Technical Report
| Published date | 01 January 1980 |
| Date | 01 January 1980 |
| DOI | http://doi.org/10.1177/0032258X8005300111 |
| Subject Matter | Technical Report |
TECHNICAL
REPORT
"Charged" cars are no danger
You occasionally see them: cars dragging a"tail" of metal braiding
and hard rubber behind them, intended to dischargethe static charge
of the vehicle. The idea comes from the big tanker trucks, which
always drag along a metal chain touching the ground. But does this
"discharge band" really serve any useful purpose?
Engineers at the Siemens high-voltage laboratory in Berlin
investigated to what extent cars are charged while being driven.
Friction between the wheels and the road surface is the main cause
for the electrical charge. The rubber-tyred vehicle is electrically
insulated as it rolls on the roadway; it is - from a physical point of
view - a capacitor with a capacitance of about 100 pF. When the
weather is dry and the insulation resistance of the tyres is thus
sufficiently high the "auto-capacitor" will be charged to
approximately 10,000 V. The energy stored equals about 0.005 Ws.
This amount of energy is so small that it can be discharged via the
human body without causing any harm whatsoever.
If
a"charged"
car is touched with the hand, the energy will almost completely
discharge in about amicrosecond. Inadarkened room this discharge
could be perceived as very thin blue sparks, but in daylight or with
street lighting it remains invisible. The whole effect is thus similar to
the static charge experienced when one walks on synthetic carpets: it
is a nuisance, but harmless all the same.
The electrical voltage between the car body and the road surface
can always be measured in terms of tens of thousands of volts even at
high speeds since pointed or sharp-edged parts of the car body
provide for local discharging and prevent higher voltages from being
attained.
Itis questionable whether conductive bands can prevent cars from
becoming charged. The vehicle can only be charged when both the
road and tyres are dry - in the rain a good electrical discharge is
always available. Adry road surface acts as a high electrical
resistance, making it difficult to bring the charge from the car to the
ground.
For
the owner of a passenger car there is really only one piece of
advice: since it is impossible to protect oneself against spark
discharges resulting from static charging it is better to accept them
without fuss.
If
one is prepared for the tingling sensation it is easier to
take. People are not endangered by the charge on a car - regardless
of whether or not one attaches a"lightning conductor" to the car or
not.
Siemens engineers once again advise motorists caught in a
thunderstorm to drive to the nearest parking space and remain in the
car. Since the vehicle is a closed Faraday cage passengers are best
protected against lightning inside the car.
72 Police Journal January 1980
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