I'm only replying for visibility at this point because of the upvotes and the dangerous scenarios that are being promoted with this advice. Breaker with LOTO would be more acceptable but even then I wouldn't trust it. People have been killed because something was wired up wrong and circuits were accidentally connected (meaning breakers off but currents still going from a connection down the line to another breaker, it can appear that the circuit is dead until a switch is flipped sending current and happens in home lighting constantly). Also, someone can simply plug the device into another outlet leaving them with a death on their hands.

I know it's dramatic and these are just comments, but something like this could save or take a life. I've seen too many workplace accidents and refuse to work in unsafe conditions or environments anymore even if it costs me the job. Last unsafe worksite I was in, nearly all of the workers walked out together when we arrived in the morning to start at a new site and saw what the project manager had in store for us (probably helped that the owner was hated and payroll had been getting screwed up for weeks). I'll never forget that day and still keep in contact with most of them. We are all waiting and scared for the day a mass casualty is reported from the shitty work they were trying to make us do (some did report it to the authorities, but never heard what came of those reports).

Exactly this. Everyone working on that machine slaps their lock on it, and every last lock needs to be removed before the tag can come off. The welders might finish in half a day, but the electrical or water or hydraulic guys might need a whole weekend to get done, so this makes sure someone doesn't say "oh the lock is gone" and make mincemeat out of some dude's head.