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.


July 16, 2008

Letter # 20070626-7800

Re: Oil-filled transformers and underground construction

Question: Do all filled transformers, including transformers filled with non or less flammable dielectrics, used in underground construction have to comply with the requirements of 29 CFR 1926.800(s)(3)?

Answer: Filled transformers being used in underground construction must comply with 29 CFR 1926.800(s)(3) if the material that they are filled with is in any way flammable. 29 CFR 1926.800(s)(3) states:


Oil-filled transformers shall not be used underground unless they are located in a fire-resistant enclosure suitably vented to the outside and surrounded by a dike to retain the contents of the transformers in the event of rupture.



The hazard associated with 29 CFR 1926.800(s)(3) is a fire hazard. In order to protect against fire hazards in underground construction, employers utilizing transformers filled with flammable material must adhere to the requirements of the standard. The standard requires using a fire-resistant enclosure to cover the transformer, suitably venting the enclosure to the outside, and surrounding the enclosure by a dike. Accordingly, any underground transformer filled with a material that is flammable in any way must comply with 29 CFR 1926.800(s)(3).

Noah Connell, Acting Director
Directorate of Construction