Previous Page  43 / 108 Next Page
Information
Show Menu
Previous Page 43 / 108 Next Page
Page Background

www. e l e c t r i c a l c o n n e c t i o n . c om . a u

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.