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INTRINSICALLY SAFE BARRIERS
MTL7700 Series
MTL700 Series
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DIN rail mountingsafety barriers
MTL7700 SERIES
Since its introduction in 1984 the MTL700Series barrier has established itself as the worldwide
standard for safety barriers. Known for its quality
and reliability, the MTL700 Series is widely used in
applications all over the world.
The MTL7700 Series follows closely in thefootsteps of the MTL700 but as a DIN rail mounting
barrier providing quick and easy installation without
the need for special hardware.
Removable terminals are used for ease ofinstallation, maintenance and for providing a loop
disconnect by simply unplugging the terminals from
the side of the module. Wire entry is also angled to
assist wiring within limited space enclosures.
MTL7700 barriers clamp simply and securelyonto standard T-section DIN rail, simultaneously
making a reliable IS earth connection.
For applications where field power isrequired for switch inputs or 2-wire transmitters, the
MTL7700 Series provides a bussed power feed
facility. W hen used in conjunction with the MTL7798power feed module the user has a fully protected,
electronically fused supply to many barriers with no
additional wiring required.
MTL7700 active modules are protected with anelectronic fuse for the majority of applications. The
MTL7798 active fused, power feed module can
protect up to 40 other modules using the bussed
power facility and provides a clear indication of a
trip condition via a red LED.
The MTL774X range of barriers offer a NAMURcompatible input and a choice of relay or solid state
output. The solid state outputs are floating so
switching to ground or from a power rail into an
input is also possible. The solid state interface also
provides a high frequency transfer for use in flow or
rotation applications.
Dual channel relay or solid state modules offerthe highest packing density with only 6.3mm per
channel and when used in conjunction with the
power bus, offer users the minimum of wiring with
the maximum packing density and the lowest cost
per channel.
Proximity detector inputs
Electronic fusing
Direct replacement forMTL700 Series barriers
Compatible terminal numberingand safety descriptions
Removable terminals
Bussed power feed to othermodules
Relay and solid state
switch modules Dual channel variants
6.3mm per channel
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June 2004
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June 2004
MTL V mA
7706+ 28 300 93 Transmitters 35 507707+ 28 300 93 Switches 35 50
28 diode 35 50
7707P+ 28 164 171 Transmitters, switches, 35 5028 diode controller outputs IIB 35 50
7710+ 10 50 200 6V dc & 4V ac systems 75 6.0 7.0 507715+ 15 100 150 12V systems 119 12.0 13.1 1007715P+ 15 50 291 12V dc systems 64 12.6 13.7 1007722+ 22 150 147 18V dc systems 174 19.6 20.2 507728+ 28 300 93 Controller outputs, solenoids 333 25.9 26.5 507728- 28 300 93 Transmitters 333 25.9 26.5 507728ac 28 300 93 333 25.0 25.9 507728P+ 28 234 119 Controller outputs, solenoid valves 252 24.9 25.9 1007729P+ 28 164 170 Controller outputs, solenoid valves IIB 184 24.9 25.9 100
Prox sw input,
774X 10 19 solid state output 30/ 35 50
and line fault detect
7755ac 3 10 300 2 or 3- Wire RTDs 19.0 (1) 3.4 2503 10 300 (floating bridge) 19.0 (1) 3.4 250
7756ac 3 10 300 3 - Wire RTDs 19.0 (0.7) 2.7 2503 10 300 (grounded bridge) 19.0 (0.7) 2.7 2503 10 300 19.0 (0.7) 2.7 250
7758+/ - 7.5 10 750 Gas detectors 17 6.0 7.3 2007.5 10 750 17 6.0 7.3 200
7761ac 9 90 100 107 6.0 7.0 1009 90 100 107 6.0 7.0 100
7761Pac 9 350 26 Strain-gauge bridges 378 6.8 7.5 509 350 26 378 6.8 7.5 50
7764+/ ac 12 1k 12 Strain-gauge bridges 1050 10.0 10.9 5012 1k 12 1050 10.0 10.6 50
7766ac 12 150 80 174 10.0 10.6 5012 150 80 174 10.0 10.6 50
7766Pac 12 75 157 Strain-gauge bridges 92 9.6 10.5 10012 75 157 Strain-gauge bridges 92 9.6 10.5 100
7767+ 15 100 150 12V dc systems 119 12.0 13.1 10015 100 150 12V dc systems 119 12.0 13.1 100
