Electrical System Basics
Electricity is supplied by a local utility to a house through three underground or overhead wires (two "hot" leads and one neutral lead) that enter the house through a conduit and a meter.
Those wires connect to their respective buss bars inside the service panel -- usually two hot, one neutral, and one ground buss.
Circuit breakers slide/snap onto the hot buss bars. They act as safeguards against short circuits and overloads by "tripping" off. A breaker also functions as a switch; turning the circuit on and off as desired.
Breakers also connect to outgoing "hot" wires. The hot wires deliver power to a device (like a light) and normally have black insulation. Cable with two hot leads also have a red hot lead.
Once the electricity has done it's work, it goes back to complete the circuit on the "neutral" wire, which is most often white. Electricity needs this completed circuit to work properly -- a way OUT through the hot wire, and a way BACK through the neutral wire.
In addition to the neutral, the green (or bare copper) ground wire offers current another path back should an electrical short or overload happen.
From the service panel, the ground has two safe paths to divert electricity: connected to a long metal rod buried outside the house and/or the house's water pipes.
All the wires, called cable, are often housed by a flexible plastic sheathing. It's nonmetallic-sheathed (NM) cable, but is often mistakenly called "Romex" which is a brand name made by General Cable Corporation.
Cable is also identified by gauge (thickness) and the number of leads it has. For example, NM 14-2G means that the cable is nonmetallic, 14 gauge, has two leads (1 neutral, 1 hot), and a ground wire.
If working with electricity scares you, a voltage/neon-light tester can help change that. It's an inexpensive, but invaluable tool for determining if a circuit is "live" or "dead."
It's basically just a small neon light bulb attached to two wires. When the contacts on the wires are touched to a live circuit, the light goes on. And when there's no juice coming to the circuit, the light stays off.
But it's important to get into a good habit of always testing switches, outlets and wires before your hand actually touches them.
You can test an outlet without taking off the cover plate, but also check its screw terminals. For that and for switches, you'll have to take off the cover plate.
To make sure the power is off before you work on an outlet, test between the screws on each side, and between the screw on the shorter slot side and the green ground screw. No light means the "juice" is off.
To check for proper grounding, test between the shorter slot (hot) and the round hole (ground). If the tester lights up, there's probably proper grounding. It should also light when you test between the shorter slot and the cover plate screw.
To check that the power is off to a switch, check between both terminals on the switch and then between the copper ground lead and each terminal.
For bare wires, hold one tester lead on the bare ground wire (or box if it's grounded) and the other test lead on the hot, then neutral wire. Also check between the two leads. If the light stays off, the circuit is off. Plug-in circuit testers that fit right into outlets can tell you a lot. On our tester, two amber lights mean everything's ok.
Other combinations of lights indicate different potential problems with a circuit -- like an improper ground. They're handy for checking and diagnosing connections when you're installing several new cable runs and circuits.