2.1 The kinds of energy on a machine

1 Electrical: line power is only the first of four

Almost every machine has line power, and almost every crew locks it. The other three electrical sources are where people get hurt.

Variable frequency drives deserve a specific mention. A VFD holds a DC bus capacitance that stays dangerous after the disconnect opens; manufacturers publish a discharge time, often several minutes, and print it on the drive. "The light went out" is not the discharge time. This is exactly what the definition of energized in 1910.147(b) means when it says "containing residual or stored energy."

2 Mechanical, and the one that is not on the list

Mechanical energy is motion and the potential for motion: rotating masses, flywheels, springs in compression or tension, belt and chain tension, a web under tension, an indexing table, a coasting fan.

Then there is gravity. It is not named in the definition of energy source, which is why it gets forgotten — but it falls inside "or other energy," and it is one of the most reliable killers in maintenance work. A raised platen, a suspended die, a boom, an elevated dump body, a lift table, a counterweight, a chute full of product. Cutting the power does nothing to any of them. 1910.147(a)(3)(i) speaks of affixing devices to energy isolating devices "and to otherwise disable machines or equipment," and blocking against gravity is what "otherwise disable" means in practice.

3 Hydraulic and pneumatic: pressure that outlives the pump

Fluid power is where the phrase "the machine was off" does the most damage. Shutting off the pump or the compressor stops new energy going in; it does nothing about what is already stored in the circuit.

The isolating device for fluid power is a line valve — named explicitly in the 1910.147(b) definition — closed and locked. The bleed or vent that follows is a separate action and belongs to 1910.147(d)(5), covered in Chapter 3 of this part. Two habits are worth building: bleed to a gauge that reads zero rather than to a sound, and remember that closing a valve on the supply side may trap pressure downstream rather than release it.

Pneumatics carry an extra trap. Air is compressible, so a cylinder can hold enough stored energy to move a heavy member quickly even after the supply valve is shut. Most pneumatic lockout valves are combination shut-off-and-exhaust valves for exactly this reason: they close the supply and dump the downstream line in one movement.

4 Thermal and chemical

Thermal energy covers hot process fluids, steam, jacketed vessels, heated platens, furnaces, dryers, and their opposites — cryogenic lines and refrigerant. A press platen at 350 °F is a burn hazard for a long time after the heaters are locked out, and a procedure that does not include a cool-down step sends someone to work on it too early. Steam adds a second problem: a steam line that has been isolated but not drained can hold condensate and hammer when it is broken.

Chemical energy covers process fluids that are corrosive, flammable, toxic or reactive, along with inerting gases such as nitrogen that create an oxygen-deficient atmosphere. On a piping system, closing a valve is often not enough: 1910.147(b) includes blank flanges and bolted slip blinds in the definition of a lockout device precisely because a positive physical break is sometimes the only acceptable isolation. Where a line is blinded, the blind itself is the lockout device.

5 Type and magnitude — the phrase in the standard

1910.147(d)(1) requires that before a machine is turned off, the authorized employee "shall have knowledge of the type and magnitude of the energy, the hazards of the energy to be controlled, and the method or means to control the energy." The same phrase appears in the training requirement at (c)(7)(i)(A), and step 2 of the sequence in 1910.147 Appendix A gives it a blank line to write on: "Type(s) and magnitude(s) of energy, its hazards and the methods to control the energy."

Type is the category — electrical, hydraulic, thermal. Magnitude is the number. 480 V three-phase. 2,000 psi. 110 psi plant air. 350 °F. 15 psig steam. A 300-pound platen at 42 inches. Writing the number down changes behaviour, because it tells the person reading the procedure what will happen if this step is skipped, and it tells them what test equipment and PPE they need to bring.

A procedure that says "electrical" in the energy column and nothing else has met the letter of nothing. A procedure that says "480 VAC 3-phase, 60 A, at MDP-4 disconnect DS-12; VFD DC bus, 5 minute discharge per nameplate" tells you what to isolate, where, and what to wait for.

6 Building the energy inventory for one machine

Everything above turns into a table you fill in once per machine, at the machine, with the manuals open. It becomes section B of the written procedure required by 1910.147(c)(4)(ii)(B), and it is the single most valuable document a maintenance department can own.

1. 1
   List every energy type present, using the categories from 1910.147(b) plus gravity.
2. 2
   Write the magnitude for each: volts and phases, psi, temperature, weight and height.
3. 3
   Name the isolating device for each, with its identifier and physical location — "DS-12, north wall of MCC room," not "the disconnect."
4. 4
   Note the stored energy associated with each, and how it is relieved, restrained or blocked.
5. 5
   Note how each isolation will be verified, which is where 1910.147(c)(4)(ii)(D) is satisfied.

Do this once, honestly, and the rest of the procedure almost writes itself. Skip it, and you get the generic document that OSHA's 2003 interpretation warns is "of little or no use to the employee" who has to follow it. Chapter 2 of this part is about the walkdown that produces column three.