7779+ 28 300 93 Controller outputs requires channels 333 25.9 26.5 5028 300 93 Controller outputs separate in IIC 333 25.9 26.5 50
7796+ 26 300 87 Vibration probes 333 23.9 24.5 5020 390 51 428 18.3 18.9 50
7796- 26 300 87 Vibration probes 333 23.9 24.5 5020 390 51 428 18.3 18.9 50
7760ac 10 50 200 Active dc & ac sensors 75 6.0 6.7 5010 50 200 Thermocouples 75 6.0 6.7 50
7765ac 15 100 150 124 12.0 12.5 50
15 100 150 124 12.0 12.5 507778ac 28 600 47 651 24.0 25.4 50
28 600 47 651 24.0 25.4 50
7789+ 28 300 93a Switch inputs / 651 26.6 27.2 50Signal returns 651 26.6 27.2 50
28 diode 0.9V+26 26.6 27.2 5028 diode 0.9V+26 26.6 27.2 50
7787+/ - 28 300 93 Transmitters 333 26.6 27.2 50
28 diode Controller outputs, switches 0.9V+26 26.6 27.2 50
7787P+ 28 234 119 253 26.4 27.2 80
28 diode 0.9V+21 26.4 27.2 80
7788+ 28 300 93 333 25.9 26.5 5010 50 200 75 6.0 7.0 50
7788R+ 28 300 93 Transmitters 333 25.9 26.5 5010 50 200 75 6.0 7.0 50
}
Vmax
V
SPECIFICATIONSKey barriers shown in blue
For notes 1 to 7 see 'Terminology' (later in this section)
Model
No.Safety description Polarities
available
+ ac
Basic circuit
Hazardous Safe
Fuse
rating
mA
Vwkg at 10
(1)A
V
Max. end-
to-end
resistance
Application1 2 3 4 5 6
}
a Terminals 3 & 7 connected together*Diagrams show positive versions. All diodes reversed on negative versions. Additional diodes fitted on ac versions.
See 'HOW THEY WORK'
and
'ACTIVE / ELECTRON ICALLY
PROTECTED BARRIERS'
See 'ACTIVE / ELECTRON ICALLYPROTECTED BARRIERS'
See additional
specification
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June 2004
GENERAL SPECIFICATION
Ambient temperature and humidity limits20 to +60C continuous working
40 to +80C storage
595% RH
Leakage currentFor 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.
TerminationsRemovable terminals accommodate conductors up to 2.5mm2
(13AWG). Hazardous-area terminals are identified by blue
labels. Removal force >15N
Colour coding of barrier labelGrey: non-polarised
Red: positive polarity (MTL7706 negative to transmitter )
Black: negative polarity
White: dummy barrier, MTL7799
Weight140g approx
Mounting and earthingBy 35mm Top Hat DIN rail
DIMENSIONS (mm)
KEY MTL7700 SERIES BARRIERS SUMMARISED
HOW THEY WORK
All MTL7700 Series 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').
TERMINOLOGY
1. Safety descriptionThe safety description of a barrier, eg 10V 50 200mA, refers tothe 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. PolarityBarriers may be polarised + or , or non-polarised (ac). Polarised
barriers accept and/ or deliver safe-area voltages of the specifiedpolarity only. Non-polarised barriers support voltages of eitherpolarity applied at either end.
3. End-to-end resistanceThe resistance between the two ends of a barrier channel at 20C, 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 20C 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 20C 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 ratingThe greatest current that can be passed continuously (for 1000 hours
at 35C) through the fuse.
7. Star connectionIn 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 Seriesbarriers is 250V ac/ dc. APPLICATION KEY BARRIER
Resistance temperature detectorsThermocouples, ac sensors
Transmitters, 2-wire, 4/ 20mA
Controller outputs, one line earthedController outputs, neither line earthed
Switches
Solenoids, alarms, LEDs
7756ac7760ac
7706+7787+
7728+7787+
7787+7741/3
7728
TYPE
Analogueinput (low-level)
Analogueinput (high-level)
Analogueoutput
Digital (on/off)input
Digital (on/off)output
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June 2004
ACTIVE / ELECTRONICALLY
PROTECTED BARRIERS
ACTIVE / ELECTRONICALLY PROTECTED BARRIERS
The following barriers have built-in overvolt protection, allowing their
use with unregulated power supplies. In many applications, eg, sensor
inputs or controller outputs, there is insufficient power available to
blow the barrier fuse and this additional protection is not necessary.
