It might seem that if you lock out the supply breaker for the equipment you’re working on, that equipment is safe to work on. The reality is that locking out and tagging the supply breaker is only one step of many required to make that equipment safe to work on. And your approach to lockout/tagout can mean that breaker still poses a potential hazard to you.
Lockout/tagout is more than just a matter of making sure a lock is on the supply breaker. For starters, is that your lock?
Suppose you need to replace a feeder conductor. You go to the supply breaker and see it’s already locked out. To put your own lock on it, you’d have to find the owner of that lock, get him to remove that lock, put a multi-lock device on the breaker, then add both your locks. And then restoring the breaker to service involves the same rigamarole. Why not save time by just telling them to add you to their lockout?
For one thing, it’s illegal. According to OSHA 1910.147, you must use your own lock when isolating an energy source. And then there’s the flimsiness of relying on verbal communication, human memory, and someone else’s assumptions.