Another thing that can be done is to use what is called a crowbar. Basicly, totally shorting out the wiring and also shorting it to Gnd. There's different ways to do this depending on what your dealing with, and even when properly applied, a crowbar, may only protect you from having the power turned back on and remaining on before you work on it. But, may not be enough to prevent you from being electrocuted. It only takes milli-amps across a human heart to kill a person. So, while a crowbar is a good idea, its not a guaranteed of protection. Still, many use them, and in different forms for different applications. Sometimes its clamped on and very heavy wire with very heavy clamps are used. Sometimes a heavy cable or chain is thrown over a dead bare power line.
Their not always used. But they do have their place to help improve safety. The idea being that sure, you checked that the power is off before you start working on it. But if somehow the circuit was to become hot while your working on it, a crowbar would trip what ever brakes or fuse that feeds it. It might save you, it might not. Power should never come back on to a circuit being worked on. But theres people killed while working on circuits. It happens.
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Another thing to keep in mind, only one 120 VAC side of the 240 should be used per area / floor / room, what have you. When I worked at the power company, there was a worker there who used both sides of 240 to make two 120 outlets in the same room. He then used both for Christmas tree lights, got shocked and killed by those lights. If he had not put both sides of thar 240 in the same area, he may have not been killed.
Better yet would be all of one side ( 120 ), of the 240 per floor AND GROUND FAULTS ON ALL CIRCUITS THAT HUMANS INTERACT WITH.
those are the same gloves my wife wears in the flower bed.
