Lockout or tagout is the process to isolate electrical power, or other sources of energy, to devices that may cause inadvertent movement of valves or other potentially harmful events.

Means for assuring that machines are not turned on while being serviced. A tag is placed on the power switch to warn operators and others not to turn on the machine until the tag is removed by the person performing the service. Tagout is considered less reliable than "lockout". See Lockout.

The placement of a tagout device on an energy-isolating device, in accordance with an established procedure, to indicate that the energy-isolating device and the equipment being controlled may NOT be operated until the tagout device is removed.

Refers to the procedures established for Control of Hazardous Energy as described by the Occupational Safety and Health Administration (OSHA) Act under Federal Registry 29 CFR1910.147. Tagout refers to application of warning tags to energy sources to ensure the equipment is not restarted until service has been completed. Tags may be used with locks for enhanced safety assurance where lockout is not possible tagout may be an alternative. Tagout alone is not as secure as is lockout.