I'm just learning about lockout systems from convos on the internet, but the idea is you have a device that locks the equipment in shutdown mode. Apparently fairly common in power generation. The hasp (the part the shackle goes into) has multiple holes, and everyone doing maintenance has their own lock, which they all put on while they're in the danger zone.

You're supposed to remove your lock when you finish up, but it sounds like sometimes angry phone calls have to be made.