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  • For use in conventional fire detection systems for protection within hazardous areas
  • Reverse polarity protection
  • Simple installation onto standard DIN rail
  • Removable colour-coded terminals
  • For conductors up to 2.5 mm2

Approvals & Conformities

Approvals & Conformities
Categories in this range.


MTL 7700 series

Shunt-diode safety barriers


All MTL7700 Series Shunt-diode safety barriers are based on the same simple principle. Each channel contains two stages of pulse-tested Zener or forward connected 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 output current limiting circuits are employed. All models are certified ‘ia’ for all zones and ‘IIC’ for all explosive atmospheres (except MTL7707P+ and MTL7729P+, ‘ia’ ‘IIB’).


Supplied by Moflash:-

7728+ Controller Outputs & Solenoids
7728AC AC and DC Sensors & Thermocouples


  1. Safety description
    The safety description of a barrier, eg ‘10V 50& 200mA’, 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.
  2. Polarity
    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.
  3. End-to-end resistance
    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.
  4. 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.
  5. 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.
  6. Fuse rating
    The greatest current that can be passed continuously (for 1000 hours at 35°C) through the fuse.
  7. Maximum safe-area voltage (Um)
    The maximum permissible safe-area voltage (Um) for MTL7700 Series barriers is 250V ac/dc.



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Additional information

Maximum Temperature

+60 °C

Minimum Temperature

-20 °C

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