Verifying zero energy and controlling stored energy

Locks prove that someone operated an isolating device and secured it. They do not prove that the energy is gone. Disconnects fail, breakers are mislabeled, valves leak, a second feed was missed, a capacitor is still charged or a part is held up only by hydraulic pressure. Verification is the step that turns an assumption into a fact. This lesson explains how to verify a zero energy state and how to control the stored energy that remains after isolation.

Why verification matters

Investigations of hazardous energy incidents often find that the worker did lock something out, but the wrong thing, or not everything. Common findings include a breaker that was labeled for the wrong machine, a disconnect whose handle moved to off but whose internal blades stayed closed, a separate control circuit or heater supply that was never isolated, and a pneumatic valve that was closed but left air trapped downstream. In each case, a proper verification step would have revealed the problem before anyone was exposed.

The try step

For machines, the first verification is usually the try step: attempting to start the equipment with its normal controls after isolation and lockout. Done properly, it looks like this:

• make sure everyone is clear of the machine and its danger zones, and tell them you are about to try it;
• check that no one else is working on connected equipment that the try could affect;
• press the start button, operate the controls or call for the cycle, including any secondary or remote controls;
• watch and listen for any movement, sound, indicator light or pressure change;
• return all controls to the off or neutral position, so that the equipment will not start unexpectedly when the locks are later removed.

If anything moves or energizes, stop. Something has been missed. Recheck the procedure and the isolation before going further, and report the finding. A try step only tests the circuits the controls use; it does not prove that every conductor or line is dead, so it is combined with other checks.

Verifying electrical isolation

For electrical work, the verification must be done by a qualified person with a properly rated voltage tester. The widely used method is often called live-dead-live:

• test the tester on a known live source of similar voltage to prove it works;
• test the circuit you will work on, phase to phase and phase to ground on every conductor, to confirm there is no voltage;
• test the tester again on the known live source, to prove it did not fail during the test.

The tester must be rated for the voltage and measurement category of the installation; meters listed to UL 61010-1 and marked with an appropriate category (CAT III or CAT IV) are used for industrial distribution equipment. OSHA 1910.333(b)(2)(iv)(B) requires a qualified person to verify de-energization with test equipment and to check the tester before and after where the voltage is over 600 volts. NFPA 70E sets out the full process for an electrically safe work condition, including PPE during testing, covered in HSE-214. Some installations have permanently mounted absence-of-voltage testers listed to UL 1436.

Unqualified persons should never open electrical enclosures to test. In many workplaces the electrician performs the electrical verification while the mechanical trades verify their own sources.

Controlling stored energy by type

Stored energy must be released or restrained, and the result verified, before work. The method depends on the type:

• Pneumatic: close and lock the supply valve, then open a bleed or exhaust valve to vent the system to atmosphere; watch the gauge fall to zero. Remember that air can be trapped in cylinders between closed valves; some systems need bleeding at several points.
• Hydraulic: stop the pump, isolate it, then open the accumulator dump valve or bleed valve and confirm the gauge reads zero. Lower or block anything held up by hydraulic pressure before releasing it.
• Gravity: lower raised parts to their mechanical rest position or install rated blocks, pins or props, then lock or tag the blocks.
• Springs: release tension or compression using the manufacturer's method, or restrain the spring so it cannot move.
• Rotating parts: wait for a complete stop, apply a brake or block, and confirm by observation or with a tachometer.
• Capacitors: wait the time stated by the manufacturer, then verify with a suitable meter that the voltage has fallen to a safe level; some equipment has discharge points or grounding devices.
• Thermal: isolate heat sources and allow the equipment to cool or warm to a safe temperature, confirmed by measurement.
• Chemical and process: drain, vent, purge and flush lines and vessels; where people will enter, test the atmosphere under confined space rules (HSE-211).

Where stored energy could build up again, for example pressure from a leaking valve, gas from a reaction or charge from induced voltage, verification must continue at intervals during the work, or the system must be kept open to atmosphere or grounded.

Who verifies, and how it is recorded

Each authorized person is responsible for satisfying themselves that the equipment is safe before they start work. In a group lockout, the person in charge usually performs or witnesses verification, and each worker may also verify or confirm it before applying their lock to the group box. Many procedures include a verification column where each check is initialed, and some sites use a lockout permit or tag that records the verification. Records do not replace the act of verifying, but they help prevent the step from being skipped.

When verification fails

If the try step causes movement, a gauge does not fall to zero, or a tester shows voltage, treat it as a near miss. Stop, keep everyone clear, recheck all isolation points, and report it. Find out why: was a source missing from the procedure, was a device mislabeled, did an isolating device fail? Fix the cause, update the procedure if needed, and repeat the full verification before starting work.

Verification for process systems

Pipes and vessels need their own kind of verification. After isolating and draining a line, the authorized person typically opens a vent or drain valve between the isolation and the work point to confirm there is no pressure and no flow, checks pressure gauges, and, where hazardous chemicals were present, confirms that the line has been flushed or purged. For entry into tanks and vessels, atmospheric testing for oxygen, flammable gases and toxic substances is required before and during entry. If a valve leaks through, the vent will show it, and stronger isolation such as a blank may be needed.

Verification and new technology

Modern equipment brings new challenges and new tools. Programmable controllers can hold a machine in a stopped state while still energized, and network connections can allow remote commands, so verification must confirm that energy, not just the control signal, has been removed. On the other hand, permanently mounted voltage indicators, pressure transmitters visible at the isolation point and absence-of-voltage testers can make verification faster and safer, because the worker can confirm isolation without opening enclosures. These devices support verification but do not replace the procedure: each one must itself be checked and maintained.

If you are not qualified to perform a particular verification, such as an electrical test, you still have a role: confirm with the qualified person that the test was done and what it showed, look for the verification record, and do not start until you are satisfied. Never accept 'it should be dead' as verification.