October 27, 2005

Mr. Carl Morgan
Los Alamos National Laboratory
P.O. Box 698
Los Alamos, NM 87544

Dear Mr. Morgan:

Thank you for your March 8, 2005 letter to the Occupational Safety and Health Administration's (OSHA) Directorate of Construction. Your letter has been transferred to the Directorate of Enforcement Programs (DEP) as it relates to general industry activities. You had questions regarding OSHA's electrical Lockout/Tagout standard, §1910.333 and the use of light switches as disconnecting means. Your paraphrased questions and our responses follow.

Question 1: Is it permissible to consider a light switch a disconnecting means and lock out the light switch instead of the breaker when changing ballasts in fluorescent lights? This procedure breaches only one lead and if the switch is wired incorrectly may leave the ballast with a phase lead and ground.

Response: Yes. It is permissible to use a light switch as a disconnecting means if the switch meets the requirements for a disconnecting means found in the National Electrical Code (NEC), NFPA 70-2005. If it does, then it would be acceptable as a disconnecting means with regard to OSHA's electrical standards (Subpart S). A disconnecting means is defined by the NEC (and mirrored in §1910.399), as "a device or group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply." A circuit breaker and a light switch are functionally similar in that they both disconnect the ungrounded conductors, thus disconnecting the downstream conductors and utilization devices from their electrical source. Therefore, in many cases, a light switch can function as a disconnecting means for the purposes of Subpart S. It must be emphasized, however, that all of the requirements for locking and tagging out circuits under §1910.333(b)(2) apply (e.g., written procedures, application of locks and tags, etc.) whether you are using a circuit breaker as a disconnecting means or a light switch.

As you noted in your letter, an incorrectly wired switch does present a potential hazard; however, incorrect wiring presents a hazard in every electric utilization circuit. NEC and OSHA requirements are predicated on the correct wiring of the conductors and the electric utilization device(s). Additionally, OSHA requires at §1910.303(b)(2) that listed and labeled equipment be used in accordance with any instructions included in the listing and labeling, which would include the manner in which devices are to be wired. Also note that §1910.333(b)(2)(iv) requires that a qualified person verify, using test equipment, that the circuit elements and equipment parts have been de-energized.¹

Question 2: If a light switch is used to disconnect power to lights when changing the ballast, must the circuit be treated as energized and other safety-related work practices implemented, as required by §1910.333(a)(2)?

Response: No. If the circuit is disconnected from its source by way of a disconnecting means, any hazardous stored energy is released, and the disconnecting means is properly locked and tagged out, then it can be considered de-energized after it has been verified as such by a qualified person using test equipment, as required by §1910.333(b)(2)(iv). It must be noted that according to §1910.333(b)(1), conductors and electric equipment cannot be considered de-energized unless they are locked and tagged out, even if the disconnecting means has been used.

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 www.osha.gov. If you have any further questions, please feel free to contact the Office of General Industry Enforcement at (202) 693-1850.

Sincerely,



Richard E. Fairfax, Director
Directorate of Enforcement Programs

¹ A "qualified person" is defined at 1910.399 as "one familiar with the construction and operation of the equipment and hazards involved." The definition is accompanied by two notes, the first of which discusses the concept that whether a person is "qualified" will depend on the circumstances and the equipment involved. The second note discusses that an employee undergoing on-the-job training that has demonstrated an ability to perform duties safely at his or her level, and that is under the direct supervision of a qualified person, is considered to be qualified for the purposes of those duties.[ back to text ]