However, where the barrier is connected to a power supply, eg, for
energising transmitters, switches, solenoids or local alarms, overvolt
protection allows the barriers to be used with unregulated supplies
and also gives protection against faulty wiring during commissioning.
MTL7706+ for 'smart' 2-wire 4/20mA transmitters
The MTL7706+ is a 1-channel shunt-diode safety barrier, with built-in
electronic overvolt protection, for energising a 2-wire, 4/ 20mA
transmitter in a hazardous area. It is powered from a positive supplyof 2035V dc and delivers a 4/ 20mA signal into an earthed load in
the safe area. It is proof against short circuits in the field and in the
safe area and is extremely accurate. The MTL7706+ will pass
incoming communication signals up to 10kHz from a smart
transmitter, while in the outgoing direction it will pass signals of any
frequency likely to be encountered.
Since the MTL7706+ has no return channel for energising the load,
the entire output of the single 28V channel is available to power the
transmitter, providing high output capability. This channel is negatively
polarised, and the safe-area signal is in fact the very current that
returns through it from the hazardous area, the novel circuit being
energised by a built-in floating dc supply derived from the external dc
source of power.
To prevent any leakage through the zener diodes and maximise the
output voltage available at 20mA, the floating supply is given a rising
voltage/ current characteristic. A separate circuit limits the current to
protect the fuse in the event of a short circuit in the hazardous area.
With a 20V supply, the barrier will deliver 16.2V minimum at 20mA
for the transmitter and lines and consumes typically 45mA at 24V
operation.
BASIC CIRCUIT
ADDITIONAL SPECIFICATION
Safety description28V 300 93mA
Supply voltage20 to 35V dc w.r.t earth
Output current4 to 20 mA
Voltage available to transmitter and lines16.2V @ 20mA with 250 load (negative w.r.t. earth)
11.0V @ 20mA with 500 load (negative w.r.t. earth)
Hazardous
Area
Safe
Area
Accuracy2A under all conditions
Safe-area load resistance0 to 500
Supply current45mA typical at 20mA and 24V supply
60mA maximum at 20mA and 20V supply
MTL7707+ for switch inputs and switched outputs
The MTL7707+ is a 2-channel shunt-diode safety barrier similar to the
MTL7787+ but with built-in electronic overvolt protection. It is
intended primarily for safeguarding a hazardous-area switch
controlling a relay, opto-coupler or other safe-area load from an
unregulated dc supply in the safe area.
The outgoing channel accepts supply voltages up to +35V and is
protected against reverse voltages: the return channel is unaffected by
voltages up to +250V.
In normal operation the protection circuit introduces only a small
voltage drop and shunts less than 1mA to earth, so its overall effect is
minimal. If the supply voltage exceeds about 27V, however, causing
the Zener diodes to conduct or if the safe-area load has a very low
resistance the supply current is limited automatically to 50mA,
protecting the fuse and power supply and enabling the loop to
continue working.
BASIC CIRCUIT
ADDITIONAL SPECIFICATION
Safety description28V 300 93mA, terminals 1 to 328V Diode, terminals 2 -4
Supply voltage10 to 35V dc with respect to earth
Output currentUp to 35mA available
Maximum voltage drop
(at 20C, current not limited)Iout x 345 + 0.3V, terminals 1 to 3Iout x 25 + 0.9V, terminals 4 to 2
Supply currentIout + 1.6mA, supply 28V or low load resistance
MTL7707P+ for switch inputs and switchedoutputs, 2W Transmitters (IIB gases)
The MTL7707P+ is a two-channel shunt-diode safety barrier similar to
the MTL7787P+, but is designed for use with group IIB gases and
features built-in electronic overvolt protection allowing use with
unregulated power supplies up to 35V dc. It is intended primarily as
a low cost solution for driving IIB certified 2-wire 4/ 20mA
transmitters, but can also be used with controller outputs with current
monitoring, solenoid valves and switches. To protect the fuse and
enable the loop to continue working, the supply current is limited
automatically at 50mA should the output be short-circuited or excess
voltage applied.
