Zero energy state is the condition in which all hazardous energy sources in equipment have been isolated, released, or blocked so it cannot move or energize.
The condition of a machine or piece of equipment in which every source of hazardous energy — electrical, mechanical, hydraulic, pneumatic, chemical, thermal, and gravitational — has been isolated, and any stored or residual energy has been released, dissipated, or restrained. Achieving and verifying a zero energy state is the core goal of lockout/tagout procedures under OSHA's control of hazardous energy standard (29 CFR 1910.147).
Before servicing a hydraulic press, a maintenance technician shuts it down, locks out the electrical disconnect, bleeds off hydraulic pressure, blocks the ram, and then attempts to start the machine to verify it has reached a zero energy state.
Equipment can retain dangerous stored energy after shutdown — in springs, capacitors, pressurized lines, or elevated parts held up against gravity — so each source must be identified and controlled, not just the main power supply.
Workers typically perform a "try" step, attempting to operate the equipment's normal controls and, where appropriate, testing for voltage or pressure to confirm that isolation was effective before any work begins.
Commonly missed sources include compressed springs, trapped hydraulic or pneumatic pressure, charged capacitors, residual heat, and parts that could fall or rotate under gravity once released.
Part of SafetyIQ's EHS glossary: plain-English definitions of workplace health and safety terms.
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