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High voltage probes:
beware the dangers
High voltage probes can be lethal if
used incorrectly or for servicing
equipment other than that for which
they were designed. This coronial
report of a recent accident in Victoria
highlights the dangers.
RECORD OF INVESTIGATION
INTO DEATH
I, Susan Adele Blashki, Coroner,
having investigated the death of ...... .
with inquest held at the Coronial Services Centre, South Melbourne on the
12th June, 1992, find that the identity
of the deceased was ....... and thatthe
death occurred on 25th July, 1991 at
the premises of HY-DU-EX, 15 Ormond Avenue, Sunshine from the effects of electrocution in the following
circumstances.
(1). The deceased, on 25thJuly 1991,
was repairing a radio frequency welder
at the premises of HY-DU-EX, 15
Ormond Avenue, Sunshine. He was a
contractor, employed as an "A" Grade
Electrical Mechanic by K. T.V.. Electronics, a small electrical -contracting
company. He was the holder of an
"A" Grade electrical mechanic's licence and had 28 years experience in
this capacity.
(2). The welder is designed for the
purpose of welding PVC. It is between
15 and 20 years old. On the previous
day the deceased had replaced the
high voltage transformer and on the
day of the accident he had tested two
rectifier valves and found them faulty.
He had then replaced the valves and
was in the process of fine tuning the
radio frequency welder.
At the time of the accident it is
probable that the deceased was testing the high voltage transforrner, us64
SILICON CHIP
ing a "Leader High Voltage Probe",
LHM - 80 Serial No. 5050061. This
probe was found under his body after
the accident. The deceased had completely overhauled and rebuilt this
radio frequency welder some 3 years
prior to this accident. The machine
had then operated effectively for a
number of years before K.T.V. Electronics was again called and Mr ..... ..
attended to service the machine.
Editorial Comment
This coronial report has been
published in full for the information of our readers . While the
exact circumstances may not be
certain, several cautions can be
drawn from the report.
(1 ). Use the utmost care when
measuring high voltages in big
power supplies, such as in microwave ovens or RF welders. They
are far more dangerous than the
EHT supply in a TV set.
(2). Wear rubber soled shoes
and rubber gloves while making
high voltage measurements.
(3). If at all possible; do not work
alone.
Footnote: the name of the deceased has been omitted to prevent further distress to relatives.
He was sufficiently familiar with this
particular machine to produce a set of
technical, electrical drawings on it.
(3). It is probable that the accident
was caused either by the earth lead
of the probe dislodging and coming
into contact with high voltage, or the
deceased may have been attempting
to put the earth clip on the earth centre tap of the 6000 volt secondary
connections and inadvertently put
the clip on one of the positive terminals.
It appears that the electrical current may have passed up the earth/
return lead and exited through the
potentiometer adjustment screw access hole to the hand of the deceased,
thereby delivering a high voltage
shock to him. This shock would have
thrown him back when he then struck
his head on the cutting ·board of an
adjoining click press and fell to the
floor.
No other person was present at the
time this incident occurred, but both
the proprietor and another employee .
at the building at the time heard a
loud bang and discovered the deceased with scorch marks on his right
fingers and marks on the back of his
head.
(4). The deceased, whilst working
at these premises the previous day,
had complained of being faint and
requested a chair. He had also complained to members of his family that
he was very cold on the eve·n ing of the
· 24th July, 1991 and to the factory manager at HY-DU-EX on 25th July, 1991
that his legs were very cold just prior
to th!:) accident. About 6-8 months
previously the deceased had collapsed
whilst at work and been absent for
approximately three months, but was
at the time of the accident considered
by his doctor to be fit for normal duties.
(5). The general area of HY-DU-EX
measures 31 metres long by 9.5 metres wide. It was constructed by a
steel frame with corrugated iron walls
and roof sheets. The area where the
deceased was located had a concrete
floor, and there was a section of what
appeared to be rubber conveyor belting covering the concrete in the form
of a floor mat. The temperature on the
25th July, 1991 at approximately 2pm
was between 7°C and 10°C, with even
lower temperatures being recorded
earlier that morning.
(6). At the time the deceased was
working on this machine there were a
number of tools scattered around including screwdrivers, pliers and electrical type tools, but the only one considered to be dangerous was the high
voltage probe which was found to
have recent burn marks on the handle. It is probable that the electric
shock originated· from this piece of
equipment as the burn mark on the
handle matched the burn marks on
the hand of the deceased.
