EFFECTS OF ELECTRIC CURRENTS ON HUMAN BODY

Bruner (1967) states that the threshold of perception of electric shock is about 1 m A. At this level a tingling sensation is felt by the subject when there is contact with an electrified object through intact skin. With the increase in the magnitude of ac, the sensation of tingling gives way to contraction of muscles. The muscular contractions increase as the current is increased and finally a value of current is reached at which the subject cannot release his grip on the current-carrying conductor, The maximum current at which the subject is still capable of releasing a conductor by using muscles directly stimulated by the current is called "let go current". The value of this current is significant because an individual can withstand, without serious after effects, repeated exposures to his 'let go current' for at least the time required for him to release the conductor. Also, currents slightly in excess of 'let go current' would not permit the individual to release his grip from the conductor supplying current.

Based on the experiments conducted on males and females, it is generally accepted that the safe 'let go current' could be taken approximately 9 mA and 6 mA for men and women respectively.

At current levels higher than the 'let go current' the subject loses the ability to control his own muscle actions and he is unable to release his grip on the electrical conductor. Such currents are very painful and hard to bear. This type of accident is called 'hold-on-type' accident and is caused by currents in the range 20-100 mA. These currents may also cause physical injury due to the powerful contraction of the skeletal muscles. However, the heart and respiratory function usually continue because of the uniform spread of current through the trunk of the body.

If current contacts contact the skin and passed through the trunk, at about 100 mA and above, there is a likelihood of pulling the heart into ventricular fibrillation. In this condition, the rhythmic action of the heart ceases, pumping action stops and the pulse disappears. Ventricular fibrillation is a serious cardiac emergency because once it starts, it practically never stops spontaneously. It proves fatal unless corrected within minutes since the brain begins to die 2 to 4 minutes after it is robbed of its supply of oxygenated blood.

At very high currents of the order of 6 A and above, there is a danger of temporary respiratory paralysis and also of serious burns. However, if the shock duration is of only a very few seconds, there is a possibility of heart reverting to the normal rhythmic action.

The threshold of perception depends largely on the current density in the body tissues. It may vary widely depending upon the size of the current contact. At very small point contact, it is probable that even 0.3 mA current may be felt whereas a current in excess of perhaps 1 mA may not produce sensation if the contacts are somewhat larger. Similarly, depending on the size of contact, the threshold of pain may also be considerably above 1 mA, probably 10 mA if the contacts are large enough.

Besides the magnitude of current, the current duration and the relationship of current flow resistance are also important. Duration of less than 10 ms typically does not produce fibrillation whereas the duration of 0.1 s or longer does. It has been found experimentally that the safe value of current in amperes (RMS) which a human body can tolerate is given as

where t is the time duration in seconds of the flow of current.

The tolerable currents mentioned above-are for power-frequency currents. It has been found that the human body can tolerate about 5 times higher current. At high frequencies (3-10 kHz) still higher currents are tolerable.