Temperature Class VS AIT

Electrical equipment generates heat when in use and hot surfaces can ignite explosive atmospheres.

It is therefore especially critical to consider the maximum surface temperature that an electrical device may reach in service when installed in a hazardous environment where explosive gases are present.

In order to prevent an explosion, it is necessary to understand the temperature classes of electrical devices and the auto-ignition temperature of explosive gases.

Temperature class (known as T-class) is one of the characteristics used to classify an electrical device so it may be safely installed in a hazardous location. It is the maximum temperature of the exposed surface of the electrical apparatus, based on the ambient temperature range allocated for that device.

There are six main temperature classes named as  T6, T5, T4, T3, T2, and T1.

What this table means is that a device classified as T5 will never attain a surface temperature higher than 100°C, which can be exposed to explosive atmospheres and cause an explosion.

At the same time, we need to know the auto-ignition temperature of the gas in our hazardous location. This is the temperature at which a mixture of air and that gas will spontaneously ignite without any additional source of ignition. For instance, Hydrogen has an auto ignition temperature of 560°C, Ethylene 425°C, Diesel 240°C, diethyl Ether 160°C and Carbon disulphide has 95°C. 

Let’s use an example to clarify these concepts.  We have an electrical motor rated as T4 for installation in a hazardous location. If this motor were installed in a location in which carbon disulphide with an auto-ignition temperature of 95° C is present, the result would be an explosion.

Now this T4 rated electrical motor can be safety installed in an area when diethyl ether, diesel or hydrogen are present because the minimum ignition temperature of these gases is higher than maximum surface temperature rise of our T4 motor.

To summarize, the maximum temperature of the exposed surface of electrical apparatus must always be lower than the ignition temperature of the explosive atmospheres where it is to be used.

All equipment with a T6 rating which has a maximum surface temperature of 85°C can be also used in T5, T4, T3, T2 and T1 environments. But If a hazardous location application requires a T4 rated device, a T3 rated product will not be suitable.

We should always remember that temperature class plays a significant role in the safe installation of electrical devices in hazardous locations.

We hope you enjoyed this presentation. If you still have questions about temperature classes,  please contact the Hazcon team.

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