HOW THEY WORK
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').
1. Safety description
The safety description of a barrier, eg ‘10V 50Ω 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.
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,
i.e. 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 hazardousarea 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. Star connection
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.
8. Maximum safe-area voltage (Um)
The maximum permissible safe-area voltage (Um) for MTL7700 Series
barriers is 250V ac/dc.
ambient temperature and humidity limits
–20 to +60°C continuous working
–40 to +80°C storage
For ‘basic’ barriers with a working voltage of 5V or more, the
leakage current decreases by at least one decade per volt
reduction in applied voltage below the working voltage, over two
decades. For the MTL7755ac/7756ac it decreases by at least one
decade for a 0.4V reduction in applied voltage.
Removable terminals accommodate conductors up to 2.5mm
(13AWG). Hazardous-area terminals are identified by blue labels.
Removal force >15N
colour coding of barrier label
Red: positive polarity (MTL7706 negative to transmitter )
Black: negative polarity
White: dummy barrier, MTL7799
Mounting and earthing
By 35mm Top Hat DIN rail