1.2 The energies in a plant, and where each one hides
29 CFR 1910.147(b) defines an energy source in eleven words: "Any source of electrical, mechanical, hydraulic, pneumatic, chemical, thermal, or other energy." Six named types and a catch-all. The catch-all is not filler — gravity, the most reliable killer on a maintenance job, is not one of the six. This chapter walks the six plus the two the list implies, and asks the same question of each: where in a machine does it stay after the disconnect is open?
1 The six named types, and the two the list implies
A maintenance tech who can name six energy types will still walk into a machine holding a seventh. Two are worth adding out loud because they behave differently from everything else: gravity, which never runs out and cannot be bled off, and stored energy generally, which is not a type at all but a state — energy that the process put in and that has nowhere to go.
Appendix A to 1910.147 is a non-mandatory guideline, but its step 6 is the best short list OSHA publishes of where stored energy actually sits: "capacitors, springs, elevated machine members, rotating flywheels, hydraulic systems, and air, gas, steam, or water pressure." Six items. On most production machines you will find at least three of them.
2 Electrical: the one that has two directions
Everyone looks upstream at the disconnect. The energy that surprises people comes from somewhere else. A machine can be fed by more than one panel — a 480 V main feed plus a 120 V control circuit from the lighting panel is a classic combination, and opening one leaves the other live. Control transformers backfeed. Variable frequency drives hold a charge on the DC bus long after the input is open, which is why the drive manual states a discharge time in minutes rather than seconds.
And a circuit that has been deenergized can be re-energized by something that is not electrical at all. 1910.333(b)(2)(ii)(D) is written for exactly that: "Stored non-electrical energy in devices that could reenergize electric circuit parts shall be blocked or relieved to the extent that the circuit parts could not be accidentally energized by the device." A motor turned by a coasting load, or by product weight on an inclined conveyor, is a generator.
3 Mechanical and gravity: energy that needs no supply
This is the category that kills maintenance workers most reliably, because it survives every kind of shutdown. A flywheel spins down for a long time. A gearbox under load unwinds when you break a coupling. A tensioned belt or chain snaps back. A spring in a brake or a clamp is holding load right now, and opening a valve does nothing to it.
Gravity is worse, because it has no source to isolate. A raised ram, a tilted hopper, a lifted platen, a die in an open press, an elevated carriage, a boom, a counterweight — all of them are loaded, and they will stay loaded until something physical holds them. That "something physical" is what 1910.147(c)(5)(i) means when it requires the employer to provide "chains, wedges, key blocks, adapter pins, self-locking fasteners, or other hardware" for "isolating, securing or blocking of machines or equipment."
The baler fatality of November 6, 2024 sits at the intersection of the two: an employee was reattaching the hydraulic cylinder and ram of a baler and was crushed when the machine is believed to have cycled while he was inside it (OSHA Accident Summary Nr 171780.015). Hydraulic pressure and a heavy ram, in a space a person has to enter to do the job at all.
4 Fluid power: hydraulic, pneumatic, steam and water
A closed hydraulic circuit holds pressure with the pump off. An accumulator holds it on purpose, with a gas charge behind a piston or a bladder, and it will drive a cylinder after every valve upstream is shut. Air receivers, pneumatic cylinders with a captive volume, and any line trapped between two closed valves are the same problem in a lighter fluid.
Two details matter to the person doing the work. First, a bleed is not an isolation: a bleed relieves the pressure that is there now, and does nothing about the pressure a check valve or an accumulator will restore. 1910.147(d)(5)(ii) is written for that case — "If there is a possibility of reaccumulation of stored energy to a hazardous level, verification of isolation shall be continued until the servicing or maintenance is completed, or until the possibility of such accumulation no longer exists." Continued, not checked once at the start.
