The three basic requirements for an explosion are a combustible gas, oxygen (ambient atmosphere) and an effective ignition source. However, an explosive atmosphere only occurs with a defined mixture ratio of gas and oxygen, which must not be too rich or too weak. This means that if the concentration of combustible gas is too high or too low, there can be no explosion. If the ratio is somewhere inbetween these values, then it is explosive. The mixture can be ignited in different ways, for example by heated surfaces, mechanically or electrically produced sparks or static electricity.
Depending on the explosion hazard, the relevant industrial areas are divided into three zones. In the IEC 60079-10-1 standard these are defined as follows:
- Zone 0: Area in which an explosive gas-air mixture is continuously present or present for long periods.
- Zone 1: Area in which an explosive gas-air mixture is likely to occur for short periods in normal operation.
- Zone 2: Area in which an explosive gas-air mixture is not likely to occur, and if it occurs it will only exist for a very short time due to an abnormal condition.
Thus, instruments for Zone 0 are naturally subject to higher requirements than those for Zone 1 or Zone 2. One key consideration for the use of test instruments is the ignition temperature of the respective gas-air mixture. The surface temperature of the calibrator components, such as the circuit board, must be below this value. To enable the devices to be categorised, six temperature classes are defined:
- T1: Ignition temperature > 450 °C, maximum permissible surface temperature at the calibrator 450 °C
- T2: Ignition temperature > 300 to 450 °C, maximum permissible surface temperature at the calibrator 300 °C
- T3: Ignition temperature > 200 to +300 °C, maximum permissible surface temperature at the calibrator 200 °C
- T4: Ignition temperature > 135 to 200 °C, maximum permissible surface temperature at the calibrator 135 °C
- T5: Ignition temperature > 100 to 135 °C, maximum permissible surface temperature at the calibrator 100 °C
- T6: Ignition temperature > 85 to 100 °C, maximum permissible surface temperature at the calibrator 85 °C
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