Sat 25 Aug, 2007 10:24 am
We've all been hearing about the floods in the Midwest. One report said a person was electrocuted when he stepped into some water where a live electrical power line had fallen.
My teenage grandson posed a question I couldn't answer. He asked just how far away will such a live electric line charge the surrounding water? Three or four feet? 10 feet? 25 feet? Is it safe to wade in water if the live wire is a 100 feet or so away?
A swimming pool for example. If a live wire fell into a pool of say 10 feet wide, 25 feet long and an average of six feet deep, would the whole pool be dangerously charged with electricity?
I suppose that a lot of factors would have to be considered, such as current flow, muddy or clear water, depth, etc., etc. But I don't know.
Like I always tell him when he stumps me, I say I don't know, but I'll try to find out.
Thanking everyone ahead of time for answers, comments, links, information, and opinions.
When a live conductor falls to earth, in a power system where there is an earthed neutral, current will flow through the earth back to the substation until the line is disconnected. On the surface of the ground around the point of contact there will be a live zone. In it there will be a "potential gradient", measured in volts (or thousands of volts if you like) per unit of length, feet, metres, whatever. A lot depends on how wet or dry the ground is, how salty the water is etc.
This potential gradient could be hundreds or thousands of volts per foot. That means that two points on the ground a foot apart may have a difference of voltage ("step potential") sufficient to drive a current through anything (or anyone) that bridges them. Such as a walking or standing person's legs. Up one and down the other. Given that 100 - 200 volts will cause fibrillation of the heart, it is possible for a lethal voltage to exist quite a long way from a fallen high voltage cable. Dead cattle are often found near fallen cables for this reason.
Power workers are trained to avoid potential gradient shocks by keeping their feet together and hopping like a rabbit in a non-radial direction to get out of the roughly circular potential gradient zone.
A person might be OK walking along dry ground but get a shock when they put one foot in a puddle.
The swimming pool would definitely be dangerous. People are killed every year by faulty wiring to pumps, underwater lights, etc. That's at 110 volts, which is quite low. The human body has a low resistance when wet, which increases the danger.
The answer to your grandson would be that it all depends on the voltage. Since power lines often carry thousands of volts, the danger could extend long way from the fallen wire. Easily 100 feet.