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43
BY
GEORGE
GEORGEVITS
was usually serviced. The driver made
it to the workshop and the battery was
quickly disconnected.
(NB: With any form of electrical
fire, the first action is to disconnect
the supply.)
Inspection of the fire-damaged area
in the engine bay revealed that the fire
was associated with wiring supplying
one of the electric radiator fans (this car
had two).
Electric radiator fans are invariably
high-speed, high-power devices. This is
necessary for sufficient radiator cooling
in hot weather.
Closer inspection of the fan revealed
that one of the push-on spade
connectors was only half mated. The
high-resistance joint caused by this
defect generated enough heat to
melt the wire’s plastic insulation and
subsequently cause ignition.
The incident occurred while the
vehicle was idling in traffic on a hot
day. The fire started when the fan
thermostat cut in and quickly spread to
nearby plastic parts in the engine bay.
The driver was lucky to make it back to
the workshop.
In another case, I was called upon to
inspect a fire-damaged semi-trailer.
The driver had been travelling at
high speed when he noticed the
smell of burnt plastic in the cab. He
pulled over as quickly as he could, but
the cab had filled with acrid smoke. He
left the vehicle to burn, fearing for his
safety. Had he disconnected the
battery, the damage would have been
much reduced.
When I inspected the prime mover, it
was obvious that the fire had started in
the engine bay.
Inspection of the wiring remnants
revealed a short length of heavy gauge
wire that was connected directly (ie:
unfused) to the positive supply rail
on the engine bay firewall. The other
end of this wire was just a melted blob
of copper.
The wire seemed to have been
added after the truck was built,
probably to supply some form of
aftermarket accessory.
Unfused wiring like this is a bad idea.
A short circuit to the chassis can create a
very large fault current that will quickly
melt and burn away the wire insulation.
It will keep on supplying heat until the
wire eventually melts and the circuit is
broken, just like a fuse.
In this case, I calculated that the
wire could readily carry more than
500A before melting.
In all likelihood, the fire started when
this wire rubbed against part of the
vehicle’s hot chassis until the plastic
insulation was worn away, thereby
creating a dead short to the chassis.
The 270Ah battery could easily
supply many hundreds of amps. The
wire would have glowed white hot
before melting. All nearby plastic parts
would also have caught fire.
LIGHTNINGSTRIKE
One inspection involved a new,
unoccupied house that had caught fire.
The damage appeared concentrated
around a run of fixed mains cabling
containing six or so cables inside a
plasterboard wall.
Part of my brief was to advise
whether the fire was electrical, whether
the wiring complied with the Wiring
Rules, and whether it might have
been damaged by another trade
after installation.
The burn pattern indicated the fire
was associated with electrical cables
passing through a hole in a stud. The
wiring could not have been damaged
after installation, as any such fault
would have tripped the relevant
circuit breaker.
The damage appeared most severe
near one of the studs, and was much
worse for a ~100mm length along a
particular cable. Most peculiar!
A neighbour said the fire had started
during a violent thunderstorm, soon
after a very loud thunderclap.
An online lightning strike database
revealed that there had been a very
close lightning strike at the time of the
fire. That afternoon there were more
than 1,000 within 50km of the site.
In my view, lighting had struck the
roof, followed the sarking and stud
down the wall and arced across to the
mains cable, thereby burning away
the insulation on that one cable. The
breaker for that circuit (and only that
circuit) had tripped.
Fire damage near a particular wall stud.