Temperature Class or Temperature Rating (Body Temperature Classification)

Body Temperature Classification – TEMPERATURE CLASS

The IEC, in publication IEC 79-8, has classified the maximum permissible surface temperature of electrical equipment into six groups, labeled from T1 to T6. These labels, referred to as TEMPERATURE CLASS, must be marked on electrical equipment and control devices along with the designation mentioned above.

T1 means that no part of the body of an electrical device, under any conditions such as inrush current, possible overcurrents, or faults such as short circuits, will exceed 450°C (or in other words, it will heat up to a maximum of 450°C). Similarly, T2 up to 300°C, T3 up to 200°C, T4 up to 135°C, T5 up to 100°C, and T6 up to 85°C.

The temperature of devices, designated as T1 through T6, must always be lower than the temperature at which gases present in the surrounding atmosphere auto-ignite.

The ignition temperature of gases, referred to by IEC as IGNITION TEMPERATURE, is above 200°C for most hydrocarbons present in the oil, gas, and petrochemical industries. Therefore, selecting T3 as the temperature class (TEMPERATURE CLASS) is suitable for the majority of electrical equipment in these industries.

In units where materials other than hydrocarbons are used, as seen in the examples below, the TEMPERATURE CLASS of equipment must be selected according to the ignition temperature of the specific materials. The ignition temperature of a number of flammable substances is listed in IEC 79-4 (with Iran's representative participating in the preparation of this publication). The attached table shows the ignition temperatures of several combustible materials, some of which are present in the oil, gas, and petrochemical industries. This information has been extracted from various sources, including IEC 79-4A and NFPA 497.

The ignition temperature determined by the (NATIONAL FIRE PROTECTION ASSOCIATION) NFPA is generally slightly lower than the temperature specified by the IEC. Therefore, considering that the IEC publication has tested only a limited number of flammable substances, it is recommended to refer to the NFPA data.

Note: It should be noted that if flammable materials are heated up to their (IGNITION TEMPERATURE) as provided in the attached table, they can ignite spontaneously without a flame or spark. In the presence of a flame or spark, the situation is different, as gases ignite easily and liquids can ignite at a temperature slightly above their FLASH POINT.

For example, the ignition temperature of gasoline with an octane rating of 60 is approximately 280°C, while its FLASH POINT is -43°C. Therefore, gasoline will ignite spontaneously at 280°C, while it can also ignite at -43°C in the presence of a spark or flame.

Example 1: If butane gas is present in a space classified as ZONE 1, and considering that butane belongs to the gas group GROUP IIA, the electrical equipment used in this area should be selected as Exd IIA. Butane ignites at 287°C, so the surface temperature of the electrical device in such an environment must always be below 287°C, which corresponds to T3. Therefore, the device should be selected as Exd IA T3 (T3 = 200°C).

Example 2: In a classified area of type ZONE 1 GROUP IIA where aldehyde is present, electrical equipment should be selected as Exd IIA T4. This selection is because aldehyde ignites spontaneously at 175°C. (T4 = 135°C)

Example 3: In a space classified as ZONE 1 GROUP IIA where trimethylamine is used, this substance ignites at 190°C. Therefore, the electrical equipment suitable for such an area should be selected as Exd IIA T4 (T4 = 135°C).

Example 4: If electrical equipment is installed in a classified area of type ZONE 1 GROUP IIC where carbon disulfide is present, the device must be selected as Exd IIC T6, because carbon disulfide ignites at 90°C, and the selected equipment must never exceed 90°C under any conditions. (T6 = 85°C)