If a person comes into contact with metal which is carrying
electric current and at the same time is earthed either through
the other hand or the feet, a current will flow through the
body dependant on the resistance of the body and the voltage
of the supply. At 240 volts this current could vary between
240 thousandths of an amp. shortened to 240 milliamps and
The first effect of such a shock is for
the hands - ( if gripping the live metal ) - to close due
to muscular contraction and prevent the hand being removed
even though it is very painful.
If the live metal is touched by some other
part of the hand, the hand will be thrown off violently which
may also cause the body to be thrown with consequent physical
If the current is limited by shoes or
clothing the first effect on the body may be muscular contraction
and restriction of breathing, causing unconsciousness. Providing
the person is removed from the electrical supply or the supply
is switched off soon enough recovery may be effected by artificial
For higher currents and / or longer periods
of contact the heart may be affected and go into fibrillation
which causes it to beat out of sequence. This in turn restricts
the circulation of oxygen in the blood round the brain and
death can occur within a couple of minutes.
Unfortunately, once fibrillation occurs
the heart may not recover even if the supply is removed and
only the use of a defibrillator which, of course, would not
usually be available except in a hospital, will stop the heart
and then allow it to start again in its normal rhythm.
At higher currents, which would normally
need higher voltages, burning of the skin could occur serious
enough to cause death. This last phenomenon would not occur
at voltages up to 415 volts.
Firstly to have a low
enough voltage that a dangerous current could not flow though
the body. This would be 50 volts or below. Although this is
used in special cases it is not practical for normal domestic
or commercial premises.
Secondly to prevent people coming
into contact with dangerous voltages. this is the object of
barriers and enclosures. As we know from experience these
are not foolproof.
The third and only practical way
on mains voltage is the RCD which will detect that
a fault current is leaking to earth and then cut off the supply
rapidly enough to prevent dangerous effects.