What is the difference between T class and T ambient?

Does the “T” class of hazardous area equipment (e.g. T4, T5, T6 for gas hazards or T135°C, T100°C, T85°C for dust hazards) define the maximum temperature of the surrounding environment in which the equipment can be installed? For example in the case of T4, is the maximum ambient temperature 135°C? The answer is “No”.

For gas hazards the “T” class is interpreted in one of two ways:

1. The maximum surface temperature of the apparatus - In the case of T4 the maximum surface temperature of the apparatus must not exceed 135°C degrees C.

2. The auto ignition temperature of a particular gas - The auto ignition temperatures of all the gasses in temperature class T4 will be greater than 1350C and less than 200°C.

For dust hazards the “T” class is also interpreted in one of two ways:

1. The maximum surface temperature of the apparatus - In the case of T135°C the maximum surface temperature of the apparatus must not exceed 135°C degrees C.

2. The minimum ignition temperature of a particular dust - The minimum ignition temperatures of all the dusts in temperature class T135°C will be greater than 135°C and less than 200°C.

The contents of the apparatus will increase the surface temperature of the apparatus when power is applied; during both normal operation and under fault conditions. As an allowance needs to be made for this temperature rise, the maximum ambient temperature (T amb) that the apparatus can operate in also needs to be defined.

This maximum ambient temperature e.g.+ 40°C or + 80°C, will therefore be lower than the temperature defined by the relevant T class, e.g. 135°C (T4). A given piece of electrical equipment will therefore be approved for a stated range of ambient temperatures in which it is safe to operate. Normally this would be T amb -20°C to +40°C, unless otherwise stated on the product certification.

K Controls can offer certain products with a T amb as low as -60°C or as high as +120°C.

In summary you will need to know the T class and the T amb (maximum and minimum) in order to specify the correct equipment.

K Controls Ltd
Tel: 01895 449 601
Email: sales@k-controls.co.uk

Published: 18th November 2014