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4 8
E L E CT R I C AL CONNE CT I ON
S P R I NG 2 01 5
INSPECTORATE
Protection convention
A
safe and reliable electrical
installation depends on
overcurrent, short-circuit and
earth leakage protection as required by the
Wiring Rules.
This comes at a cost to the customer, but
inadequate protection will have serious
implications for those using the installation
if something goes wrong – and for the
electrician responsible.
Let us look at Wiring Rules requirements
for the three types of protection mentioned.
OVERCURRENT PROTECTION
This seeks to automatically disconnect
the supply if an overload or short circuit
occurs (Clause 2.5.1).
Either situation could cause injury or
damage due to excessive temperatures
or electromechanical stresses in the
installation.
Protection these days is more commonly
provided by circuit breakers, and with some
use of high rupturing capacity (HRC) fuses.
However, the days of the rewireable fuse
are long gone. Electricians should always
seek to replace rewireable fuses, using a
residual current breaker with overcurrent
protection (RCBO). This will help to ensure
safety and perhaps save a life.
Electricians can never know how many
lives are saved by their work, but those
they fail to save can be easily tallied.
OVERLOAD PROTECTION
This must be sized so that it is no larger
than the maximum demand of the circuit
or the continuous current rating of the
conductor (Clause 2.5.3.1).
It doesn’t necessarily have to be at the
start of the circuit (Clauses 2.5.3.3). It can
even be omitted in certain circumstances,
such as where there is a fixed load not
capable of causing an overload, eg: a
heating appliance (Clause 2.5.3.4).
That also means the load must be suitably
matched with the cable and does not
negate the need for short-circuit protection.
SHORT-CIRCUIT PROTECTION
A short-circuit current must be
interrupted to prevent the conductors
becoming too hot, and to limit
electromechanical forces.
The designer or electrician must know
the prospective short-circuit current at the
protection location (Clause 2.5.4.1). This can
be determined by calculation. However,
the easiest way for an electrician to obtain
a value is to measure the level using a
loop impedance tester (most loop testers
are capable of measuring the prospective
short-circuit current).
The circuit breaker or fuse used must
have a kA rating greater than the calculated
or measured prospective short-circuit
current at that location (Clause 2.5.4.5).
The Wiring Rules allow for two situations
in which short-circuit protection doesn’t
have to be at the start of the circuit or a
reduction in size of the current-carrying
capacity of a cable (Clause 2.5.4.3).
The first is where the length of the circuit
is less than 3m, it is mechanically protected,
the risk of short-circuit is reduced to a
minimum, and it is installed in a way that
minimises the risk of fire or other danger.
The second is where the short-circuit
device is upstream of a reduction in
cross-sectional area or other change, but
the protective device must protect the
smaller conductor.
Short-circuit protection can even be
omitted in certain circumstance (Clause
2.5.4.4) such as:
In the first part of this two-
part series,
Chris Halliday
looked at the more obscure but
nevertheless important types of
protection. He now examines
the conventional forms.
with
Chris Halliday