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.

December 29, 1988

Mr. Joseph Bode
Occupational Safety and
Health Administration
Washington, DC 20210

Dear Mr. Bode:

This letter is in response to your request for information concerning Mine Safety and Health Administration (MSHA) testing and approval of bearing temperature equipment manufactured by Topf Enterprises of Tyrone, Pennsylvania.

MSHA has evaluated two types of bearing temperature equipment manufactured by Topf Enterprises. The first is a portable, battery-powered instrument, the Model No. TE-4320 Portable Temperature Monitor. This instrument was issued MSHA Approval No. 2G-3696-0 and is approved for use in gassy mines in accordance with the conditions stated in our letter to Topf Enterprises dated December 19, 1986, (of which you have a copy in your files).

The other type of Topf Enterprises bearing temperature equipment accepted by MSHA includes a family of temperature monitors included under Topf Model Nos. TPD-2 and TE-4320. These are stationary equipment which have been classified as sensors under the MSHA Mine Wide Monitoring Systems (MWMS) Program. Each monitor holds several MWMS letter classifications (of which you also have copies) based on the operating voltage requirements of the sensors. The MWMS program is structured in such a way so as to insure the intrinsic safety of a sensor circuit by requiring that all electrical connections (power and signal) to safe area or outby equipment are protected by intrinsic safety barriers. These barriers, many of which have been certified by Underwriter's Laboratory (UL) or Factory Mutual Research Corporation (FMRC), include voltage and current limiting components which limit the electrical and thermal energy which can be stored or released in the sensor circuit based on the particular component values in the circuit. The sensor letter classification indicates the corresponding barrier letter classification to which the sensor may be safely connected.

Both types of Topf bearing temperature equipment described above have been determined by MSHA testing and evaluation to be intrinsically safe in the most easily ignitable concentration of methane gas in air and to be incapable of igniting a layer of coal dust via thermal effects. The latter implies that no external surface of the equipment or any point within the equipment not enclosed in a dust-tight enclosure will exceed 150 degrees Celsius.

My understanding of the Occupational Safety and Health Administration (OSHA) regulations in 20 CFR 1910.307(b)(1) is that intrinsically safe equipment and associated wiring is permitted in any hazardous location for which it is specifically approved. Therefore in order to comply with the provisions of 29 CFR 1910.272(p)(4)(ii) for temperature monitoring of bearing in grain handling facilities, it would be necessary to use equipment which meets the requirements for equipment which is to be installed in a Class II, Division 1, Group G (Dusts composed of agricultural products) location based on the area classification system of the National Electrical Code (NEC).

MSHA has not specifically approved the Topf Enterprises equipment for use in Class II, Division 1, Group G atmospheres. However, the ANSI-UL 913-1988 standard for intrinsically safe apparatus and associated apparatus requires, in essence, that intrinsically safe and associated apparatus intended for use in Class II, Division 1, Group G atmospheres meets the following:

(1) Spark ignition requirements for Class 1, Group D or Methane.

(2) Maximum temperature of exposed surfaces not to exceed 165 degrees C.

(3) Dust-tight enclosures, or for conductive dusts it is assumed that all spaces do not comply with creepage and clearance distances.

The Topf Enterprises equipment tested and approved by MSHA meets the spark ignition requirements for methane, meets the MSHA maximum surface temperature for coal dust of 150 degrees C and is enclosed in a NEMA 4 (watertight and dust-tight) enclosure. Therefore, when used under the conditions described above for MSHA Approval No. 2G-3696-0 or the MWMS requirements for letter classified sensors, the Topf bearing temperature equipments tested and approved by MSHA would meet the requirements for Class II, Division I, Group G Dust Atmospheres.

I trust that this information will be of assistance in determining the suitability of the Topf equipment for grain handling facility applications. If I can be of further assistance, please call me at (304) 547-0400 or FTS 723-1361.


Kenneth A. Sproul
Chief, Intrinsic Safety and
Instrumentation Branch