John Turner, of Kings High School, asks :-

Why can birds stand on power lines but humans get electrocuted?

John Campbell, who used to give a sobering talk, entitled 'Death by Electric Shock', to physics students at the University of Canterbury, responded.

Humans can safely sit on power lines provided they are highly trained and know exactly what they are doing. Transpower's skilled employees regularly do this in order to service the high voltage transmission lines without disturbing the supply of electricity to users.

First, what kills animals during an electrical shock? The main criteria is the flow of electrical current through the body. As reference figures, the current flowing in a one bar electric heater is about 4 amperes and the current flowing in a household light bulb is about half of an ampere. For humans a current of about one thousandth of an ampere (a milliamp) is on the limit of sensation. About 10 milliamps is painful, about 20 milliamps we cannot let go of the wire, about 30 milliamps causes muscular paralysis (a severe shock) and about 40 milliamps causes severe breathing difficulties.

For currents of between 100 and 200 millamps we generally die because the heart muscles no longer work as one, producing an unco-ordinated twitching of the walls of the heart's venticles (ventricular fibrillation), a condition which doesn't pump blood round our body. Surprisingly, humans, if resuscitated, survive higher currents because the heart muscles get mercifully clamped during the current flow so dont fibrillate. However in these cases humans are likely to be severely burned and die later of complications from the burns.

I dont know the figures for birds. However birds are usually small and stand on one wire with their feet close together. In this case there is little electrical force to drive electrical current through their body. Larger birds with their wings spread can span from one wire to its neighbour and then they are in severe danger.

It also depends on whether the wires are insulated with plastic which inhibits current flow. Many large birds and small animals do die when shorting out two wires. Repair people who know what they are doing can contact one wire but if they make a mistake they die.