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OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at


OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed.


December 16, 1999

Thomas L. Weekley, Ph.D.
Assistant Director
UAW General Motors Department
8000 East Jefferson Avenue
Detroit, MI 48214

Dear Dr. Weekley:

Thank you for your March 25, 1999 letter to the Occupational Safety and Health Administration's (OSHA's) Directorate of Compliance Programs. You have requested an interpretation of the energy isolation device requirements contained in 29 CFR §1910.147. Additional information concerning your inquiry was provided in a June 1, 1999 letter from Mr. Lewis Schultz and Mr. Michael Douglas, at the United Automobile, Aerospace and Agricultural Implement Workers of America (UAW) and the General Motors Corporation (GM) Center for Human Resources, Health and Safety Center. Your background information, question, and our reply follow:

Background: The UAW and GM recognize the importance of designing processes and equipment with effective health and safety controls. Therefore, a pilot program was conducted to determine the effectiveness of employing a Monitored Power System (MPS) design approach that removes all hazardous motion energy from devices, equipment, and machinery in a designated area, such as a zone or cell. The four key elements in the MPS are: (1) redundancy (parallel or series); (2) a self-monitoring feature; (3) fail-to-safe condition with self-testing capability; and (4) hardware integration so the MPS may not be bypassed by other control circuitry or hardware. The MPS mode control devices cannot be overridden or by-passed because the authorized person secures his or her plant approved lock on the MPS gate box before entering the safeguarded area. The gate box serves as a single point of control and accomplishes the same de-energized state as placing individual locks on multiple energy isolating devices to meet the requirements of §1910.147(b) and (c)(1).

The pilot program involved a Task Based Risk Assessment (TaBRA) process through which the MPS was incorporated into specific machines and equipment at the pilot sites. TaBRA has three basic output categories consisting of lockout, MPS, and other solutions. The use of the Monitored Power Systems are integrated into the existing hazardous energy control procedures and a validation process is in place to verify that the key safety requirements in the TaBRA are met. It is understood that lockout must still be performed whenever the exposure cannot be controlled or eliminated as determined by the risk assessment process.

Furthermore, the MPS meets the control reliability (Section 5.5) and control component failure protection (Section 6.13) requirements, respectively, of the American National Standards for machine tools (ANSI B11.19-1990) and manufacturing systems/cells (ANSI B11.20-1991). Hazardous motion from the parts transfer and/or conveyance systems (that are an integral part of the cell) shall be stopped by the MPS. This requirement, in conformance with ANSI/ASME B20.1-1996 on the Safety Standards for Conveyors and Related Equipment, ensures that no motion may occur, which could cause injury to anyone servicing part of the production system.

Question: Does the current standard, §1910.147, permit the use of the Monitored Power Systems through a task based risk assessment process for production and troubleshooting tasks or is a variance required?

Reply: Section 1910.147(c) requires that before any employee performs servicing or maintenance on a machine or equipment where the unexpected energizing, start up or release of stored energy could occur and cause injury, the machine or equipment shall be isolated from the energy source, and rendered inoperative. Machines and equipment are isolated from energy sources by "energy isolating devices." Since the MPS relies on electrical control circuits to isolate potentially hazardous energy, and the standard specifically excludes such control circuits in the definition of "energy isolating devices" (1910.147(b)), application of the MPS alone would not constitute compliance with the requirements of §1910.147.

Moreover, variance applications are traditionally related to particular pieces of machinery and/or defined processes or systems. The TaBRA process, as we understand it, would be applied to a wide variety of machines and processes throughout General Motors' entire operation. Until the TaBRA is applied in specific cases, and a determination made that the MPS will be used to control hazardous energy associated with specific pieces of equipment or machinery, it is extremely difficult, if not impossible, for the Agency to determine whether the application of MPS would provide a level of protection that is equivalent to that which is achieved through the use of energy isolating devices. See 29 U.S.C. §655(d). Until the Agency is assured that the MPS would provide an equivalent level of protection in all cases in which MPS potentially would be applied, the Agency cannot not approve a variance request.

However, an MPS, which meets the above referenced ANSI consensus standards on control reliability and control component failure protection, would provide alternative safeguarding measures, which constitute effective employee protection. Thus, such an MPS may be used to protect employees who are performing minor tool changes and adjustments, and other minor servicing activities, which take place during normal production operations, provided that each element of the §1910.147(a)(2)(ii) exception is met. In other words, the MPS system may be used in cases in which minor tool changes and adjustments, and other minor servicing activities, are performed during normal production operations, and are routine, repetitive, and integral to the use of the equipment for production. It is important, as you have stated, to apply this safeguarding technique (MPS) through a hazard analysis process (TaBRA) on a case-by-case basis in order to assure that it, in fact, provides effective employee protection.

In regard to your enclosed MPS design information, our office has not performed a detailed review, nor approved, the adequacy of such design in meeting the above referenced ANSI or any other consensus standards. OSHA does not test, approve, certify, or endorse any equipment or product, including machinery.

Thank you for your interest in occupational safety and health. We hope that you find this information helpful. Please be aware that OSHA's enforcement guidance is subject to periodic review and clarification, amplification or correction. Such guidance could also be affected by subsequent rulemaking. In the future, should you wish to verify that the guidance provided herein remains current, you may wish to consult OSHA's website at If you have any questions, please feel free to contact the [Office of General Industry Enforcement] at (202) 693-1850.


Richard E. Fairfax, Director
Directorate of Compliance Programs

[This document was edited on March 2008 to reflect policy contained in OSHA instruction CPL 02-00-147]