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MODULE V
Electrical engineering - Basics Page 38
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6. Safety regulations, protection types, safety measures according to DIN/VDE
and UVV VBG 4
Under point 1. (introduction into electrical engineering) different effects
of electric current had been discussed.
The most common reasons for accidents with electric current are lack of knowledge,
negligence and carelessness. This is because human beings mostly are only able
to recognise the effects of electric current with their senses and not the causes
(the electric current itself).
Statistics and research studies that had been conducted by trade/professional
associations prove that only a small part of the accidents with electric current
had been caused by defective electrical installations or devices.
This means that in order to avoid accidents it is necessary that
- one acquires a good
knowledge about the possible dangers and most frequent causes for accidents
- one follows the rules
for the prevention of accidents (UVV)
6.1 Dangers of electric current
If a human being touches an electric conductor through which electric current
is flowing then this current will also flow through his/her body since the human
body also conducts electric current. In such a case, the applied voltage and the
total resistance of the body determine the size of the occurring amperage. The
total resistance depends on:
- the transition resistance
between the conductor and the human body
- the resistance of the
human body
- the transition resistance
between the human body and the conductor and/or between the human body and
the earth
Humidity strongly reduces
the transition resistance. The amperage is increasing with a decreasing total
resistance. For this reason, especially in humid rooms where the transition resistance
is usually very small the danger is particularly high.
International safety regulations specify that human beings can only be exposed
for a very short period of time:
- to an amperage of 30mA
- and to a voltage that
is smaller than 42V
If these limits are exceeded,
electric current can have the following dangerous effects:
- An electric current
that flows through a human body can have a very negative effect on important
body functions. As a result, e.g. muscle cramping, heart flutter and cardiac
arrest can occur.
- If the human body comes
in contact with very high amperages, the electric heat effect causes burns,
especially at the points where the current enters and leaves the body.
- If an electric current
flows for a longer period of time through a human body, the body fluids (among
other things the blood) are being decomposed by electrolytic processes. As
a result intoxications can occur which sometimes break out with a time delay
of several days. For this reason, a medical examination is absolutely necessary
after accidents with electric current.
- Accidents with electric
current very often cause so called "secondary accidents". An example for a
secondary accident would e.g. be if a service technician gets an electric
shock and is reacting uncontrolled and in panic. Because of this uncontrolled
wrong reaction he falls off a ladder or hurts himself with the machine he
was working with, etc....