Hazardous
Area
Safe
Area
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June 2004
MTL7742 proximity sensor or switch inputwith solid state output
The MTL7742 is a single channel switch/ prox input barrier with an
open collector solid state interface to the safe area equipment. The
solid state switch is especially useful for high frequency switching
apparatus including pulse and rotational sensors. The power bus
terminal can be used to connect power to the module and the input
power supply range makes the module suitable for use with
unregulated supplies
BASIC CIRCUIT
ADDITIONAL SPECIFICATION
Safety description10V 19mA
Supply voltage20 to 35V dc with respect to earth
Input characteristicsOutput energised if input >2.1mA(
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June 2004
MTL7744 2 channel proximity sensor or switchinputs with solid state outputs
A dual channel version of the MTL7742. This module provides two
solid state interfaces for prox/ switch inputs. Power is connected via
the power bus.
BASIC CIRCUIT
ADDITIONAL SPECIFICATION
Safety description10V 19mA
10V 19mA
Supply voltage
20 to 35V dc with respect to earthInput characteristics
Output energised if input >2.1mA(
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June 2004
MTL7700 Series barriers protect devices located in all normally
occurring explosive atmospheres, including air/ flammable gas
mixtures, dusts and fibres. Applications covered include the
protection of installations incorporating uncertified devices (simple
apparatus) such as thermocouples, switches and resistive sensors, or
separately certified energy storing (or voltage producing)
apparatus including ac sensors, transmitters and current-to-pneumatic
(I/ P) converters. Recommended choices for specific applications are
discussed briefly in the following pages.
MTL7700 SERIES
BARRIER APPLICATIONS
ANALOGUE INPUTS (HIGH LEVEL)
2-wire transmitters, 4/ 20mA, conventional and smartThe recommended barrier for use with 'conventional' and 'smart'
4/ 20mA transmitters (fed by a 26V regulated supply) is the
MTL7787+. This provides up to 12.9V (14.6V for MTL7787P+) at
Vwkg and 20mA for a transmitter and its lines as well as 5V for the
typical 250 load. This application and this barrier is suitable for usewith the optional power bus facility.
The MTL7706+ is recommended for applications where an
unregulated supply of up to 35V is used. It provides 16.0V for
conventional and Smart transmitters at 20mA, as well as 5V for a
typical 250 load. W ith the MTL7706+ terminal 3 is negative withrespect to earth, so the connections to terminals 3 and 4 should be
reversed.
3
4
1
2MTL7760acMTL7765
MTL7778
RecorderControllerData loggerComputer
Compensating ca e
3
4
1
2MTL7760ac Earth free
signal
PhotocellMicrophoneTurbineFlowmeter
+6.0V3
4
1
2MTL7760acV
out0V
3
4
1
2MTL7788R+
250
1-5V
Power +26V
Power Bus
3
4
1
2MTL7796-Power -24V
Vout
0V
Probedriver
Transducer
ontor
3
4
1
2MTL7706+
MTL7787+
MTL7787P+ 2504 -20mA
Power +26V
Signal
Power Bus
1-5V
ANALOGUE INPUTS (LOW-LEVEL)
Thermocouples and mV sourcesThe recommended barrier for thermocouples and mV sources is the
MTL7760ac. This 2-channel non-polarised barrier retains the earth-
free nature of the signal and, providing the receivers input floats,
rejects common-mode ac and dc interference up to at least 7V and is
unaffected by earth faults on the primary element.
Hazardous
Area
Safe
Area
Vibration probesThe 3-wire transmitters used with vibration monitoring equipment are
invariably supplied by a 24V dc power supply hence the
recommended barrier choice is the negatively-polarised MTL7796.
Slidewire displacement transducersThe simplest choice is the MTL7760ac. This barrier supplies power
and brings back a unipolar signal.
ac sensors, photocells, microphones and turbine flowmetersThe MTL7760ac is the recommended choice for these devices. While
many of these are designated simple apparatus and thus do not
need certification, note that some ac sensors may be subject to a
significant level of inductance and will therefore need to be designed
and certified for hazardous-area locations.
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June 2004
RTDsFor 3-wire RTDs, a single MTL7755ac barrier is the most economical
choice. This is suitable for use with a floating bridge the two leads
from the bridge arms are protected by the barrier with the third
(supply) lead being earthed through the barrier. The barrier has a low
end-to-end resistance of only 19/ channel to minimise span changesand its channels track within 0.15 (between 20 to +60C) tominimise zero shift with temperature.
STRAIN-GAUGE BRIDGES
Single strain-gauge bridgesThis shows an arrangement using two or three barriers, which is safe
in IIC gases. W ith the MTL7761ac, the circuit is powered from a 14V,
230 source; if the bridge resistance is 230, then the bridgevoltage is 7V. If the bridge resistance is 350, then the bridgevoltage is 8.4V.
