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Standard Interpretations - Table of Contents
• Standard Number: 1910.147; 1910.147(c)(3); 1910.147(c)(2)(iii); 1910.303(a); 1910.399; 1910.303

May 13, 2003

Mr. Thomas L. Jones
Senior Safety Consultant
1014 Vine Street, Suite 1100
Cincinnati, Ohio 45202-1195

Dear Mr. Jones:

Thank you for your October 15, 2002 letter to the Occupational Safety and Health Administration's (OSHA's) Directorate of Enforcement Programs (DEP). This letter constitutes OSHA's interpretation only of the requirements discussed and may not be applicable to any scenario or questions not delineated within your original correspondence. You had specific questions regarding the Control of hazardous energy (lockout/tagout) standard, 29 CFR 1910.147. Your scenario, question, and our reply follow:

Scenario: To control hazardous energy, your client uses a modified standard off/on switch mounted in a 4" x 4" electrical box. Your client has modified the electrical box by installing a metal clip bracket to the electrical box's faceplate, which permits the off/on switch to operate under normal conditions.

The add-on bracket also permits a lock to be installed so that the switch may be locked in the "Off" position. The lock and bracket assembly are designed to prevent anyone from turning the switch to the "On" position, hence isolating electricity to the screw-threading machine while servicing and maintenance are being performed.

Question: Is this locking mechanism an acceptable device to meet the "Lockout" provisions detailed in 29 CFR 1910.147?

Reply: The bracket may be acceptable for lockout purposes if the referenced switch is an energy-isolating device
1 and the bracket reliably ensures that the switch and the related screw-threading machine cannot be operated until the lockout device is removed. However, it is important to point out that your client's installation of this bracket constitutes a modification to the electrical box and the dead-front switch. An electrical box cover and switch that are modified in this manner do not comply with the 29 CFR Part 1910, Subpart S, Design Safety Standards for Electrical Systems, requirements for electrical utilization systems. Specifically, paragraph 1910.303(a), Approval, requires electric equipment to be approved.2 Pursuant to Subpart S standards, independent, third party testing is required for certain equipment and materials to be acceptable for workplace use.

To achieve compliance with 1910.147 and 1910.303(a), your client could install a separate, approved disconnect switch box, which is capable of being locked out in accordance with the 1910.147 standard.
3 OSHA always encourages such modifications, but the standard does not require energy-isolating devices to accept a lock unless the associated machine or equipment was subject to major repair, renovation or modification, or was installed after the January 2, 1990 "grandfather date."4 See paragraph 1910.147(c)(2)(iii).

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

1 An electrical switch may be an energy-isolating device if it physically prevents the transmission of electrical energy. Additionally, the mechanical isolating device must also simultaneously open all ungrounded conductors of the supply circuit and no pole can be operated independently. [back to text]

2 See the definitions of approved, acceptable, certified and listed in 1910.399. [back to text]

3 If the switch were replaced with a lockable isolating device, your client would need to comply with all of the requirements in the standard, including the Full employee protection provisions addressing the use of tagout devices. See paragraph 1910.147(c)(3). [back to text]

4 If the machine was installed before January 3, 1990, and no major repair, renovation, or modification has occurred since that date, your client may use a tagout system that complies with all of the tagout-related provisions of the standard. It would not be necessary to modify the electrical boxes and the switches to use a tagout system. [back to text]

Standard Interpretations - Table of Contents

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