Hazardous energy is any source of electrical, mechanical, hydraulic, pneumatic, chemical, or thermal energy that could injure a worker if unexpectedly released during equipment servicing or maintenance. It’s the reason a machine can hurt or kill someone even after being “turned off,” and it’s one of the most serious and commonly cited workplace safety hazards in the United States. Failure to properly control hazardous energy causes an estimated 120 deaths and 50,000 injuries every year in American workplaces.
The Seven Types of Hazardous Energy
OSHA recognizes seven broad categories of energy that can be hazardous. In practice, a single piece of equipment often involves more than one type at the same time.
- Electrical: current flowing through wiring, circuits, or components. Even after a machine is powered down, capacitors can hold a dangerous charge.
- Mechanical: energy from moving parts like gears, belts, flywheels, or springs. A compressed spring stores energy that releases as sudden, forceful movement when the tension is freed.
- Hydraulic: energy stored in pressurized liquid used to move heavy loads. Examples include power presses, hydraulic lift systems, and the raised beds on grain trucks.
- Pneumatic: energy stored in pressurized air or gas. Air compressors, spray devices, and air cylinders all hold pneumatic energy.
- Chemical: energy released through reactions, such as combustion, corrosion, or toxic gas generation when chemicals come into contact with each other or with air.
- Thermal: extreme heat or cold in systems like boilers, furnaces, freezers, or steam lines that can cause burns or cold injuries.
- Gravitational: the potential energy in any elevated or suspended object. The heavier the load and the higher it sits, the more dangerous it becomes if it falls.
Why “Off” Doesn’t Always Mean Safe
The core danger of hazardous energy is that it can persist after a machine’s power switch is flipped. This leftover energy, called stored or residual energy, is what catches workers off guard. A flywheel keeps spinning after the motor stops. A hydraulic cylinder still holds a load in the air. Pressurized air remains in a line even after the compressor shuts down. Capacitors in electrical panels can deliver a fatal shock minutes after disconnection.
Workers injured by the unexpected release of hazardous energy lose an average of 24 workdays recovering. Many of these injuries involve amputations, crushing, or electrocution, and they disproportionately happen during routine tasks like cleaning, unjamming, adjusting, or lubricating equipment.
How Hazardous Energy Is Controlled
The primary method for controlling hazardous energy is a procedure called lockout/tagout (LOTO). The idea is straightforward: before anyone works on a machine, every energy source feeding that machine gets physically disconnected and locked in the off position. A personal lock, placed by the worker doing the job, prevents anyone else from turning the energy back on while work is underway. A tag identifies who placed the lock and why.
OSHA’s standard for hazardous energy control (29 CFR 1910.147) lays out six steps that must happen in order:
- Preparation: The worker identifies every type and source of energy connected to the machine, along with the specific hazards and the methods to control them.
- Shutdown: The machine is powered down using its normal stopping procedure, done carefully to avoid creating new hazards from an abrupt stop.
- Isolation: Every energy-isolating device (circuit breakers, valves, disconnects) is physically located and switched to cut the machine off from its energy sources.
- Lock and tag application: The authorized worker attaches a lock and tag to each isolation point, making it impossible for someone to re-energize the equipment without removing the lock first.
- Stored energy release: Any residual energy still in the system gets relieved, disconnected, or restrained. This might mean bleeding hydraulic pressure, grounding electrical capacitors, blocking elevated parts so they can’t fall, or letting a flywheel coast to a full stop.
- Verification: Before any hands touch the machine, the worker confirms that the isolation actually worked, typically by trying the normal operating controls and confirming the machine does not respond.
That last step is critical. Skipping verification is one of the most common ways LOTO procedures fail, because a worker assumes isolation was successful without actually testing it.
Who Needs to Know About It
OSHA’s standard applies to general industry workplaces where machines and equipment are serviced or maintained. It covers a wide range of activities: installing, adjusting, inspecting, modifying, cleaning, unjamming, and performing tool changes on equipment where unexpected startup or energy release could hurt someone.
The standard draws a distinction between two types of workers. “Authorized” employees are the ones who actually perform the lockout. They need detailed training on recognizing every type of hazardous energy in their workplace, understanding how much energy is present, and knowing exactly how to isolate and control it. “Affected” employees are those who operate or work near the locked-out equipment but don’t perform the lockout themselves. They need to understand the purpose of the procedure and know never to attempt restarting or re-energizing equipment that has been locked out.
Some industries fall under separate standards rather than the general industry rule. Construction, agriculture, oil and gas drilling, maritime work, and electric utility operations for power generation and transmission are excluded from the 1910.147 standard, though they have their own hazardous energy requirements.
Why Violations Are So Common
Despite being well-established, the hazardous energy control standard consistently ranks among OSHA’s top 10 most frequently cited violations year after year. The most common failures tend to be procedural rather than technical. Companies either lack written, machine-specific LOTO procedures altogether, or the procedures they have are too generic to actually guide a worker through isolating a particular piece of equipment. Other frequent issues include failing to conduct periodic inspections of the program, not retraining workers when procedures or equipment change, and allowing shortcuts like using tags without locks.
The pattern behind many serious injuries follows a similar script: a worker assumes a machine is safe because it appears to be off, skips one or more steps in the LOTO process, and then stored or residual energy releases without warning. The 24-day average recovery period masks the severity at the extremes. Many of these incidents result in permanent disability or death, particularly when electrical or mechanical energy is involved.
Stored Energy Deserves Extra Attention
Of all the hazards in the LOTO process, stored energy is the one most likely to be overlooked. Turning off a machine and locking out the power supply handles the primary energy source, but it does nothing about the energy already trapped in the system. A raised forklift platform still holds gravitational energy. A compressed spring in a stamping press still holds mechanical energy. Steam in a pipe still holds thermal and pneumatic energy.
Dealing with stored energy requires specific action for each type. Hydraulic and pneumatic systems need to be bled or vented so pressure drops to zero. Springs need to be repositioned or physically blocked so they cannot release. Elevated components need to be lowered to resting position or mechanically secured. Electrical capacitors need to be discharged and grounded. Each of these steps has to be part of the written procedure for that specific machine, because the stored energy profile varies from one piece of equipment to the next.