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.

August 15, 1996

Mr. Larry Brown R.S. Technology Services. Inc. 1327 Clegg St. Petaluma, CA 94954

Dear Mr. Brown:

This is in response to your letter of September 25, 1995 requesting interpretative guidance relative to the Permit-Required Confined Spaces (PRCS) standard and equipment approved for hazardous locations (sewer system inspections and maintenance). Please excuse the delay in responding to your inquiry.

The response to the questions raised are set forth below.

Question 1: Under what circumstances can non-explosion proof electrical devices be used in a sewer system (manholes and distribution piping) relative to the 1910.146 standard?

Reply: The circumstances where non-explosion proof electrical devices can be used in sewer system manholes and distribution piping with employees present are: 1) when atmospheric testing of the permit space indicates flammable gases, vapors, and mists are not present: and 2) when tests indicate that flammables are present but the electrical device is not energized when employees are in the space. Where tests indicate that flammables are present and the device is energized employees cannot be in the permit space or around the sewer opening.

Question 2: Can a sewer be made temporarily non-hazardous by purging, inerting, flushing, or ventilating to control atmospheric hazards making it acceptable to use electric equipment not classified as explosion-proof?

Reply: Yes. Paragraph 1910.146(d)(3)(iii) lists these procedures as a means to eliminate or control atmospheric hazards. The operative term with regard to flammable or explosive atmospheres is "control." If less than 10% of the Lower Flammable Limit (LFL) is maintained, employees would be able to use spark-producing tools and non-explosion proof equipment in the permit space. Where there are areas adjacent to the space (piping) which cannot be documented as being below the 10% level, it is presumed that a hazardous atmosphere is present and spark-producing equipment must be prohibited.

Question 3: Does 29 CFR 1910.146(c)(5) or some other section allow the sewer to be temporally classified as non-hazardous if atmospheric monitoring indicated no potential hazard?

Reply: "No", for (c)(5), and "Yes" for (c)(7) under very strictly defined circumstances. The alternate entry procedures allowed by paragraph (c)(5) can only be instituted when the permit space is totally isolated and contains atmospheric hazards which cannot be eliminated but which can be controlled through the use of continuous mechanical ventilation. In order for an employer to apply (c)(5) to a sewer, it would have to demonstrate that the sewer has been totally isolated from other potential sources of hazard (i.e., the sewer distribution system) and then demonstrate that the ventilation system is maintaining the space below the trigger limits for the atmospheric hazard (i.e., 10% LFL). Normally, total isolation of sewer manholes or selected sections of piping is not practical or even feasible to prevent the inherent hazards (flammable gases) from entering the space because entries are performed with the system in service.

When the atmospheric monitoring indicates there are no atmospheric hazards, paragraph (c)(7) may be used to temporarily reclassify a sewer as a non-permit space as long as there are no other hazards present. Again, paragraph (c)(7) can only be applied in those situations where there are no actual or potential atmospheric hazards. If the sewer manhole has an unsealed connection to the sewage distribution, a potential for atmospheric hazards exists. Once a space is determined to contain no actual or potential hazards, it can be reclassified as a non-permit space.

Question 4: Is it acceptable to use electrical equipment not classified as explosion-proof while the entry is allowed since the atmospheric monitoring indicates there are not high enough concentrations of flammable gas or vapors to ignite?

Reply: Yes and No. Yes, if the flammable concentration is below 10% of the lower flammable limit. The referenced concentration of flammable gas or vapor in the question is "not high enough to ignite." The PRCS standard has a limit (below 10% LFL). However, the standard does not prohibit entry into a permit space where a hazardous flammable atmosphere is at or above the 10% LFL. For concentrations at or above 10% LFL, paragraph (d) would require the employer to incorporate safe work procedures to address the flammable/explosive hazard (such as equipment approved for hazardous locations) in the entry plan for those spaces. Electrical equipment as well as other sources of ignition (non-sparking hand tools) must be considered and addressed for the hazardous atmosphere.

Question 5: Does the same apply if no person actually enters the sewer while the atmospheric monitoring was in progress and the electrical equipment was in operation?

Reply: The standard addresses the protective measures the employer must take to assure safe entry into a permit-required confined space. Where employees are not in the permit space during a procedure or process the standard requires an employer to take effective measures to prevent entry into the space. These measures could include physical barriers, permanently closing the space, as well as bolting and locking the space, supplemented by training employees and posting danger signs. The steps taken by the employer must be capable of preventing employees from entering the permit space. A copy of the Compliance Directive which gives a more detailed explanation has been included for your information.

If you have further questions on this response please contact Don Kallstrom of my staff at (202)219-8031 ext. 109.


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