Thermal evaluation of junction and connection boxes in explosion protection

Abstract

To reliably avoid potential ignition sources and thus ignition of the potentially explosive atmosphere in junction and connection boxes of type of protection Increased Safety ‘e', the self-heating shall not exceed a specified level depending on the temperature class. For the conformity assessment of such products, complex thermal tests are necessary due to the great variety of mounting types and arrangements of terminal blocks in the enclosure, depending on the enclosure size. These tests are very time-consuming for the manufacturer of junction and connection boxes or for the testing laboratory and are therefore associated with considerable costs. To reduce this effort and to ensure a uniform assessment in the conformity evaluation by a certification body, it is therefore essential to create fitting charts depending on the enclosure size.This work introduces a calculation tool by means of systematic investigations on different enclosure sizes and with different assemblies, which enables the calculation of fitting charts with justifiable effort. For this purpose, the maximum temperature in the enclosure was determined as a function of the current and the assembly at 20 ± 5 °C ambient temperature. From this, electrical and enclosure-specific constants such as the maximum permissible current per conductor and a conductor specific factor were determined and combined with an exponential dependence of the power dissipation. It is shown that this relationship is valid for an overtemperature of 40 K for compliance with temperature class T6 up to an ambient temperature of +40 °C. Finally, to verify the reliability of the calculation tool, the results are compared with the enclosure-specific rated value of the maximum power dissipation according to IEC 60079-7, 5.7.