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 https://www.osha.gov.

January 14, 1997

Mr. Brian Heise
Heise's Online Thermographic Services
1900 Northshore Hills Blvd.
Knoxville, TN 37922

Dear Mr. Heise:

This is in response to your January 6 request for clarification of the Occupational Safety and Health Administration (OSHA) standards applicable to H. VIR (inspection) Windows installed in electrical equipment enclosures. These windows provide for visual monitoring of electrical equipment inside the enclosure.

Under paragraph 1910.303(a), workplace electrical conductors and equipment must be approved to be acceptable to OSHA. Under the paragraph 1910.399 definition of "acceptable," an electrical installation or equipment is acceptable to OSHA and approved within the meaning of 29 CFR 1910 Subpart S-Electrical if it is accepted, or certified, or listed, or labeled, or otherwise determined to be safe by a nationally recognized testing laboratory (NRTL).

Apparently, the inspection windows you make are installed:

1. at the factory when the enclosure is being fabricated,

2. at the factory after electrical components have been installed within the enclosure, or

3. at the workplace.

Any modifications to electrical equipment, for example, at the workplace, after it has been certified, listed, or labeled by a nationally recognized testing laboratory must be in accordance with the conditions of the NRTL certification in order to be acceptable to OSHA.

Thank you for your interest in employee safety and health. If we can be of further assistance, please contact the Office of Safety Compliance Assistance, Mr. Ronald J. Davies, telephone # (202) 219-8031, extension 110.


John B. Miles, Jr., Director
Directorate of Compliance Programs