Apprentice path · Stage 1: Staying alive on the job · Lesson 3

Lockout / tagout: proving it is dead

Turning something off is not the same as knowing it is off -- you have to test it, and then prove your tester was working.

Why this matters on the job

Every year someone is hurt on a circuit that was switched off. The switch was in the OFF position. It was off. It was also still live, because of a second feed nobody looked for, or a breaker that failed, or a meter that had quietly died in the van.

How to think about it

  1. Find every source feeding the equipment -- including backfeed and stored energy.
  2. Isolate it and lock it, following the lockout procedure for that specific equipment.
  3. Have a qualified person test all the relevant exposed parts for absence of voltage.
  4. Prove the tester on a known live source before the absence test, and again after it.

Worked through

The test sequence is three steps, not one: known source, then absence test, then known source again. You check your meter on something you know is live, you test the equipment and read zero, and then you go back to the known live source and check the meter again. That last step is the one people skip, and it is the whole point.

Check yourself: You test the equipment and the meter reads zero. Are you done?

Answer: No. You have to prove the tester again on a known live source. Until you do, a reading of zero could mean the equipment is dead, or it could mean your meter is dead.

A dead meter reads zero on everything, including a live 480 volt bus. Proving it works after the test is the only way to tell the two zeros apart.

Watch out for: Off is not proof. A tag is not proof. A tag is a message to other people; it is not a test result, and it does not stop electricity.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.