Second, the isolating device for a fluid is a line valve, a block or a blind — and 1910.147(b) is explicit that a lockout device includes "blank flanges and bolted slip blinds." A closed valve with a lock on it is an isolation. A closed valve that somebody can crack open, or that leaks past its seat, is a hope. Which of the two the job needs is a decision the procedure should already have made.
| Energy | Where it stays after the disconnect is open | What holds it safe |
|---|---|---|
| Electrical | Second feeds, control circuits, VFD DC bus and capacitors, UPS, backfeed through a coasting motor | Every disconnecting means locked, capacitors discharged under 1910.333(b)(2)(ii)(C), then a test at the point of work |
| Mechanical | Flywheels, tensioned belts and chains, wound gear trains, springs in brakes and clamps | Coast-down time, then blocks, pins and restraints — 1910.147(c)(5)(i) |
| Gravity | Raised rams, platens, carriages, booms, hoppers, counterweights, anything above you | Mechanical support that takes the load. There is nothing to bleed |
| Hydraulic | Closed circuits, accumulators, cylinders holding a load, lines between closed valves | Isolate, then bleed to atmosphere; watch reaccumulation under (d)(5)(ii) |
| Pneumatic | Receivers, captive cylinder volumes, plant air with no local isolation | Lockable line valve, then vent — plus blocks for whatever the air was holding up |
| Chemical | Product in the vessel, residue in dead legs, a reaction in progress, purge gas | Double block and bleed or a blind, then the confined space rules of Part 4 |
| Thermal | Steam lines and jackets, ovens, dryers, molten material, surfaces that were running an hour ago | Isolate and drain, then a cool-down interval somebody measured rather than guessed |
5 Chemical and thermal: the two that hurt after everything looks safe
Chemical energy is the one a mechanic is least trained for and most likely to meet. A tank that is drained still has a heel, a residue and a film on the walls. A dead leg holds product that never moved. A line that carried caustic yesterday will hand it to you out of a fitting today. And a purge gas that made a space safe from fire made it lethal from oxygen deficiency in the same motion. Part 4, Chapter 2 works through what that means in practice.
Thermal energy is quieter. Steam traps and jackets hold pressure and heat long after the boiler is off. A heated platen, a dryer, an oven or an extruder barrel takes hours. A cool-down interval belongs in the written procedure with a number and a way to check it, not in somebody's memory of how long it usually takes.
Where a plant is covered by 1910.119, the process safety information required by (d) is where the real properties live: what the material is, what it does, and the design limits of the equipment holding it. 1910.119(f)(2) requires operating procedures to be "readily accessible to employees who work in or maintain a process" — maintenance is named in that sentence for a reason.
6 Isolating device, or just a control?
One definition in 1910.147(b) settles more arguments than any other. An energy isolating device is "a mechanical device that physically prevents the transmission or release of energy," and the list includes a manually operated circuit breaker, a disconnect switch, a manually operated switch that disconnects all ungrounded supply conductors with no pole operable independently, a line valve, and a block. Then the last sentence: "Push buttons, selector switches and other control circuit type devices are not energy isolating devices."
That one sentence rules out the E-stop, the HMI, the mode selector, the safety relay and the PLC output. Every one of them is excellent at stopping a machine, and none of them is an isolation. 1910.333(b)(2)(ii)(B) says the same thing on the electrical side and then adds the interlock: control circuit devices "may not be used as the sole means for deenergizing circuits or equipment. Interlocks for electric equipment may not be used as a substitute for lockout and tagging procedures."
One more line in the definitions is worth carrying. "Capable of being locked out" means the device has a hasp, an attachment point or a built-in locking mechanism — or that lockout can be achieved "without the need to dismantle, rebuild, or replace the energy isolating device or permanently alter its energy control capability." And 1910.147(c)(2)(iii) closes the loop going forward: since January 2, 1990, whenever equipment is replaced, majorly repaired, renovated or modified, and whenever new equipment is installed, its energy isolating devices "shall be designed to accept a lockout device." A machine that cannot be locked out is a project, not an excuse.
- 1910.147(b) names six energy types — electrical, mechanical, hydraulic, pneumatic, chemical, thermal — plus "other energy," which is where gravity lives.
- Appendix A step 6 lists where stored energy sits: capacitors, springs, elevated machine members, rotating flywheels, hydraulic systems, and air, gas, steam or water pressure.
- Gravity cannot be bled or switched off. A raised ram, platen, carriage or boom needs a physical support — the hardware 1910.147(c)(5)(i) requires the employer to provide.
- 1910.147(d)(5)(ii) requires verification of isolation to be continued, not done once, wherever stored energy can reaccumulate to a hazardous level.
- Push buttons, selector switches and other control circuit devices are not energy isolating devices, and 1910.333(b)(2)(ii)(B) adds that interlocks are not a substitute for lockout.
- Since January 2, 1990, replaced, majorly repaired, modified or newly installed equipment must have energy isolating devices designed to accept a lockout device — 1910.147(c)(2)(iii).
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