Electrical Connection Spring 2015

27 / 116

www. e l e c t r i c a l c o nn e c t i o n . c om . a u 2 7
with Chris Halliday
of circuits. Rating factors are shown in Tables
27 to 29 for such factors as cables in heated
concrete, soil temperature, burial depth and
soil thermal resistivity.
In using Tables 4 to 21, circuit designers or
electricians should consider the following
cable situations:
> Vertically or horizontally spaced;
> touching the surface of, say, wall lining
material;
> exposure to the sun;
> enclosed or not;
> partly or completely surrounded by
thermal insulation; and,
> buried directly or in an enclosure such as
conduit.
They should then go to the correct
column of the table.
VOLTAGE DROP
Too long a cable run with too much load
will cause excessive voltage drop, which
may result in unsafe or non-operation of the
equipment on that circuit.
This will breach the Wiring Rules, so we
must consider voltage drop when selecting
a cable (see Electrical Connection Autumn
2015, page 44).
FAULT LOOP IMPEDANCE
The loop impedance of circuits must
be considered for all circuits and should
comply with Tables 8.1 and/or 8.2 of
the Wiring Rules.
Table B1 provides guidance on circuit
lengths. Voltage drop requirements
will generally, but not always, be
more stringent than loop impedance
requirements. Further detail is provided in
the aforementioned article.
SHORT-CIRCUIT TEMPERATURE
Cables may have to endure the thermal
effect and mechanical forces caused by fault
currents for up to five seconds.
Therefore we must check the minimum
cable size that can withstand these effects
by applying the following formula:
S = √ I2t
K2
where
S = size of the current-carrying
component (mm²).
I = short-circuit current (amps).
t = duration of the short circuit current
(seconds) – check protective device
information for the fault level.
K = temperature constant depending on
the material of the current-carrying
component.
Some examples are provided in
AS/NZS 3008.1.1 for reference.
REPUTABLE SUPPLY
If the Infinity and Olsent debacle has
taught us anything, it is that we should buy
only reputable brand cables from reputable
suppliers.
This goes for all other electrical
equipment and components. The message
from government fair trade departments
is: “If it sounds too good to be true – then it
probably is”.
CONCLUSION
Selecting a particular type and size of
cable is not easy, and there will be other
factors not included here.
Failure to consider all requirements may
mean that people are put at risk, or the
electrician must bear the cost of returning to
remedy the installation.
> Power Logic
EMONA
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