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3 8
E L E CT R I C AL CONNE CT I ON
W I NT E R 2 01 5
METHOD IN THE MADNESS
To protect and serve
T
he protection of installations,
circuits and electrical equipment
is crucial for the safety of clients,
their customers, families and friends.
For the electrician and electrical
contractor, such protection helps to avoid
personal or company fines, loss of licence
or damage to individual and company
reputation if something goes wrong.
Much of the protection requirements
for installations, circuits and electrical
equipment is detailed in the Wiring Rules,
AS/NZS 3000.
These rules are all about ensuring
safety, having been based on experience,
and are called up in legislation (the
Rules are law). So, in addition to a
moral obligation to comply, there’s a
legal obligation.
In this article we investigate the
requirements and practical implications
for the more obscure protection issues
of undervoltage, overvoltage, impulsive
transient and unbalance (voltage
and current).
Power supplies in rural areas are more
affected by power quality problems than
those in urban areas, so there is a greater
need for protection.
Rural powerlines are more exposed
to lightning, storms, bushfires, trees and
fauna. They are also constructed with
longer spans between poles, therefore
wires are more easily knocked together.
UNDERVOLTAGE
Although Australia has a 230V nominal
voltage, the supply is rarely at 230V.
Undervoltage is when the supply is
low for a sustained period – in power
quality terms, that means longer than one
minute – and delivers less than 90% of the
nominal, that is, below 207V.
Undervoltage can occur for several
reasons, including:
>
poorly designed systems with long
runs of undersized cables that might be
overloaded;
>
poorly set or faulty voltage regulation on
the grid supply;
>
burnt supply connections; or,
>
loss of a phase on the high-voltage grid
(what could be called a ‘brownout’).
The vulnerability of appliances depends
on the level of undervoltage and the type
of equipment. (A power supply recently
ordered for a computer monitor is rated
100-240V, so it is unlikely to be affected.)
However, motors will be badly affected
by sustained undervoltage, with increased
heating due to increased losses. This may
result in catastrophic failure of the motor if
it is unprotected by thermistors, suitably set
overload protection or undervoltage/phase
fail protection. The extent of the problem
will depend on how mechanically loaded
the motor is.
The Wiring Rules at Clause 2.8.1 require
undervoltage protection where loss and
subsequent restoration of voltage or drop
in voltage could cause a danger to persons
or property. However, protection is not
required if damage to electrical equipment
is considered an acceptable risk.
What is an acceptable risk and who
makes this call? Often the sparkie makes
the decision, but this is an issue to discuss
with the client.
An acceptable risk might include
situations in which the risk of fire to other
parts of a building are highly unlikely, and
perhaps whether the equipment is ‘throw
away’ if damaged. The term ‘acceptable
risk’ suggests that some form of risk
assessment is to be carried out.
Failure to discuss omitting protection
equipment and not conducting a risk
assessment could leave the sparkie footing
the bill for any damage. A court case may
ensue if the owner seeks compensation for
damage and lost production.
OVERVOLTAGE
The Wiring Rules deals with overvoltage,
such as that caused by lightning or
switching operations.
These events are generally discussed
in microsecond timeframes and not the
sustained overvoltage being discussed in
this section.
Sustained overvoltage occurs when the
supply is high for typically longer than
one minute and is more than 10% above
nominal, that is, above 253V.
Overvoltage is a common problem. It
will degrade most electrical equipment
over time and shorten its life. The
most noticeable effect is that halogen
downlights blow too soon. The more
insidious effects will go largely unnoticed.
The causes of overvoltage are both grid
and installation based.
Grid distribution transformers are
generally set to provide adequate voltage
at peak loads. At light loads they may
deliver too high a voltage. Also, problems
can occur in the grid, such as a faulty
voltage regulator or loss of a phase on the
high voltage (this often causes low voltage
but high voltage can occur).
Customer generation, such as by
photovoltaic systems, needs a higher
voltage to push excess current out into
The Los Angeles Police motto
is also a good one for sparkies.
In the first of two articles,
Chris Halliday
examines
the more obscure types of
protection against harm
from electricity.