The hazard
A live wire looks exactly like a dead one, and your body can become part of the circuit. Electrical contact can cause shock, burns and electrocution, and even a small shock can make you fall from a ladder. How badly you are hurt depends on how much current flows, the path it takes through your body and how long the contact lasts.
The action
In general industry, a lock and a tag go on each disconnecting means used to shut off the equipment, with a few exceptions in the rule. The lock keeps anyone from operating the switch, and the tag prohibits operating the switch or removing the tag without authorization. Each lock and tag is removed by the person who put it on, or under that person's direct supervision (29 CFR 1910.333(b)(2)(v)(C)). Nobody works on the equipment until a qualified person has tested it and verified it is deenergized (29 CFR 1910.333(b)(2)(iv)(B)). (29 CFR 1910.333(b)(2)(iii)(A))
ASK THE CREW
What do you look for on a cord or tool before you plug it in?
From the talk: Electrical safety. Read it in full, free, with the questions and the sign-in sheet.