An MTL7764ac can be used to sense the bridge supply voltage.
An MTL7761ac is used here for the mV output.
An MTL7766Pac provides 12.3V for a 350 bridge with a 20Vsupply. MTL7761Pac's can be used for the sense and pick-off circuits.
Hazardous
Area
Safe
Area
3
4
1
2MTL7756ac
Receiver
7 5
3
4
1
2MTL7755ac
Receiver
3
4
1
2
Receiver
3
4
1
2MTL7755ac
MTL7761ac
MTL7755ac
MTL7761ac
4-wire constant-current circuits do not need matched barrier
resistances and can be protected by two MTL7761ac barriers. If the
increase in loop resistance is too great, use two MTL7755ac barriers
instead.
If the bridge circuit is already earthed, the third barrier channel
provided by an MTL7756ac is needed. For extreme accuracy, 3
channels and an earth-free bridge can be used, a configuration that
cancels out the small errors due to barrier leakage.
Channels 1 and 2 (those between terminals 1 & 2, and 2 & 4
respectively) track to within 0.15 between 20 and 60C.
3
4
1
2
+7V3
4
1
2MTL7761ac
MTL7764ac
3
4
1
2MTL7761ac
--7V
0V
Output (mV)
Sense
+
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June 2004
STRAIN-GAUGE BRIDGES (cont)
Double strain-gauge bridgesQuite frequently there is a demand to monitor two load cells, and a
possible circuit, safe in IIC, is shown.
Here, the lower voltage drop of the MTL7766Pac is an advantage.
The MTL7766Pac supplies power to the bridge(s) whilst two
MTL7761Pac barriers interface with the sense and pick-off circuits.
Using 350 bridge systems, the following voltages are availablefrom an MTL7766Pac with a 10V supply:
1 bridge: 13.11V
2 bridges: 9.75V
ANALOGUE OUTPUTS
Controller outputs (I/ P converters)The single-channel MTL7728+ with a voltage drop of 6.66V at 20mA
is the recommended choice for most controller outputs. Higher-power
versions are available: the MTL7728P+ (5.1V drop) is suitable for IIC
applications; the MTL7729P+ (3.68V drop) for IIB applications.
Hazardous
Area
Safe
Area
3
4
1
2
+10V3
4
1
2MTL7766Pac
MTL7761Pac
3
4
1
2MTL7761Pac
--10V
0V
Sense
+
Output (mV)
3
4
1
2MTL7728+
3
4
1
2MTL7787+
MTL7787P+
3
4
1
2MTL7787+
MTL7787P+
The MTL7787+ and MTL7787P+ are also suitable for controllers
containing a resistor which enables the return current to be monitored
for high-integrity operation.
For controllers with an output circuit separated from the 0V rail by the
control transistor, the 2-channel MTL7787+ is the preferred choice as
the return channel can handle up to 26.6V allowing the control signal
to be turned off completely. The voltage drop is 8.1V at 20mA. A
higher-power version of the latter, the MTL7787P+, is also available.
The return channel of these barriers handle up to 26.4V and the
maximum voltage drop is only 6.38V
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June 2004
DIGITAL (ON/ OFF) INPUTS
SwitchesThe normal choice is the MTL7787+/ 7787P+ with a regulated
supply. The MTL774X modules are recommended for applications
where an unregulated supply of up to 30V for relay output modules,
or 35V for solid state output modules, is used.
The MTL7789+ offers a dual channel passive barrier for switch inputs
where the input current for each channel is
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June 2004
SPARE CABLE CORES AND SCREENS
The MTL7799 dummy barrier is used primarily for securing and
earthing unused cables and screen connections. Hazardous area
terminals 3 and 4 are internally connected to the DIN-rail
mounting/ earth connection. It also provides a power bus connection
for direct connection of power for modules such as the MTL7743 and
MTL7744 where no power supply screw terminal is provided.
POSITIVE DC SYSTEMS
Low-level to 12V dc systems
The two channels of the MTL7764+ and MTL7767+ can be combined
safely in IIC.
The MTL7764+ can be used for low-level logic return signals whilst
the MTL7767+ is used for 6V dc and 12V dc systems.
18V dc systems
The single-channel MTL7722+ is recommended for 18V dc systems.
