To avoid the potential for electrical shock, make sure a circuit is dead before working on it.
Electrical current flows in a continuous closed path from the source through a device that uses the power and then back to the source. But electricity need not flow in wires to make the return trip to the source. It can return through any conducting body—including a person—that contacts the earth directly or touches a conductive material that in turn enters the earth.
If you accidentally become a link in an electrically live circuit, you’ll get a shock—or worse. The key word is “link.” To get an electrical shock, you must be touching a live wire or device at the same time you’re touching a grounded object or another live wire.
This may sound like a rather unlikely situation, but consider that whenever you’re touching any metal plumbing fixture, standing on the ground or on a damp concrete floor or patio, or partially immersed in water, you’re in contact with a grounded object. In other words, you’re satisfying one of the two requirements for getting a shock.
There may be two requirements for getting a shock, but there’s only one requirement for not getting one: Always make sure that the circuit you intend to work on is dead.