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I recently replaced a couple of the really old lightswitches in my house. Shortly afterwards I heard Shirlock Homes on the StackExchange podcast and he mentioned that you can shock yourself if you wire it up incorrectly.

How can I verify that I have in fact wired my lightswitches safely?

NOTE: I'm sorry I don't know what information to include, I bought lightswitches that looked the same, and copied the wiring.

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If you used the existing wiring, you're likely fine. @Shirlock was talking about switching on the neutral wire instead of the hot. If a professional wired up the house originally, there shouldn't be a problem. –  Tester101 Sep 21 '11 at 18:11
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I'm sure glad you guys were listening to the podcast. Tester and BMitch got you covered Nathan –  shirlock homes Sep 22 '11 at 5:05

2 Answers 2

up vote 4 down vote accepted

As Tester says, Shirlock is referring to a switched neutral. The switch should be stopping the current on the black line from getting to the light when turned off, but there are lots of ways to wire a switch where the wire colors don't help. In particular, if the wiring begins in the light fixture and uses a single pair of wires to the switch, you'll only have a black and white wire in the switch J-box.

To know if you have a switched neutral, you'd need to check if there is any voltage to ground from inside the switched outlet or light socket when the switch is turned off. If it's a light socket and you don't have a ground, you can use the large pin on an extension cord. Just use a voltage tester to see if there's any current, they are cheap and I'd consider it a required tool if you're doing any electrical work.

But Tester is also right that if you had a semi-competent electrician wire your home, and replaced the switch with another and using the same wires, you are unlikely to have this issue. And if you never stick your finger in the light socket, you'll be unlikely to ever realize you had this problem.

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I'm not sure exactly what you have in your switchboxes and light fixture boxes but I'll explain some simple ways, provided we're not talking about three-way switches. Also, some switch boxes won't have a neutral wire. Those switch boxes will have a single black wire going to one screw of the switch and the white wire going to the other screw on the switch. The most common configuration in a switchbox is a source wire (black/white/ground) coming from the breaker panel and another wire (black/white/ground) going to the light.

One way to check is to turn off the power and pull the switches you replaced out of their boxes - but don't unwire them. The black wires should be on the switch and the white wires should be twisted together in a wire nut. Black is (almost) always hot and white is (almost) always neutral on a single pole switched circuit.

Perhaps the best way to check is to turn off the power and unwire the lights that are controlled by the switches you replaced. Next, turn the power back on and make sure the light switches are in the off position. Then test both the black and the white wires at the light fixtures and make sure they are dead. If either wire at the light fixture is still hot then the switch is wired incorrectly. If one of the wires is still hot at the light fixture with the switch in the off position, then you most likely have the light switch on the neutral wire. In which case you will need to turn the power back off and unwire the switch and the other wire that's twisted together in the wire nut. Separate the bare ends of the wires so you can safely turn the power back on. With the power back on, test the wires in the switch box and determine which one is hot. Whichever wire is hot, regardless of color, should be the one you connect to the switch. Always place the switch on the hot, never the neutral as this allows the light fixture box to be energized at all times. Some people will turn a light switch off to replace a light fixture and this is how accidents happen.

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