AC AND DC SYSTEMS
High-level ac and dc systemsThe versatile star-connected MTL7765ac and MTL7778ac allow Vwkg
to be developed from each channel to ground but only allow Vwkg to
be developed between channels. This provides some common-mode
voltage capability and can allow higher cable parameters to be used.
NEGATIVE AND FLOATING POWER SUPPLIES
Digital (on/ off) outputs
The MTL7728 is used with a negative power supply and positive
earth. Typically used for digital inputs or outputs, as shown.
The MTL7728-- can also be used with floating power supplies, for
transmitters.
Hazardous
Area
Safe
Area
3
4
1
2
0V
MTL7764+
MTL7767+
+10V/ +12V
+10V/ +12V
3
4
1
20VMTL7722+
+19.6V
3
4
1
2MTL7765acMTL7778ac
12V/ 24V
0V
+26V
3
4
1
2
MTL7728
3
4
MTL7799
1
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June 2004
POWER BUS APPLICATIONS
The PB7700 power bus is invaluable for saving installation time and
wiring when connecting a 24V dc power source to a number of
barriers.
Typical applications include hazardous-area switches, 4/ 20mA
transmitters and proximity detectors. The diagram illustrates the
configuration for 4 barriers but up to 40 barriers can be served by
this method.
The MTL7798 power feed module would normally be used with
standard barriers such as MTL7787+ and MTL7787P+ because the
current/ voltage trip protection mechanism of the MTL7798 protects
the fuses in the barriers.
The MTL7799 dummy barrier can be used instead of the MTL7798
for direct feed-through connection of a 24V dc supply onto the
power bus. Looping the power feed to each end of the bussed power
allows the removal of individual barriers without loss of power to
others in the chain.
Other units which can use the power bus facility:
MTL7706
MTL7707+
MTL7707P+
MTL7741
MTL7742
MTL7743
MTL7744
MTL7745
MTL7787+
MTL7787P+
MTL7788+
MTL7788R+
MTL7789+
Hazardous
Area
Safe
Area
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June 2004
MTL7700 SERIES
ACCESSORIES
MOUNTING/ EARTHING ACCESSORIES
MTL7700 Series barriers mount easily and quickly onto standard DIN
rail which also acts as the intrinsically safe earth.
THR2 standard DIN rail
THR7000 plated rail
Specially nickel-plated T-section (35mm x 7.5mm) DIN rail for use in
potentially corrosive atmospheres. Supplied in 1meter lengths.
ISP7000 insulating spacers
Attached to the base of a DIN rail at either end or at intervals
(depending upon DIN rail length) to isolate the IS earth from a
structural earth.
ERB57S Earth-rail bracket, straightNickel-plated; supplied with two push fasteners, one 14mm earth-rail
clamp and one 10mm earth clamp for cables 16mm2.ETM7 earth terminal
For terminating cable screens and 0V earth returns and securing
spare cores to the earth rail. A maximum of two ETM7s per barrier
can be accommodated.
TGL7700
ERB57O
TAG57
ERL7ETM7
14mm earth-railclamp
THR2/THR7000
ERB57Sin lower position
ISP7000
ETL7000
10mm earthclamp
IMB57
Snap off extensionwhen using IMB57as a central support
27
7.5
35
14.6
62
50
20
M6 x 16
4.4 M4 x 16
IMB57 Insulating mounting blockOne required at each end of a tagging strip/ earth rail. Suitable for
low-profile (7.5mm) and high-profile (15mm) symmetrical DIN rail.
ERB57O Earth-rail bracket, offset
Nickel-plated; supplied with two push fasteners, one 14mm earth-
rail clamp and one 10mm earth clamp for cables 16mm2.
ETL7000 earth terminal
Provides connection for routing the IS earth from the DIN rail to an
appropriate plant earth. Maximum cable cross-section is 10mm2
. Tworecommended per discrete length of DIN rail. See instruction manual
INM7700 for more details.
BPL7700 Power Bus linkWhen a number of barriers use a common power supply, the optional
power link (BPL7700) can be used. Typical applications include
hazardous area switches, solenoids and 420mA transmitters. The
barriers it can be used with are the MTL7706, MTL7707+,
MTL7787+, MTL7787P+, MTL7789P+ and MTL774X. See next page
for further details.
