Lockout / Tagout

Lockout/Tagout is the procedure for isolating a piece of equipment from all energy sources, locking that isolation in place so nobody else can re-energize while you’re working, tagging it so they know why it’s isolated, and verifying the equipment is actually at zero energy before you start. OSHA 29 CFR 1910.147 makes it federal law for employers to have a LOTO program; beyond legality, it’s the single most important habit between you and a survivable career in HVAC.

You will work on live equipment sometimes — voltage drop testing, amp draw measurement, and other diagnostic tasks require it. That’s covered in Chapter 58. This chapter is about every other time, which is most of the time: when you’re replacing, disassembling, or physically handling anything that can move, get hot, carry voltage, or discharge stored energy.

Why LOTO exists

The story is always some variant of:

Two techs on a rooftop unit. One is working inside the panel. The other, not knowing someone is inside, flips the breaker back on to test something. The tech inside gets hit with 240V.

Or:

Homeowner gets impatient, walks to the basement while the tech is working, and resets the breaker “to help.”

Or:

Tech assumes the thermostat at “off” means the equipment is de-energized. Reaches into the air handler blower compartment. The blower, running off a time-delay relay, spins up on its own and takes a finger.

Every LOTO procedure exists because a specific version of one of these stories ended with someone dead or maimed. It’s tedious and it’s inconvenient and it’s completely non-negotiable.

The core procedure

Basic single-source LOTO (one tech, one disconnect)

procedure

The try-test-try sequence is the step most often skipped and most often the one that kills someone. A meter with a blown fuse or dead battery reads 0 VAC on a live 480V service. Prove the meter works on known-live voltage before trusting the zero-reading.

Multi-source situations

Most residential HVAC has one primary energy source (the electrical disconnect), which simplifies LOTO. But some situations have more:

A dual-fuel heat pump with gas backup — which is increasingly common — requires locking out the outdoor electrical disconnect, the indoor electrical disconnect, AND closing the gas valve before any work. Three energy sources, three lockouts.

Capacitor energy — the “I thought it was dead” hazard

Motor-run capacitors on AC condensers and some blower motors can hold hundreds of volts after power is removed. The capacitor behaves like a small battery: isolation doesn’t discharge it, time alone often doesn’t fully discharge it, and touching the terminals can deliver a painful shock or — with larger caps — worse.

Always discharge capacitors before handling them:

Discharging a capacitor

procedure

The common direct-screwdriver discharge makes a visible spark and an audible snap on a charged run capacitor. If you don’t get a spark, either the cap was already discharged or it’s failed open — meter it to find out.

The discipline of doing it every time

LOTO fails when it becomes “sometimes I do it and sometimes I skip it on the small ones.” Your brain doesn’t remember whether you locked out on this particular call — it remembers whether you always lock out. Making it automatic is the point.

The worst accidents happen on the “easy” calls: change a capacitor, swap a contactor, replace a thermostat. The tech has done it a hundred times, skips the disconnect, reaches in, and today is the one where the disconnect was still hot because someone was working at the panel and bumped a breaker, or the capacitor had 200V sitting on it from an hour ago, or the blower’s TDR fires. The longer you’ve been doing the work, the more important the discipline becomes — complacency is the real killer, not inexperience.

When LOTO is not possible or not sufficient

Some diagnostic tasks require the equipment to be energized — you can’t measure 24 VAC on a call circuit with the equipment off. For those, see Chapter 58 on energized work. The key point: LOTO is the default, energized work is the exception, and the exception must be specifically justified for each task.

Some situations LOTO alone doesn’t address:

• Heat that stays hot for hours after shutdown (boilers, furnaces, strip heat) — add wait time
• Refrigerant pressure — separate recovery procedure
• Water / hydronic pressure — isolate and drain, or pressure-control
• Gas supply — shut at appliance valve, consider upstream cock

LOTO is about electrical isolation. Physical hazards need their own controls.

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Check your understanding

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01What is 'try-test-try' in the LOTO procedure?

ATry to turn the equipment on to see if it's really off, then test, then try againBVerify your meter works on a known-live source, test the equipment for voltage, then verify your meter still works on the known-live sourceCTry the disconnect three times to make sure it's openDA hand-signal communication protocol between two techs

02Another tech needs to work on the same equipment you've locked out. What do they do?

AAsk you to remove your lock so they can use their ownBWork without a lock since yours is already thereCAdd their own personal lock to the same hasp — your lock staysDAsk the supervisor to approve removing your lock

03You're about to replace a run capacitor on an AC condenser. Power has been locked out for 10 minutes. What's your next step before handling the cap?

AJust remove it — ten minutes is plenty for dischargeBDischarge it with an insulated screwdriver or dedicated discharge tool, then verify 0 VDC across terminals with your meterCSmell it to check if it's leakingDWait another ten minutes to be sure