Take care servicing
•
m.1crowave ovens
all, the old TVs used valves in the
high voltage section, and a magnetron is just another kind of valve.
Without some knowledge of circuit
theory, one may not realise that the
high impedance of the 17kV circuit
would limit the current to a relatively
safe level.
·
On the other hand, the microwave's
high voltage is supplied via a low
impedance transformer, with only a
hundred ohms or so of resistance in
the secondary winding to limit whatever current might flow when the cirBy JIM LAWLER
cuit was completed. In normal operation, the naturally high impedance of
On 15th November 1989, a service- the Coroner's suggestions for correct the magnetron is all that prevents
catastrophic currents from wrecking
man was killed while repairing a tools and clothing are followed.
One clue to the origin of the prac- the oven.
microwave oven. To give the full deMake no mistake! The high voltage
tails, we reproduce a report by the tice came from the Service Manager
Victorian Deputy State Coroner into of a company well known in the mi- circuit in a microwave oven is lethal!
the death. We suggest that you read crowave field. He suggested that the The power transformer delivers
habit of sparking the ultor cap of an around 4000 volts AC, at anything up
the report before continuing.
It's not an easy matter to comment old black and white TV to check for to half an amp. And the high voltage
on this tragic accident, yet comment high voltage has been carried over capacitor ensures that every joule of
into the new technology by some that energy is available, ready to kill
needs to be made.
Accidents don't just happen. They people who lack proper knowledge the unwary serviceman. There is no
of the circuits they are handling.
hope of surviving contact with that
are caused by someone or something.
It might be thought that if it was sort of energy and no properly trained
And somewhere there has to be a
safe to spark the 17kV in a TV set, it serviceman would ever knowingly
cause for this one.
In preparing this article, I have must also be safe to do the same with put himself at risk by working on a
asked a number of microwave manu- only 4kV in a microwave oven. After live microwave oven.
So be warned. No matter
facturers if they had ever rehow blase anyone may be
commended the procedure folabout working on mains powlowed by the deceased at the
time of his death. None of them
ered appliances such as TV
sets, washing machines and
has done so.
dishwashers, microwave ovI have also spoken to many
technicians about their servicens are an entirely different
matter. They are extremely
ing methods and none of them
dangerous.
have ever used the method
Editor's notE:1: we do not
described in the Coroner's reagree with the coroner's recport, although one had heard
that there were those who used
ommendations for working on
the method. In fact, no-one to
live microwave ovens, in the
whom I spoke would ever conuse of insulated clothing or
sider working on a live oven in
tools. Microwave ovens are
any way, shape or form.
just too dangerous for that. In
So it puzzles me where the
our opinion, they should not
practice arose. It seems to be a The large power transformer inside a microwave oven be worked at all on while they
risk-filled procedure, even if generates around 4000V AC & is potentially lethal.
are live.
While everyone appreciates the convenience
and speed of microwave ovens, they can be
very dangerous in the wrong circumstances,
as can any electric or electronic appliance.
We feature this report as a timely reminder
that doing any service work on a microwave
oven, while it is connected to the mains
power, is extremely dangerous.
6
SILICON CHIP
The Coroner's Report
Case No. 4851/89
14th May, 1990
RECORD OF INVESTIGATION INTO DEATH
I, GRAEME DOUGLAS JOHNSTONE, Deputy State Coroner HAVING INVESTIGATED the death of JEFFREY ALLAN DOHERTY with inquest held at the Coronial Services Centre, South
Melbourne on the 7th May, 1990
FIND that the identity of the deceased was JEFFREY ALLAN DOHERTY and that death occurred on the 15th November, 1989 at
5 Harwell Street, Ferntree Gully from electrocution in the following circumstances.
(1 ). At approximately 4.30 pm on the 15th November the deceased a self employed Electrical Appliance Serviceman was called to
5 Harwell Street to repair a faulty Toshiba Microwave Oven . The deceased was repairing the oven using faulty pliers when he received a high voltage electric shock.
(2). The deceased had disconnected the cables from the high voltage transformer at the terminals of the magnetron unit and
touched the lug on the end of the cable against the magnetron terminal whilst waiting for a spark. He had undertaken this procedure on many occasions previously.
He was holding the lug with a pair of Insulated Sidchrome Combination Pliers. The pliers had a crack and small hole in the
insulation to the handle which resulted in an insulation breakdown.
The crack resulted in the insulation being ineffective with the high voltage.
The deceased was wearing jeans, a short sleeved shirt and leather boots. No insulating gloves were being worn.
(3). The deceased was the director of J. A. Appliances Pty Ltd and that company was engaged in the repair and servicing of
washing machines, dishwashers and microwave ovens.
The deceased had attended various courses on the repair of microwave ovens being run by Toshiba and National. It appears that
his general training was by practical experience in the appliance repair industry.
AND I FURTHER FIND that the deceased contributed to his own death by repairing the microwave oven using faulty pliers with the
power on and thereby receiving a high voltage electric shock.
RECOMMENDATIONS AND COMMENTS
I propose to forward my Findings, Recommendations and Comments to the Attorney-General for dissemination to the Minister
Department of Labour, the Chairman of the State Electricity Commission , the Electrical Development Association, the Electrical
Contractors Association, the Electrical Contractors Federation (Victoria), the Electrical Wholesale Association (Victoria), the Electronic Technicians Institute, the Victorian Employers Federation, the Trades Hall Council and Toshiba (Australia) Pty Ltd.
It is essential that the facts of this case be widely distributed by the State Electricity Commission to all appliance manufacturers
and distributors (especially of microwave ovens and television sets), all licensed electrical mechanics and contractors, and all persons working within the appliance service/repair industry. (An obvious source of dissemination of the information would be the
yellow pages).
There is no requirement for electrical appliance repair or service personnel dealing with plug-in appliances such as televisions,
washing machines, dish washers, microwaves and the like to have any form of State Electricity Commission License. It is noted
that restricted licenses are required where personnel are working on appliances that are permanently connected to supply.
It would appear that electrical appliance repair/service personnel learn their trade through personal experience and interest or occasional training in the event that they work with a large appliance repair company.
·
There is an obvious need as highlighted by this case for both formal training requirements followed by restricted licenses for all
electrical appliance repair/service personnel. It is understood that this matter is currently being examined by the State Electricity
Commission.
This case also highlights the dangers associated with the failure to ensure that appropriate equipment is used in all electrical work
(especially with high voltages) in that:
(1 ). It is imperative that the insulation on pliers, screw.drivers and the like be regularly checked. In the event that work is being conducted with high voltage appliances the appropriate type of pliers should be used.
(2). Insulated gloves should always be used when working with high voltage appliances.
(3) . Clothing should be appropriate for the job at hand in that the following items should be worn :
(a) long sleeve shirts and pants;
(b) insulating type shoes (not leather).
The technique used by the deceased to repair the microwave oven was also inappropriate.
Where high voltage is involved in the repair of microwaves, high voltage testers should be used. The fuse in the high voltage circuit should be tested. A failure of the fuse in the magnetron circuit indicates a failure of the magnetron.
A simple magnetic test will indicate operation or otherwise of the high voltage transformer.
It is therefore essential that all repairs be undertaken without supply connected in all high voltage situations (microwaves/TV) and
testing equipment should be used to check voltage.
Graeme Johnstone
Deputy State Coroner
JANUARY 1991
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