82mm
135mm
28mm
66mm 15mm
150mm 10mm
3m
9mm 14mm
5mm
133.5mm
10mm
3m
10mm
57mm
14mm
10
43.548 overDIN rail
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June 2004
TAGGING ACCESSORIES
Two methods of tagging are available which can be used separately
or together:
1) Individual barrier identificationTH7700 barrier identifiersTH7700 barrier identifiers are supplied clipped on
to the tops of individual barriers to provide
transparent holders for identification labels.
2) Tagging strip method
TAG57 Tagging strip, 1m lengthCut to size. Supplied with reversible tagging strip label
suitable for either MTL5000 or MTL7000 Series module
spacing.
TGL7700 Tagging strip labels, set of 10 x 0.5mFor use with TAG57 tagging strip. Tags are reversible -
one side for MTL7700, the other for MTL700.
HOW TO ORDER
MTL7700 barriers
Select by barrier number and polarity,
e.g. MTL7728+
Mounting accessories
THR2 Standard DIN-rail, 35 x 7.5mm
THR7000 T-section DIN-rail, specially-plated,
35 x 7.5mm, 1m lengthISP7000 Insulating spacer
Standard earthing/earth-rail accessories
ETL7000 Earth terminal, DIN-rail mounted
IMB57 Insulating mounting block
ERB57S Earth-rail bracket, straight
ERB57O Earth-rail bracket, offset
ERL7 Earth rail, 1m length
ETM7 Earth terminal, pack of 50
Standard tagging accessories
TAG57 Tagging strip, 1m length
TGL7700 Tagging strip labels, set of 10 x
0.5m
Bussed power links
BPL7700 Pack of 100
Enclosures
DX070 Enclosure, for MTL7700 x 5
DX170 Enclosure, for MTL7700 x 13
DX430 Enclosure, for MTL7700 x 33
Spares (all in packs of 10)
SAF7712 Safe-area terminals 1&2
HAZ7734 Hazardous area terminals 3&4
SAF7756 Safe-area terminals 5&6
HAZ7778 Hazardous area terminals 7&8
TH7700 Tag holder
Literature
INM7700 Instruction manual, MTL7700 Series
INM57ENC Instruction manual, MTL5000/ 7000
Series Enclosures
CD7700... Customer drawings
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June 2004
Region UK UK USA Canada/ USA Japan
(Authority) (BASEEFA) (BASEEFA) (FM) (CSA) (TIIS)
Systems
Standard EN 50014 EN 50039 3600, 3610 entity CAN/ CSA E60079 See Certificates
EN 50020 3611, 3810 UL698, UL913, UL1604
IEC60079, C22.2
Approved for [EEx ia] IIC EEx ia IIC AIS/ I,II,III/ 1/ Entity Class 1, Div 2, Gps A,B, Ex (ia) IIC
[EEx ia] IIB EEx ia IIB ABCDEFG- SCI-942; C,D; Ex nA [iA] IIC T4 Ex (ia) IIB
NI/ I/ 2/ABCD/ T4
[I/ 0] AEx[ia]IIC Class 1, Zone 2, AEx nA
SCI-942 Entity; IIC T4
NI/ 1/2/ IIC/ T4
Ta=60C except
where Ta=65C
Model No. Certificate No.
MTL7706+ BAS01ATEX7217 Ex01E2219 3010737 1345550
MTL7707+ BAS01ATEX7217 Ex01E2219 3010737 1345550
MTL7707P+ BAS01ATEX7218 Ex01E2220 3010737 1345550MTL7710+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16433MTL7715+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16434
MTL7715P+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16611
MTL7722+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16435
MTL7728+/ / ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16397
MTL7728P+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16436MTL7729P+ BAS01ATEX7218 Ex01E2220 3010737 1345550 C16612
MTL7741 BAS01ATEX7217 Ex01E2219 3010737 1345550 C16613
MTL7742 BAS01ATEX7217 Ex01E2219 3010737 1345550 C16614
MTL7743 BAS01ATEX7217 Ex01E2219 3010737 1345550 C16615
MTL7744 BAS01ATEX7217 Ex01E2219 3010737 1345550 C16616MTL7745 BAS01ATEX7217 Ex01E2219 3010737 1345550 C16617
MTL7755ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16450
MTL7756ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16437
MTL7758+/ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16618
MTL7760ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16619MTL7761ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16438
MTL7761Pac BAS01ATEX7217 Ex01E2219 3010737 1345550
MTL7764+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16440
MTL7764ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16441
MTL7765ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16620MTL7766ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16442
MTL7766Pac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16443
MTL7767+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16444
MTL7778ac BAS01ATEX7217 Ex01E2219 3010737 1345550 C16621
MTL7779+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16445MTL7787+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16447
MTL7787P+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16448
MTL7788+BAS01ATEX7217 Ex01E2219 3010737 1345550 C16449
MTL7788R+ BAS01ATEX7217 Ex01E2219 3010737 1345550
MTL7789+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16622MTL7796+ BAS01ATEX7217 Ex01E2219 3010737 1345550 C16446
MTL7796 BAS01ATEX7217 Ex01E2219 3010737 1345550
MTL7798 NI only
Note : For FM compliance, the MTL7700 Series barriers shall be installed in compliance with the enclosure, mounting, spacing andsegregation requirements of the ultimate application.
