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42 E L EC TR I C AL CONNEC T I ON
AU T UMN 20 1 6
FEELING THE HEAT
O
ver the years, I have
investigated a diverse range of
electrical fires.
The list includes house fires caused by
defective wiring, incorrectly installed
lighting and lightning strike. It also
includes vehicle fires.
WIRINGAND FITTINGSDEFECTS
Fixed mains wiring in domestic and
most commercial applications commonly
uses some form of PVC insulated cable.
The most common type is thermoplastic
sheath flat cable (cream colour).
The PVC insulation will burn when
heated and emit toxic fumes (including
chlorine gas); however, PVC is not an
accelerant – once the source of heat is
removed, it will not sustain a fire.
An electrical fire can be caused by
arcing between two conductors or by
the heat from ohmic losses in a high-
resistance joint.
Indoor mains wiring originates from
some form of switchboard, and each
conductor is connected to a protective
device (fuse or circuit breaker) by means
of a screw terminal.
If this screw terminal is not tightened
sufficiently, it will constitute a high-
resistance joint. Under a high current
load, such a joint will heat up and can be
a source of ignition.
Protection devices that plug into a
switchboard can also fail and burn if the
socket they plug into does not make a
good contact.
The wiring for power points and
lighting is usually daisy chained. Thus
power points and light fittings can
act as connection points for two or
more cables. Each joint, if not installed
correctly, can be a source of ignition
under certain conditions.
I have encountered many cases of a
tunnel terminal, used for terminating
the cable at the rear of the power point,
has not been correctly tightened.
Under heavy load (eg: a clothes
dryer) the high resistance caused by
the loose joint has caused the cable
to heat up and catch fire. This can
spread to timber in the wall and cause
serious fire and smoke damage to
the house.
Similarly, power points and power
boards can suffer internal mechanical
damage through abuse. One or more
of the socket contacts can become
splayed and may no longer make a
low-resistance contact when a power
plug is inserted.
If the load happens to be a blow
heater or hair dryer, the plastic
constituting the plug and the socket
can melt and eventually cause nearby
combustible materials to ignite.
Ceiling-mounted halogen downlights
operate at very high temperature. For
ventilation and safety, the Wiring Rules
specify clearances from adjacent timber.
If ceiling insulation covers such a fitting,
it will prevent ventilation and may
constitute a fire hazard.
This type of lighting has been known
to cause roof fires if not installed
correctly, if ventilation has subsequently
been obstructed, or if associated power
supply components have failed (eg: step-
down power transformer or ballast).
Halogen downlights are being
replaced by far more efficient and
cooler-running LED designs.
VEHICLE FIRES
Electrical wiring in vehicles differs
from mains wiring in several ways.
It operates from DC and employs a
low-voltage source (usually 12V or 24V).
By necessity, the source (the battery)
has to have a very low impedance in
order to operate the starter motor.
To deliver enough power to the
headlights or ignition system, the
operating currents are generally
much higher than those in domestic
mains circuits. Because of this, vehicle
wiring systems do not tolerate faulty
connections or unfused short circuits.
Some time ago I examined a European
sports car that had caught fire while
being driven. Fortunately, the vehicle
was close to the workshop where it
THE FIRST PART OF THIS SERIES
DESCRIBED HOW HEAT STEMS
FROM CURRENT IN A RESISTIVE
ELEMENT, AND HOW FAULTS
CAN CAUSE FIRES.
GEORGE
GEORGEVITS
NOW OUTLINES
SOME CASES.
A plug-in circuit breaker that has caught
fire due to a high-resistance joint.
ELECTRICAL FIRES