Rule 4 of 40 · Chapter I — Before You Touch Anything
Lock out and tag out the source
Why this rule exists
De-energizing protects you only while the source stays off. A colleague flips a breaker, a timer closes a contactor, or another crew restores power on schedule, and the circuit you are holding goes live. Lockout and tagout, required by OSHA for servicing and maintenance, puts a physical lock and warning tag on the disconnect so it cannot be re-energized without your knowledge. The lock is yours, the key stays in your pocket, and the tag names you. Verifying dead comes after the lock, never instead of it.
In practice
Identify every energy source, not just the obvious one. Open each disconnect and apply your own lock and tag. If several people work the same equipment, each applies a personal lock to a group hasp: one lock, one worker, one key. Try to start the equipment to confirm the isolation worked, then return controls to off. Verify dead at the point of work. When finished, only the person who applied a lock removes it. Never remove someone else's lock to save time.
When it doesn't apply
Cord-and-plug equipment can use the plug itself as the isolating means if it stays under your exclusive control. Stored energy, capacitors, springs, elevated loads, must be discharged or restrained; a lock does not drain a capacitor. When a lock must be removed and the owner is gone, follow a documented removal procedure, not bolt cutters and hope.