Temperature Classifications and Temperature Class Ratings for Areas and Instrumentation

Temperature Classifications are given to a hazardous area and Temperature Class Ratings are applied to instruments and are utilized for all hazardous areas and protection methods. This is the manner used to quantify the level of thermal energy allowed in the area or produced by the instrument.

Temperature Classification: This classification identifies the minimum ignition temperature threshold for the hazardous area. In other words, in order for the explosive or combustible environment to ignite, it would need to be subjected to a temperature in excess of this value. The values applied to this classification run from T1 to T6. A classification of T1 means the minimum ignition temperature is >450° C [842° F]. A classification of T6 means the minimum ignition temperature is >85° C [185° F].

Related: How to Perform Temperature Profiling in Hazardous Environments

Temperature Class Rating: This rating identifies the maximum surface temperature the instrument will produce at a given ambient environmental temperature [typically 40°C]. You may find higher ambient temperatures referenced in a rating but they need to be clearly identified in the instrument markings or certificate. The same T1 thru T6 values are utilized for Temperature Class Ratings as for Temperature Classifications. A rating of T1 means the maximum surface temperature generated by the instrument at 40° C is 450° C. A rating of T6 means the maximum surface temperature generated by the instrument is 85° C.

  Area Instruments
Temp Classification/ Rating Min Ignition Temperature °C [°F] Max Surface Temp °C [°F]
T1 >450 [842] 450 [842]
T2 >300 [572] 300 [572]
T3 >200 [392] 200 [392]
T4 >135 [275] 135 [275]
T5 >100 [212] 100 [212]
T6 >85 [185] 85 [185]

Can a Division 1 rated instrument be used in a hazardous area with a Division 2 classification?

Yes it can. Assuming the associated Class and Temperature rating for the instrument are appropriate for the area, you can always use an instrument with a more stringent Division rating than required for the area. However, if you use a Division 1 rated instrument it must always be installed according to Division 1 wiring procedures, regardless of the hazardous area division classification. Doing so is typically more involved and will add to the overall expense of the installation versus using a Division 2 rated instrument.

Is it permissible to field repair an instrument with a hazardous area rating?

Unfortunately this is not a clear or straightforward issue. Much of the answer depends upon the instrument in question and the repairs to be performed. In general, repair of hazardous area rated instruments fall into one of three basic categories. [In this context, repair does not include any alterations to the device]

Factory Repair Only: This typically involves instruments that must go through specific test procedures that can not be performed in the field in order to achieve / maintain their third party rating. If the repair in question requires this test to be performed again prior to the instrument being returned to service, it must be come back to the factory.

Field Repair by Authorized Factory Personnel: In this instance complicated testing with specialized equipment may not be required; however, specific procedures may need to be followed in order for the instrument to maintain its third party rating. It is not possible or practical to instruct the end user’s instrument technicians how to perform this work. If the work is not performed correctly there could also be liability concerns. This dictates that authorized factory personnel must be employed.

Field Repair by the End User: First, for this to be possible the repair in question must involve a like-for-like replacement. The new component in question must be considered a direct replacement in terms of form, fit and function. [in essence it will not change the model code] It also means that no special or unique testing of the instrument is necessary after the repair is complete.

Categorizing how the repair of instruments with a hazardous approval rating must be performed has many grey areas. Each instance can be different and must be evaluated independently. Contact the Endress+Hauser Service Department should you have any questions regarding a specific situation.

Endress+Hauser offers thermometers for a variety of applications including industrial, hygienic, high-temperature, heavy-duty, and more.

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Nationally Recognized Testing Laboratories [NRTL]

Finally this brings us to the subject of NRTL’s [Nationally Recognized Testing Laboratories]. This has been a growing issue with respect to instrument ratings in recent years. You have all heard of agencies such as FM and UL. In the past the choice between which test agency to use was largely based upon customer or market preference. While this practice may still exist in small pockets, the trend is changing.

In previous articles on hazardous approval ratings, no mention of a specific test agency was made. That is because OSHA makes no distinction between NRTL’s. The test procedures are harmonized. Every NRTL tests to the same standards to evaluate an instrument for a given approval rating. As long as an agency is classified as a NRTL and testing within its scope of registration, they are considered equal. The NRTL mark [FM, UL etc…] is not the instrument approval rating. The designation of Class, Division and Group is the approval rating! For this reason all reference to a specific test agency may one day disappear from a manufacturer’s literature. It would be replaced with a general statement that the instrument is NRTL certified to Class X, Division Y, Group Z. The only place the NRTL mark would be located is the official certificate the individual agency would issue. For further information on the visit the OSHA website at; http://www.osha.gov/dts/otpca/nrtl/index.html.


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