A 52-year-old welder was removing a jammed piece of metal from the hydraulic door of a scrap metal shredder, but did not de-energize and lock out the shredder first. He also failed to release the residual hydraulic energy in the system and block the door open.

When the welder cut away the jammed metal with his torch, the door closed on him—and killed him. Of the five preventable mistakes discussed below, this welder committed at least two. And either one could have killed him.

Lockout/tagout is intended to prevent hazardous energy from hurting or killing workers, but the process is complex and workers often miss one or more important steps. Read on to learn about common lockout/tagout mistakes and how to help your workers prevent them.

5 deadly mistakes

These deadly mistakes can maim or kill. Make sure your workers are aware of them and take precautions to avoid them.

1. Mistaking “off” for “safe”

It makes perfect sense to shut down a potentially deadly machine before you put yourself at risk of injury by inserting your hands, your head, or some other part of your body into it, and many workers do so willingly enough.

Unless they take the next, vital step of locking out the power source, though, workers are still at risk, and they may not even realize it. More than one worker has died when a well-meaning coworker came along who was unaware that a machine was being serviced—and turned it back on.

Workers need to know: If someone else could turn the machine on while they’re in the zone of danger, they’re not safe. The worker who is in danger must lock out the machine for safety and keep the key.

2. Mistaking “locked out” for safe

Wait…didn’t we just say “lock out the machine for safety”? Unfortunately, machines may also have parts that can move after the power source is locked out.

Locking out the power to a power press may stop it cycling, but a press that is raised and not supported could fall unexpectedly. Any part that could move freely after the power is locked out—whether it could slide, fall, rotate, or shift—needs to be blocked or restrained against unexpected motion.

Workers need to know: Train your workers to secure any parts that could shift, fall, or otherwise move in a way that might endanger them.

3. Misusing interlocks

Interlocks are safety devices. They stop the machine from operating when something, usually some part of the worker’s body, intrudes into the work area. Interlocks are generally designed to protect against accidental or incidental contact, though—not to protect workers performing some adjustment or removing a jam.

Workers who use interlocks when they should use lockout procedures place their bodies at risk because they haven’t taken proper precautions. They may discover too late that interlocks can be unreliable, reset unexpectedly, or fail. Lockout/tagout is far more secure for the worker who must perform work activities within the zone of danger.

Workers need to know: An interlock device will protect them against accidental or incidental contact, but when they purposely enter the zone of danger, they must use lockout/tagout.

4. Misidentifying the power source

Power sources are supposed to be clearly marked so workers can confidently lock out the machine they will be working on. However, mistakes have occurred.

Workers may lock out the wrong power source if markings are missing, incorrect, unclear, or worn. Workers could also lock out the wrong power source if modifications have been made—for example, if an additional power source has been spliced into the line beyond the point of lockout.

Workers need to know: Make sure your workers are aware of all power sources that feed into a machine they are working on.

5. Failing to account for residual energy

Not all machines stop moving immediately when the power is disengaged. Workers must wait until all parts stop moving—for example, a saw blade must be allowed to coast to a stop once turned off.

Machines may also shift or move while the worker is still inside the zone of danger—for example, after a jam is cleared. Any residual energy that could cause a machine to move must be bled off—for example, pneumatic or hydraulic pressure behind a ram that could cause the ram to continue moving once a jam is removed.

Workers need to know: Workers must determine which parts of machine will continue to move once power is disengaged, as well as the residual energy source present in the machine, and proceed accordingly.