Science Questions

Why did my light work when soaked?

Sun, 10th Feb 2008

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Question

Martin asked:

I was always told that water and electricity donít mix. After a second of stupidity Iím not sure this is entirely true. I put a nail through my central heating pipe and a vast quantity of water then began to drop into the ceiling space and then went through the light fitting. The light fittingís not a standard rose. Itís got a one-inch deep bulb where the wires are connected between the mains and the halogen bulbs. The light was on at the time and it stayed lit for well over ten minutes until a friendly plumber arrived on site and almost had a heart attack realising the electricity was still switched on. Why didnít the house short out? Why didnít the light go out?

Answer

Chris: So Martinís ceiling rose almost became a shower rose, Dave.  Why didnít it go bang?

Dave: Do you have a trip switch in your house?

Martin: I do have fuses, yeah.

Dave: Oh.  Fuses are just things which will break if you draw too much current.  Have you got one of the trip switches, sort of residual current devices which will trip if any currentís going to earth?

Martin: No.

Dave: Ok.  If you havenít got a trip switch the only thing which is going to cause the electricity to turn off is if youíre drawing more current than one of the fuses can take so that would involve more than 40 or 50 amps of current.  As long as thereís less current going through the water than forty of fifty amps then your house just thinks someoneís plugged in a whole load of lights so maybe a heater in your ceiling rose.  If you did have a trip switch and the water was connected to something earth like a water pipe or central heating system to the earth then the trip switch would detect that there was some current running from the mains to the earth down the earth wire.  A trip switch would think oh no, somethingís wrong maybe the central heating systemís exploded, weíll turn the power off now.  Without a trip switch thereís no reason why your power shouldnít carry on.

Chris: These lights are definitely not 12V, are they?

Martin: Iíve not got a clue.

Chris: Because one of the other fancy things that people tend to do in kitchens and other places round the house is that they tend to have a 12 volt system running those little mini halogen lights.  Theyíre very, very bright because all they do is draw a much bigger current but at a lower voltage.  Because water isnít a terrifically good conductor like Dave said then at 12 volts it often wonít actually make that much difference because thereís not a big enough potential difference to flow the current through the water.  So if it's 12 volts, that may be why it didnít go bang.

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