Electrical wiring in warehouses storing hazardous materials

April 8, 1997
Richard L. Freeman
Colonel, United States Army
Assistant Executive Director
Installations
Department of Defense Defense
Logistics Agency Headquarters
[Recipient address omitted]
Dear Colonel Freeman:
This letter is in response to your request for interpretation of 29 CFR 1910.399, Subpart S, classified electrical installations.
As you have explained your facilities and the activities conducted there, OSHA classifies them as "industrial plants" as defined in 29 CFR 1910.106(e). These facilities deal with container and portable tank storage as defined in 29 CFR 1910.106(d). Since you have not described the specific makeup of each facility, we are providing the following guidance for your review. In following the electrical installation requirements specified, both situations that you have described concerning "sealed" and "closed or sealed" containers will be covered.
You have stated that "new" disposal storage warehouses are designed with a mechanical ventilation system providing six air changes per hour, and automatic fire suppression systems. You have not described how "existing" facilities are ventilated and provided with protective systems. You have specified, no mixing or transfer operations are conducted in these facilities and the liquids are not used in these facilities. Since we do not know what existing facilities are like, the following guidance is provided to cover either type (new or existing) of facility, your safety, health and fire protection officials are going to have to make the final determination as to which section applies as they are "on-scene" and have the necessary knowledge to make that determination. The electrical equipment in these facilities should be installed according to 29 CFR 1910.106(e)(7), Electrical. Paragraph (e)(7)(i), Equipment, addresses the installation of equipment and recognizes three basic storage/use areas and has requirements for each area. Overall, paragraph (e)(7)(i)(a) requires all wiring to meet Subpart S. More specifically, paragraph (e)(7)(i)(b) addresses locations where flammable vapor-air mixtures are present under "normal" operations. Paragraph (e)(7)(i)(c) addresses locations where flammable vapor-air mixtures are present under "abnormal" operations. And, paragraph (e)(7)(i)(d) addresses locations where "ordinary" wiring may be used.
OSHA's primary concern is "employee" protection. In the conditions that you have described where flammable and combustible liquids (Hazardous Material and Hazardous Waste), and where no liquid transfer takes place, there is no associated generation of vapor-air mixtures, and the fire hazard from an employee exposure perspective is limited. Only in limited, "abnormal" conditions such as an accidental spill or release of a flammable/combustible liquid will the potential for a fire/explosion develop. Other OSHA requirements in 29 CFR 1910.106 for proper drainage and spill control, proper ventilation, and adequate fixed fire protection systems along with an appropriate emergency action plan for employee evacuation as specified in 29 CFR 1910.38 can provide sufficient protection to employees in the event of an accidental release. Keep in mind that the employees must be trained on the hazards to which they are exposed and in the emergency plans and actions required to protect them.
Should you have further questions, please contact [the Office of General Industry Enforcement at (202) 693-1850]. Thank you for your continuing interest in occupational safety and health.
Sincerely,
John E. Plummer, Director
Office of Federal Agency Programs
[Corrected 2/4/2004]

[Requester letter reproduced on OSHA page; omitted here.]