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Standard Interpretations - Table of Contents
• Standard Number: 1910.147; 1910.147(a)(2)(ii); 1910.147(b)

January 25, 2008

Mr. Earl Reyes
Wm. Wrigley Jr. Company
Wrigley Building
410 N. Michigan Avenue
Chicago, Illinois 60611

Dear Mr. Reyes:

Thank you for your November 1, 2007 letter to the Occupational Safety and Health Administration's (OSHA's) Directorate of Enforcement Programs (DEP) regarding the Control of hazardous energy (lockout/tagout or LOTO) standard, 29 CFR 1910.147. This letter constitutes OSHA's interpretation only of the requirements discussed and may not be applicable to any questions not delineated within your original correspondence.

Your question relates to the minor servicing exception, contained in the 29 CFR §1910.147(a)(2)(ii) note, and a programmable logic controller (PLC) that is used to control safety-related functions.

Additionally, you state that when a minor service activity is required, the PLC opens all ungrounded supply conductors by two contactors wired in series. You further state that this design isolates power to all points of operation that the operators may come in contact with during the service work. Your paraphrased question and our reply follow:

Question: With regard to the minor servicing exception [contained in §1910.147(a)(2)(ii) note], would the described PLC system meet the definition of an alternative measure which provides effective protection?

Reply: Circuit control systems, such as the PLC system you describe, are not energy isolating devices as defined at §1910.147(b). As a result, reliance on a PLC system that controls machine or equipment safety functions, such as stopping or preventing hazardous energy (motion), is prohibited by the LOTO standard and, as a result, is presumed to be ineffective employee protection from injuries resulting from hazards such as component failure, program errors, magnetic field interference, electrical surges, and improper use or maintenance.

However, if an employer can demonstrate that a PLC system is an alternative measure which provides effective protection, the PLC system may be used only to protect employees who are performing minor tool changes and adjustments, and other minor servicing activities that take place during normal production operations and are routine, repetitive, and integral to the use of the equipment for production pursuant to the minor servicing exception contained in the 29 CFR §1910.147(a)(2)(ii) note.

To meet this exception, an employer must demonstrate that there is effective employee protection, through the use of a system hazard analysis, before OSHA would accept PLCs for minor servicing activity covered in the equipment-specific analysis. In other words, the PLC system, on a case-by-case basis, would need to be designed, installed, used, and maintained in accordance with the generally-recognized good engineering practices (e.g., applicable manufacturers' recommendations; prior operating experience; reliability data) so as to protect employees from hazardous energy sources during the minor servicing activities. The improper application of a PLC system on a machine or piece of equipment would not constitute recognized good engineering practice and would not constitute effective alternative protection if, for example, it fails to protect employees (from the described hazards above) who are performing minor tool changes and adjustments.

Thank you for your interest in occupational safety and health. We hope you find this information helpful. 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 http://www.osha.gov. If you have any further questions, please feel free to contact the Office of General Industry Enforcement at (202) 693-1850.


Richard E. Fairfax, Director
Directorate of Enforcement Programs

Standard Interpretations - Table of Contents

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