resulting in severe insulation
damage, melting of conductors,
metal vaporisation, arcing and fires.
Two forms of protection are
used; circuit breakers and fuses.
Although the circuit breaker is
considered a replacement for the
fuse, both have their applications.
The key advantage of the fuse is
the response time, opening within
4-5ms, compared to that of a circuit
breaker. High fault currents that can
damage machine power electronics
are therefore prevented. The
fuse’s voltage and current rating for
both continuous operation as well
as interruption must be carefully
considered to provide the correct
protection. Help with fuse selection
is often useful, if not essential, as
the breadth of applications where
fuses can be used, together with
the depth of choice available, is
vast. Eaton, for example, catalogues
8,500 different fuse types.
Circuit breakers, however, are
resettable – in some cases even
remotely – after a fault. For some
applications, the ability to reset a
circuit breaker from another location
rather than sending a technician can
improve machine up-time. Circuit
breakers also perform better than
fuses in circuits with inductive loads
such as motors or transformers
that draw heavy transient start-
up currents. They can more easily
be set to open on genuine faults,
without ‘nuisance tripping’ during
the inductive transients.
Additionally, circuit breakers have
adjustable protection characteristics
suitable for many different
applications, whereas a fuse with
exactly the right parameters must
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