The Leader high voltage meter
probe, model LHM 80 would have
been used to check voltage within the
machine when it was operating. It
was manufactured by Leader Electronics Corporation, Japan, in approximately May, 1975. It is designed not
for checking voltage on an RF welder,
but for checking DC voltage of television sets. This model did not have
any insulating cover fitted over the
potentiometer adjustment screw, although since August 1985 this model
has been modified to include an insulating cover. Production of the modified unit was discontinued in April,
1991.
(7). It is probable that a person holding the lead of a high voltage probe
while kneeling or sitting on the concrete floor and the flexible lead or
alligator clip coming into contact with
a high voltage within the machine
could receive an electric shock sufficient to cause death.
(8). I find that based on a reasonable belief, for reasons unexplained,
the deceased has incorrectly used the
EHT probe thereby receiving an electric shock of sufficient intensity to
cause death.
(9). I further find that whilst the
external temperatures were cold, and
that the factory may also have been
cold, that there is no evidence to suggest that this was a factor contributing
to the cause of death, and that no
other person has contributed to the
cause of death.
Care required with high voltage probes
I am writing to ask for your assistance in alerting your readers to the
potential danger associated with
high voltage probes as used in testing television receivers, transmitters
and other equipment requiring high
voltage measurements.
The result of a recent coronial
inquest in this State has highlighted
the dangers associated with the use
of such test equipment. The victim
in this incident was a qualified "A
Grade" licensed electrical mechanic
who was testing a radio frequency
welding machine. He had extensive
experience in testing and repairing
this type of equipment.
This incident has highlighted the
need to ensure that: tools and equipment are kept in good working condition; equipment is used within the
manufacturers' guidelines; the earth
connection is intact and operative;
COMMENTS AND
RECOMMENDATIONS
The Leader High Voltage Probe LHM
80 is designed and manufactured by
Leader Electronics Corporation, Japan
and is marketed in Australia by AWA
Distribution. It i.s designed for measuring high voltage (+25,000 volts) in
colour TV receivers by qualified TV
technicians and is safe to do so.
Whilst the probe can be used for
other purposes, such as measuring
other high voltage sources, qualified
and experienced technicians using
this equipment should be aware of
the possible dangers involving the
measurement of dangerous high
voltages · as these high voltages can
result in the death of the operator if
used incorrectly. The dangers to the
operator could be measuring a negative supply exceeding 1,000 volts
whereby electrocution could occur
through the trimpot hole, or where
the "earth" clip falls off, where a minor electric shock could occur but
which could result in a secondary
injury occurring as a result of the operator withdrawing their hand.
This EHT probe and other similar
types are widely used throughout the
TV serving industry for the checking
of EHT in TV receivers for being ei-
extra care is taken when working in
the vicinity of live electrical equipment.
There has been an increase in
the occurrence of electrical fatalities and serious electrical accidents
involving electrical workers including engineers, technicians and mechanics in the past year.
We must all take greater care
when working with any electrical
equipment.
I request your assistance in warning your readers, many of whom are
involved with the testing, repair,
manufacture and design of electrical equipment, that caution should
always be exercised when working
on live equipment.
R.N. Clark,
Chief Electrical Inspector,
State Electricity Commission,
Victoria.
ther under or over voltage. They are
quite safe to use, and the Leader probe
falls into this category when used correctly.
It is recommended that all industries which use an EHT probe to measure high voltage be. advised of the
dangers of using an EHT probe. In
particular, attention is drawn to a new
service industry developing in microwave ovens. The high voltage in a
microwave oven is several thousand
volts, and. is negative. Its existence
can be measured with an EHT probe
used inversely which may prove fatal
if the insulation on the probe breaks
down, either through the probe handle if dirty or cracked, or through
trimpot holes, or the negative connecting lead if its insulation is damaged.
It is noted that a document has already been prepared by ·the Occupational Health and Safety Authority
alerting persons who use high voltage
meter probes of the dangers associated with their use. It is hoped that
distribution of this information to po- ·
tential users will reduce the likelihood of a further tragedy such as this
in the future.
S.A. Blashki,
Coroner
DECEMBER
1992
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