3.4.1 - The nature
of electric shock
The nervous system of the human body controls
all its movements, both conscious and unconscious. The system
carries electrical signals between the brain and the muscles,
which are thus stimulated into action
The signals are electro-chemical in nature,
with levels of a few millivolts, so when the human body
becomes part of a much more powerful external circuit, its
normal operations are swamped by the outside signals. The
current forced through the nervous system of the body by
external voltage is electric shock.
All the muscles affected receive much stronger
signals than those they normally get and operate very much
more violently as a result. This causes uncontrolled movements
and pain. Even a patient who is still conscious is usually
quite unable to counter the effects of the shock, because
the signals from his brain, which try to offset the effects
of the shock currents, are lost in the strength of the imposed
A good example is the 'no-let-go' effect.
Here, a person touches a conductor which sends shock currents
through his hand. The muscles respond by closing the fingers
on the conductor, so it is tightly grasped. The person wants
to release the conductor, which is causing pain, but the
electrical signals from his brain are swamped by the shock
current and he is unable to let go of the offending conductor.
effects of an electric shock vary considerably depending
on the current imposed on the nervous system, and the path
taken through the body. The subject is very complex but
it has become clear that the damage done to the human body
depends on two factors:
the value of shock current flowing, and
2. - the
time for which it flows.
These two factors have governed the international
movement towards making electrical installations safer.