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Understanding the Zero Mechanical and Zero Energy States in LOTO

dateDecember 12, 2023

Industrial activity is cyclic, not haphazard in nature. Equipment and processes have different modes in the activity cycle, namely production, standby, and shutdown. The operational cycle of equipment and systems provides an important basis for creating a structured approach to assessment and control of hazardous energy. The approaches needed and available to ensure safety differ according to the mode in which they are applied.

As appropriate, performing the hazardous assessment for each segment can considerably clarify the process of achieving safe work conditions because the focus could change in each part of the cycle. Though the operational cycle consists of production, standby, and shutdown mode, but here let’s focus on the shutdown mode.


Shutdown mode is the state of inability of a machine or equipment to operate or to release energy. The system can retain energy during shutdown.

ANSI Z241.1 - 1975 produced by the American Foundry men’s Society introduced the concept of the Zero Mechanical State (ZMS) in a consensus standard (ANSI 1975). The concept had existed for a considerable period of time in the industry previous to this (AFS 1961) ZMS is the state of the machine of minimum potential unexpected movement caused by stored energy.

The concept of the Zero Energy State attempts to ensure that the equipment or structure contains no source of energy that could expose an individual to harm during maintenance or disassembly. This means no introduction of energy through the inputs or outputs and no storage of energy in the equipment or structure.

What is Zero energy state?

Zero Energy State is the point at which machinery or equipment has no energy flowing to or from it and as a result, does not have the potential to cause accidental physical harm or injury if handled in this state. It is a safety standard set out by the Occupational Health and Safety Act (OSHA) as part of safety procedures to help protect workers from accidental injury or death.

The Occupational Health and Safety Act (OSHA) requires that organizations implement the necessary safety procedures for the handling of machinery or equipment during repairs, cleaning, calibration, and maintenance in order to prevent unnecessary physical injuries or fatalities. Industry best practice and equipment safety guidelines must be followed in order to ensure that machinery reaches zero energy state before being handled by workers. Zero energy state ensures that energy is isolated and eliminated from the equipment preventing injuries such as compression, pinching, trauma, and removal of limbs.

What is Zero Mechanical State?

The mechanical potential and kinetic energy of all portions of the equipment or machine is set so that the opening of pipes, tubes, hoses or actuation of any valve, lever, button, springs, raised or elevated part does not produce any movement which could cause injury.

The process of rendering equipment machines and systems safe for work incorporates the following phases: Deactivation, De-energization, Isolation, Lockout/Tagout, and Verification. These actions have stood the test of time and are incorporated directly or indirectly into consensus and regulatory standards and guidelines.

The state of zero energy in the equipment, machine, or a system is not always achievable and often is not necessary for pursuing the work to be performed. Energy retention in equipment, machines and systems occurs through deliberate storage and entrapment. In situations where energy can remain in equipment, a machine, or a system, a thorough analysis to determine the potential for expression of the energy and procedures to prevent expression of this energy are required. Therefore, it is highly recommended to develop Machine or Equipment specific procedures for each piece of equipment.

E-Square Compliance and Education Division
About the Author

E-Square Compliance and Education Division

A team of safety professionals and educators united to enhance workplace safety with essential Lockout Tagout knowledge. We offer expertise to foster safety compliance and effective LOTO protocols across industries.

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