Testing for Absence of Voltage
Turning off a breaker and putting on your lock isn’t enough. Breakers get mislabeled, circuits get backfed, and switches fail. Before anyone touches a conductor, a qualified person must test it and prove it’s dead. “Test before touch” is the last step in creating an electrically safe work condition.
- Electricians
- Maintenance
- HVAC
The steps of an electrically safe work condition
- Identify every source of power, using up-to-date drawings and labels.
- Shut off the load, then open the disconnect for each source.
- Visually verify the disconnect blades are open or the breaker is in the open position, where possible.
- Release stored energy (capacitors, springs) as needed. See Capacitors.
- Apply lockout/tagout devices. See Lockout/Tagout.
- Test for absence of voltage, wearing the PPE required for live work.
- Ground the conductors where induced voltage or stored energy could be present (especially above 1,000 volts).
Live-dead-live
- Live: test your meter on a known live source or proving unit to confirm it works.
- Dead: test the circuit you’re going to work on.
- Live: test the meter again on the known source, to make sure it didn’t fail during the test.
If the meter fails the second check, your “dead” reading means nothing. Start over with a working meter.
Test every conductor
- Test phase to phase and phase to ground for every phase, and phase to neutral where there is one.
- Test at the point of work, not just at the disconnect.
- Watch for backfeed from generators, solar, UPS units, control power from another panel, or multiple feeds.
Use the right tester
- A meter rated for the voltage and the measurement category (CAT III or CAT IV) of where you’re testing. See Test Meters.
- Leads in good condition, with finger guards and shrouded tips.
Non-contact voltage testers (pens) are not enough to prove a circuit is dead. They can miss voltage on shielded cable, DC circuits or with a weak battery. Use them only as a quick first check, then test with a contact meter.
Permanently mounted testers
Some equipment has a permanently mounted absence of voltage tester (AVT) that checks itself and the conductors with the push of a button, without opening the enclosure. When listed for the purpose and installed correctly, NFPA 70E allows it to be used for the test.
Before you touch
- All sources identified and locked out
- Meter rated for the job and verified on a known source
- Every conductor tested at the point of work
- Meter verified again after testing
- Stored energy released and grounds applied if needed
- PPE worn until absence of voltage is confirmed
Standards & references
- 29 CFR 1910.333(b)(2)(iv): Verification of de-energized condition
- NFPA 70E: Establishing an electrically safe work condition
- See also: Energized Work Permits · Arc Flash
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