Working with electricity can be dangerous unless you adhere strictly to certain rules.

CAUTION: NEVER WORK ON ANY LIVE CIRCUIT, FIXTURE, RECEPTACLE OR SWITCH.

Your life may depend on it.

Safety rules you should follow whenever you're working with electricity include:

Always shut off power at the main disconnect before changing a fuse.
Always shut off power to the circuit before repairing or replacing a switch, receptacle, or fixture.
Always tape over the main switch, empty fuse socket, or circuit breaker, empty fuse socket or circuit breaker when you're working. Leave a note there so no one will accidentally turn on the electricity. Keep any fuses you've removed in your pocket.
Always check that the circuit is actually dead before you begin working on it. Use a circuit tested or voltammeter.
Always unplug any appliance or lamp before repairing it.