Why did my light work when soaked?

10 February 2008

Question

I was always told that water and electricity don’t mix. After a second of stupidity I’m 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 fitting’s not a standard rose. It’s 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 didn’t the house short out? Why didn’t the light go out?

Answer

Chris: So Martin's ceiling rose almost became a shower rose, Dave. Why didn't 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 current's going to earth?Martin: No.Dave: Ok. If you haven't got a trip switch the only thing which is going to cause the electricity to turn off is if you're 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 there's less current going through the water than forty of fifty amps then your house just thinks someone's 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, something's wrong maybe the central heating system's exploded, we'll turn the power off now. Without a trip switch there's no reason why your power shouldn't carry on. Chris: These lights are definitely not 12V, are they?Martin: I've 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. They're very, very bright because all they do is draw a much bigger current but at a lower voltage. Because water isn't a terrifically good conductor like Dave said then at 12 volts it often won't actually make that much difference because there's not a big enough potential difference to flow the current through the water. So if it's 12 volts, that may be why it didn't go bang.

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