When an electrical problem occurs, such as a short circuit, it's important to turn off the power quickly to prevent a fire. You'll also need to shut off power whenever making repairs or replacements on an electrical circuit. The fastest way to do this is to turn off the main breaker in the panel or, if you have a fuse box, remove the main fuse. But this will shut off power to the entire house, so it's only for emergencies. The better solution is usually to shut off power to an individual circuit by shutting off the breaker or removing the fuse that controls that circuit. Always test lights, switches and outlets for power before working on them, even if you're sure you shut off the right breaker.
Get Familiar With Your Breaker Box
The control center for your home's electrical system—the main electric panel—contains circuit breakers that switch off the current to individual circuits. Circuit breakers are specialty switches that can be switched on and off manually to control power to individual circuits, but they are also safety devices that switch off automatically when they sense problems, such as short circuits, power surges, or excessive power draw. If you don't know where the main panel is, note where the electrical wires from the power lines are attached to your house. The panel should be on an inside wall very close to that point. Sometimes it's in a bedroom closet or some other unexpected place, but usually, it's in the basement, garage wall, utility room, or some other similar spot. It may even be on the outside of the building. Find it and note its location so you don't have to search for it in an emergency.
When you open the panel door, you'll see an array of individual switches. Each switch is a circuit breaker, and you turn off power to whatever individual the breaker controls by toggling the switch toward the outside of the panel. The larger breaker at top of the panel is the main circuit breaker; it controls all the power flowing down to the individual breakers. On the inside of the panel door, there should be an index that tells you which circuit each breaker controls. If you don't see this index, or it's outdated, it's time to make a new one by cataloging every device in the house that goes off when you flip off each of the breakers in the panel.
Unless they have had electrical updates, homes built before 1960 may have fuse boxes, which are the precursors to circuit breaker boxes. The fuses are heat-sensitive elements enclosed in glass and porcelain, and you switch off power to a circuit by unscrewing and removing the fuse that controls that circuit. Like breakers, fuses also serve as safety devices that burn through and stop the power flow when they sense electrical problems. Because each fuse is encased in insulating material, it's safe to touch one with your bare hand, but it's wise to wear a rubber glove just in case. You should also wear rubber-soled shoes whenever you're touching fuses or circuit breakers.
Shutting Off Power
Now that you're familiar with your panel, and it has been properly indexed, you can easily shut off power to any device in the house. Simply locate the breaker that controls the circuit and flip it toward the outside of the panel. It may have an OFF marking on the swtich, too. Be sure to toggle it far enough to produce an audible click. Turning the power back on is simply a matter of pushing the switch back to the ON position.
If an individual outlet is fitted with a GFI (ground-fault interrupter) receptacle, you can turn off the power to this outlet by tripping the GFI. To trip a GFI, press the top of the two buttons you see on the front face of the receptacle until you hear a click. Depending on how the GFI has been installed, this will shut off power to just this outlet, or it may also turn off power to lights or receptacles "downstream" from the GFI, if they have been wired to feed from the GFI. When you want to restore power, press the bottom button.
GFI outlets are required in wet locations, such as bathrooms and kitchens, as well as on outlets located outside or below grade. Each has an internal, resettable breaker built into it. There is often one or more GFIs on a circuit that also has lights and non-GFI outlets.
Always Test the Voltage
After shutting off a breaker, it's important to test for voltage before you start disconnecting wires. A mistake in the panel index or a faulty breaker could result in shock, injury or a fire if you forge ahead without checking first. A non-contact voltage tester is the safest, easiest tool to use for this. It has a single probe that detects the magnetic field around a live wire. If you hold the probe close to a live wire or terminal, an LED on the tester illuminates. A two-wire tester is also reliable, but each probe has to be in contact with a bare wire or terminal before the LED will come on. If you're working on a receptacle, you can test it with a plug-in receptacle tester or by plugging in a light or appliance you know to be working.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.