2.2 Stored and residual energy: what kills after the power is off
The power being off is the beginning of safety, not the end of it. 1910.147(d)(5)(i) is the step that acknowledges this: "Following the application of lockout or tagout devices to energy isolating devices, all potentially hazardous stored or residual energy shall be relieved, disconnected, restrained, and otherwise rendered safe." Four verbs, because stored energy comes in four kinds of problem — some can be let out, some has to be held, and some has to be waited out.
1 Four verbs, four different jobs
Relieved is a fluid: open a bleed, crack a drain, vent to atmosphere, and the energy leaves. Disconnected is a coupling, a driveshaft, a linkage — break the path so the energy has nowhere to go. Restrained is a spring or a suspended mass: nothing leaves, and something physical holds it. And otherwise rendered safe is the catch-all that covers grounding a capacitor bank, allowing a flywheel to coast to a stop, and letting an extruder barrel cool.
The reason the distinction matters is that only the first verb produces a gauge you can read. A relieved system shows zero. A restrained mass shows nothing at all — the block either is in place and rated, or it is not. Mixing the two up is how a mechanic ends up watching a pressure gauge on a machine whose real hazard is a raised platen.
2 Appendix A step 6: the best short list OSHA publishes
Appendix A to 1910.147 is non-mandatory, and its step 6 is still the most useful sentence in the standard for a mechanic: "Stored or residual energy (such as that in capacitors, springs, elevated machine members, rotating flywheels, hydraulic systems, and air, gas, steam, or water pressure, etc.) must be dissipated or restrained by methods such as grounding, repositioning, blocking, bleeding down, etc."
Take it item by item against a real machine. Capacitors: the drive, the power factor bank, the welding supply. Springs: brake springs, spring-return cylinders, clamp springs, gas struts on a guard door. Elevated machine members: the ram, the platen, the hopper, the tilted mold, the raised carriage. Rotating flywheels: the press, the fan, the shredder rotor that takes four minutes to stop. Hydraulic systems: everything from the accumulator to the cylinder holding the tool. Air, gas, steam or water pressure: every line with a closed valve at each end.
| Where it is | Why the disconnect did nothing | What makes it safe |
|---|---|---|
| VFD DC bus and capacitor banks | Charge is stored, not supplied | Manufacturer's discharge time, then test; discharge and short-circuit high-capacitance elements per 1910.333(b)(2)(ii)(C) |
| Hydraulic accumulator | Gas charge behind the piston drives fluid with the pump off | Isolate, bleed to atmosphere, then check for reaccumulation under (d)(5)(ii) |
| Cylinder holding a load | Fluid is trapped by a check or a closed valve, and gravity is still on | Block the load mechanically first, then relieve |
| Raised ram, platen, carriage or hopper | Gravity has no supply to isolate | Rated safety block, pin or stand taking the load |
| Flywheel or high-inertia rotor | Momentum outlasts the shutdown by minutes | Coast to a stop, confirm visually, then restrain if it can be turned by the process |
| Springs in brakes and clamps | They were installed loaded and stayed loaded | Restrain or relieve deliberately, with the sequence in the equipment manual |
| Steam jacket, oven, extruder barrel | Heat and pressure decay slowly | Isolate and drain, then a cool-down interval that is written down and measured |
| Product in a vessel or a dead leg | Draining is not the same as cleaning | Double block and bleed or a blind, then the Part 4 confined space rules |
3 Reaccumulation: the clause that turns a check into a watch
1910.147(d)(5)(ii) is one sentence and it changes the job: "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 a check at the start and a signature. If the machine can build pressure back up — a leaking valve upstream, an accumulator that recharges, a header that repressurizes when another line is opened, a vessel that gases off as it warms — then somebody has to keep watching, or the possibility itself has to be removed. Removing the possibility is almost always cheaper: a blind instead of a valve, a spool piece pulled out, a bleed left cracked open with a gauge in sight.
4 Blocking is a rigging problem, not a housekeeping one
1910.147(c)(5)(i) requires the employer to provide "locks, tags, chains, wedges, key blocks, adapter pins, self-locking fasteners, or other hardware" for "isolating, securing or blocking of machines or equipment from energy sources." That is the only mention of blocking hardware in the standard, and it says nothing about how strong a block has to be — because the answer comes from the equipment, not from OSHA.
Which means three practical rules that the standard implies and the manufacturer states. First, use the block the machine came with, in the pocket it was designed for: a press safety block, a boom rest, a gate pin. Second, if there is no purpose-made block, the support has to be engineered for the load — a piece of channel, a hydraulic jack or a wooden crib chosen by eye is a guess, and gravity does not negotiate with guesses. Third, the load has to be set down onto the support so the support is carrying it, not merely near it. A block under a ram that is still held by hydraulic pressure protects nothing until the pressure is relieved and the ram settles.
Appendix A carries a warning that catches people on the way out of the job, and it belongs here rather than in the release chapter: "The removal of some forms of blocking may require reenergization of the machine before safe removal." Getting a block out from under a settled ram means lifting the ram, which means energy, which means the five-step testing-and-positioning sequence of (f)(1). Plan the removal at the same time as the installation.
5 The electrical side of stored energy
Subpart S handles the same problem in its own words, and both halves are worth having. 1910.333(b)(2)(ii)(C): "Stored electric energy which might endanger personnel shall be released. Capacitors shall be discharged and high capacitance elements shall be short-circuited and grounded, if the stored electric energy might endanger personnel." A Note adds the detail that decides your PPE: "If the capacitors or associated equipment are handled in meeting this requirement, they shall be treated as energized."
Then (b)(2)(ii)(D), which is the mirror image and the one people miss: "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 with a coasting load behind it, a conveyor with product on an incline, a fan in a duct with air still moving — each of them can make voltage at terminals you have already tested dead.
6 The baler, and why this chapter exists
On November 6, 2024 at about 2:45 p.m., an employee was reattaching the hydraulic cylinder and ram of a baler on the west wall of the baler room. OSHA's report states that it is believed the baler may have been cycled to extend the cylinder while the employee was inside the machine, causing fatal crushing injuries and severe lacerations (Accident Summary Nr 171780.015). The keywords OSHA attached to that record are a summary of this whole part: Baler, Crushed, Guard, Hydraulic, Hydraulic Cylinder, Lockout/Tagout, Machine Guarding, Tagout.
The job that killed him is one of the most ordinary tasks in the trade — reinstalling a cylinder. What it required was step 4 before step 5, a block under the ram, a bleed with a telltale, and a lock that made the cycle physically impossible rather than merely unlikely. All four of those are in the paragraph above, and none of them takes long.
- 1910.147(d)(5)(i) requires stored and residual energy to be relieved, disconnected, restrained and otherwise rendered safe — and only the first of those verbs produces a gauge you can read.
- Appendix A step 6 names the six usual hiding places: capacitors, springs, elevated machine members, rotating flywheels, hydraulic systems, and air, gas, steam or water pressure.
- (d)(5)(ii) turns a one-time check into a continuing watch wherever stored energy can reaccumulate — or requires the possibility itself to be removed with a blind or a pulled spool.
- Blocking hardware is required by (c)(5)(i) but sized by the manufacturer. The load has to be set down onto a rated support, not parked above one.
- Appendix A warns that removing some forms of blocking may require reenergizing the machine, so the removal is planned with the five-step sequence of (f)(1) in mind.
- 1910.333(b)(2)(ii)(C) requires capacitors to be discharged, and high-capacitance elements short-circuited and grounded, with the handled parts treated as energized; (D) requires stored non-electrical energy that could reenergize a circuit to be blocked or relieved.
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