Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 08/01/2015 12:30:01
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Rosemarie Parsons asked the Naked Scientists:
I was in the yard and cut a branch and also the electric cord which was under the branch. The sheers I was holding has rubber grips and I was wearing gum boots. I saw the spark and loud bang but didn't feel any shock go through me. The outside power points are not connected to a cut-off switch. Why didn't I feel anything in my body?
What do you think?
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The electric cord contains two or three wires (depending on wiring practices in your country).
When the shears bit into the first wire, it may have connected the shears to:
- ground, which is very safe, and you would not have been hurt, or even felt anything
- or "live" voltage (eg 110 or 230V AC, depending on your country). However, the rubber grips on the shears, and plastic gum boots would have prevented the current passing through your body to ground. So you wouldn't have felt anything in this case, either.
When the shears bit into the second wire, they would have connected the first wire to the second wire, causing a high current to flow, producing the spark and the bang that you experienced. This current would have been broken within 1 second by a fuse blowing, or by the shears cutting all the way through the cord.
In many places, they are now making residual current breakers mandatory, and progressively replacing aluminium/aluminum ladders by fiberglass, as this provides more layers of protection against electrical shock, without totally eliminating the risk.
But it's best not to touch or cut electrical cables! A puncture in the gum boots, holding the shears by the metal arms or leaning against the tree may have produced a very different outcome.
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Also note that the modern equivalent of a fusebox is a consumer unit (http://www.expertelectrician.co.uk/installations/26-understanding-modern-consumer-units-fuse-box):
"The next point the power goes to are the RCDs (Residual Current Device) where both live and neutral cables enter the safety device. The RCD automatically cuts power if there is a difference detected between the live and neutral supply (some power is flowing somewhere other than the designed circuit). It operates in a fraction of a second in the event of a fault minimising damage to cables, appliances and most importantly keeps the user as safe as possible..."
If you have one of these, you have less chance of getting electrocuted.