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.

July 5, 1990



SUBJECT: Repair/Rebuilding of UL Listed Equipment for Hazardous Locations - Requirements for such equipment to be "Acceptable" to OSHA


This is in further response to your memorandum of September 11, 1989, regarding the above subject. Our interim response of January 25, 1990, indicated that regardless of where the equipment is repaired or rebuilt, the equipment has to be reevaluated by UL to determine if it complies with the applicable UL requirements.

From the review of information pertaining to the equipment listed for hazardous locations, it was gathered that the equipment has to meet several UL safety requirements. Some of the UL requirements are described in the following paragraphs.

When UL listed devices (specifically, items listed for hazardous locations) are constructed, they are required to be made with carefully machined joints or flanges, which are to be held within narrow tolerances or clearances. For example, an explosion-proof device for use in class I locations is required to be constructed in such a manner that, if an internal explosion occurs within the enclosure of the device, then the flame arising from that explosion is arrested within the enclosure and is not propagated to the surrounding atmosphere. The narrow clearances are to be designed such that the hot gases resulting from the internal explosion are cooled sufficiently (as they pass through the clearances) to prevent ignition of the outside flammable or combustible gases.

Another important safety measure taken by UL is their policy to conduct periodic examinations or tests of randomly selected listed products, at the manufacturing facility or at a UL testing station, to determine continued compliance of the equipment's production with the UL requirements.

[Corrected 10/22/2004]