ELECTRIC SHOCK
The number of electrocution ( i.e. deaths from electric shock ) per annum of people at work is, fortunately, small and has never yet attained the three figure, but electric shock accident are common and are frequently accompanied by burn injures at the point(s) of contact.
SHOCK PARAMETERS
Electric shock data have been compiled by a number of researchers who have experimented mainly on animals. Their results are not entirely satisfactory because if the difficulty of relating animal response to humans. Some work, up to ‘let go’ current severities, has been carried out on human volunteers and has been useful but there are still unexplored areas and the data is incomplete. What follows, therefore is based on present knowledge and logical probability.

The body is a non-linear resistor with an inverse characteristic with respect to the applied voltage. The resistance varies also with sex, state of health and between individuals. Most of the resistance is in the skin. Under dry conditions, point contacts and safety extra low voltages, the resistance may be thousand of ohms and there may be no shock sensation but a wet skin considerably reduces the resistance and a slight shock may be felt if sufficient current flows between the contact points. The shock sensation is frequency dependent and is at maximum at about the mains frequency of 50HZ and diminishes at higher or lower frequencies. The shock sensation disappears at frequencies exceeding about 1.0MHZ and also on a ripple-free d.c. However, contact with a d.c conductor produces an initial shock as the current rises from zero to a steady state value and another when contact is broken and the current again drops to zero. Any sudden change in current produces a shock sensation, so shocks are experienced from pulsating or oscillating current but not from a pure d.c and not anomalously from a high frequency a.c.

There is considerable difference in response to body currents between individuals but in general and for mains frequency, i.e. 50HZ in Nigeria, the responses to sustained current are as follows:

(1) 0.5 to 2.0mA is the threshold of perception with no deleterious effect or unpleasant sensation.

(2)Over 2.0mA the shock sensation increases with increase in current from a mild to a painful shock and at about 9.0mA for a
man and 6.0mA for a woman, it begins to become difficult to break contact, due to partial paralysis.

(3)At 20mA and over, muscular paralysis inhibits the ability to ‘let go’ and the victim is liable to become unconscious through
paralysis of the motor nerves. He may stop breathing and/ or his heart may stop beating.

(4) At current in excess of about 50mA, ventricular fibrillation, i.e. uncoordinated operation of the heart valves, may occur and
the victim’s chances of survival are poor as this condition needs immediate specialized treatment.

Time is however another factor in electric shock danger. It is known that considerably shock currents can be tolerated for a very short time and it is possible to produce safety curves of shock currents plotted against time. These effects are used by designer of RCDs(residual current device) to determine the performance characteristics of their products.

Lest I forget, harmless momentary shocks, however, can cause accident because of the involuntary reaction of the victim; for example, a momentary shock can cause the victim to fall of a ladder an injure himself.

cc: lalasticlala
dominique
sissy3