All MTL7700 Series barriers are based on the same simple principle. Each channel contains two stages of pulse-tested Zener or forwardconnected diodes and an ‘infallible’ terminating resistor. In the event of an electrical fault in the safe area, the diodes limit the voltage that can reach the hazardous area and the resistor limits the current. A fuse protects the diodes, and the two stages of voltage limitation ensure continued safety if either stage should fail. No active outputcurrent limiting circuits are employed. All models are certified ‘ia’ for all zones and ‘IIC’ for all explosive atmospheres (except MTL7707P+ and MTL7729P+, 'ia' 'IIB').
The safety description of a barrier, eg 10 V, 50 Ω, 200 mA, refers to the maximum voltage of the terminating Zener or forward diode while the fuse is blowing, the minimum value of the terminating resistor, and the corresponding maximum short-circuit current. It is an indication of the fault energy that can be developed in the hazardous area, and not of the working voltage or end-to-end resistance.
Barriers may be polarised + or –, or non-polarised (‘ac’). Polarised barriers accept and/or deliver safe-area voltages of the specified polarity only. Non-polarised barriers support voltages of either polarity applied at either end.
The resistance between the two ends of a barrier channel at 20°C, ie of the resistors and the fuse. If diodes or transistors are present, their voltage drop (transistors ON) is quoted in addition.
Working voltage (Vwkg)
The greatest steady voltage, of appropriate polarity, that can be applied between the safe-area terminal of a ‘basic’ barrier channel and earth at 20° C for the specified leakage current, with the hazardous-area terminal open circuit.
Maximum voltage (Vmax)
The greatest steady voltage, of appropriate polarity, that can be applied continuously between the safe-area terminal of any barrier channel and earth at 20° C without blowing the fuse. For ‘basic’ barriers, it is specified with the hazardous-area terminal open circuit; if current is drawn
in the hazardous area, the maximum voltage for these barriers is reduced. The ‘ac’ channels of ‘basic’ barriers and most channels of overvolt-protected barriers withstand voltages of the opposite polarity also – see circuit diagrams.
The greatest current that can be passed continuously (for 1000 hours at 35° C) through the fuse.
In star-connected barriers, the two channels are interlocked such that the voltage between them cannot exceed the working voltage, Vwkg: this allows for higher cable capacitance or inductance.
Maximum safe-area voltage (Um)
The maximum permissible safe-area voltage (Um) for MTL7700 Series barriers is 250 V AC / DC.