Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

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

December 6, 1991

Mr. Thomas Slavin
NAVISTAR International Transportation Corporation
455 North Cityfront Plaza Drive
Chicago, Illinois 60611

Dear Mr. Slavin:

Thank you for your letter of September 24, addressed to Mr. Bode of my staff, concerning the applicability of 29 CFR 1910.147, the lockout standard, and 29 CFR 1910, Subpart O, machine safeguarding requirements. You requested an opinion of the applicability of these standards to your activities relating to the use of automatic equipment for the machining of connecting rods, crankshafts, cylinder heads and engine blocks.

The integrated machine system described by you is generally referred to as a cellular manufacturing system or manufacturing systems/cells. This form of manufacturing system was recently addressed by an American National Standards Institute, Inc. committee, identified as ANSI B11.20. That committee of industry experts recently completed a safety standard applicable to these manufacturing systems. Although OSHA does not enforce industry consensus standards, such standards do provide guidance with respect to the accepted industry procedures for ensuring operator safety and compliance with the OSHA requirements. It is recommended that a review of the ANSI B11.20, American National Standard Safety requirements for construction, care, and use for manufacturing systems/cells, be accomplished by your staff. Copies of the ANSI B11.20 standard are available from:

American National Standards Institute, Inc.
1430 Broadway
New York, New York 10018
telephone: 212-642-4977 or 212-354-3300

In your letter you requested an OSHA opinion in respect to specific questions:

1. Are these servicing and maintenance activities considered to be minor tool changes and adjustments or minor servicing for the purposes of 1910.147?

2. Does the perimeter guarding, electrical interlock and restart procedure described ... provide effective protection for the purposes of 29 CFR 1910, Subpart O?

As you are aware, practically all functions performed upon a machine are essential to the use of the equipment for production. However, only those functions which are routine, repetitive, and integral to the use of the equipment and for which effective safeguarding of the employee is ensured are exempt from the additional requirements of 29 CFR 1910.147. Functions performed by employees (operators) as a normal and routine activity related to the operation of production equipment and during which the employee is safeguarded from the harmful release of energy through compliance with the requirements of 29 CFR 1910, Subpart O, are not subject to the requirements of 29 CFR 1910.147.

Procedures associated with the replacement of dulled or broken tools on automatic production equipment, such as you describe in your letter, are considered normal production operations requiring compliance with 29 CFR 1910.212. Should full compliance with the machine safeguarding requirements not be provided, then and only then do the requirements of 29 CFR 1910.147 become effective and obligatory if the employee is subject to the unexpected release of harmful energy.

The procedure described in your letter appears to provide effective safeguarding of the employee engaged in the tool change operation; however, the interlock noted at the gate may not be in the exclusive control of the employee engaged in the activity. The employee engaged in the tool change activity must be in continuous and full control of the interlock and thereby in control of the machine operation. Full evaluation of the operation described cannot be creditably accomplished without an on-site evaluation of the procedures; therefore, the opinions expressed in this letter are subject to such further evaluation.

If we may be of further assistance, please contact us.


Patricia K. Clark, Director
Directorate of Compliance Programs