... than others 3.Industrial telephone :IP67 4.EX 1 Application 1.1 The hazardous location of explosive gas mixture , ... Zone 21.2 explosive gas mixture , 1A 2B IIC* 1.3 temperature group ,T1-T61.4 Ambient temperature,-30 ... response , 300~3400 Hz2.6 output sound signal ,1.5V / 300Ω2.7 loop circuit impedance ,1KΩ ... 16Ω)3.0 Installed ,wall-mounted 3 Specification3.1 This type of explosion-proof telephone made of ...