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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 80 milliamps.

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 damage.

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 respiration.

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.


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Source - Contactum Distribution