On January 25, 2008, OSHA issued an interpretation letter regarding its “Control of Hazardous Energy (Lockout/Tagout)” standard, § 1910.147. This standard “covers the servicing and maintenance of machines and equipment in which the unexpected energization or start up of the machines or equipment, or release of stored energy could cause injury to employees.” The standard, however, contains the following exception:

Minor tool changes and adjustments, and other minor servicing activities, which take place during normal production operations, are not covered by this standard if they are routine, repetitive, and integral to the use of the equipment for production, provided that the work is performed using alternative measures which provide effective protection.

The question posed to OSHA was as follows:

“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?”

The programmable logic controller (PLC) mentioned in the question above was designed to open all ungrounded supply conductors by two contractors wired in series and to isolate power to all points of operation that the operators may come in contact with during the service work. OSHA concluded that the PLC was not an energy isolating device under § 1910.147(b), which is defined as “a mechanical device that physically prevents the transmission or release or energy.” Therefore, the PLC was presumed to be an 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, OSHA also stated a PLC could be an alternative measure if the employer can demonstrate that the PLC provides effective employee protection with a system hazard analysis. To meet this requirement, the PLC must be individually designed, installed, used and maintained in accordance with the generally recognized good engineering practices to protect employees from hazardous energy sources during the minor servicing activities. If OSHA approves the PLC, the employer can only use it to protect employees who are performing routine, repetitive and integral minor tools and adjustments or other minor servicing activities that occur during normal production operations.