Certified to CENELEC IIB/ FM Grps C-G only.
APPROVALS - for the latest certification information visit www.mtl-inst.com/certs_1.nsf
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June 2004
ac1/dc Note 3 BASEEFA Group IIC Matched FM (Grps A&B) Matched
ref. C L 2 L/ R Power C L 2 L/ R Power
Model No. (F) (mH) (H/) (W) (F) (mH) (H/) (W)MTL7706 + a 0.083 3.05 (4.2) 56 0.65 0.083 4.2 56 0.65
MTL7707 + a1/ a2/ b 0.083 3.05 (4.2) 56 0.65 0.083 4.2 56 0.65
MTL7710 + a 3 0.91 74 0.50 3 0.91 74 0.50
MTL7715 + a 0.58 1.45 66 0.56 0.58 1.45 66 0.56
MTL7715P + a 0.580 0.33 28 1.09 0.580 0.33 28 1.09
MTL7722 + a 0.165 1.45 45 0.81 0.165 1.45 45 0.81MTL7728 +/ -/ ac a 0.083 3.05 (4.2) 56 0.65 0.083 3.05 (4.2) 56 0.65
MTL7728P + a 0.083 1.82 (2.51) 44 0.83 0.083 1.82 (2.51) 44 0.83
MTL774X b3 2.86 96 742 2.86 96 742 0.039
MTL7755 ac a1/ a2 100 0.46 145 0.225 100 0.46 145 0.225
b 100 0.13 69 0.45 40 0.13 69 0.45
c 40 0.41 73 0.45 40 0.13 69 0.45
MTL7756 ac a1/ a2/ a3 100 0.46 145 0.225 100 0.46 145 0.225
b1 100 0.13 69 0.45 40 0.13 69 0.45
b2 100 0.06 44 0.675 40 0.13 69 0.45
c1 40 0.41 73 0.45 40 0.13 69 0.45
c2 40 0.23 61 0.60 40 0.06 44 0.675
MTL7758 +/ - a1/ a2 11.1 0.07 26 1.40 11.1 0.07 26 1.4
b 11.1 0.02 10 2.8 8.8 0.02 10 2.8
MTL7760 ac a1/ a2 3 0.91 74 0.5 3 0.91 74 0.5
b 3 0.20 27 1.00 3 0.20 35.6 1.00
MTL7761 ac a1/ a2 4.9 3.72 163 0.225 4.9 3.72 163 0.225
b 4.9 0.91 62 0.45 0.31 0.91 62 0.45
c 0.31 3.72 81 0.45 0.31 0.91 62 0.45
MTL7761P ac a1/ a2 4.9 56 613 0.058 4.9 56 613 0.058
b 4.9 14 236 0.115 0.31 14.0 236 0.115
c 0.31 56 306 0.115 0.31 14.0 236 0.115
MTL7764 + a1/ a2 1.41 240 1000 0.036 1.41 240 1000 0.036
b 1.41 61 360 0.072 1.0 61 360 0.072
MTL7764 ac a1/ a2 1.41 240 1000 0.036 1.41 240 1000 0.036
b 1.41 61 360 0.072 0.125 61 360 0.072
c 0.125 240 500 0.072 0.125 61 360 0.072
MTL7765 ac a1/ a2 0.580 1.45 66 0.56 0.580 1.45 66 0.56
b 0.580 0.32 22 1.125 0.58 0.32 31.6 1.12
MTL7766 ac a1/ a2 1.41 5.8 151 0.24 1.41 5.8 151 0.24
b 1.41 1.47 58 0.48 0.125 1.47 58 0.48
c 0.125 5.8 75 0.48 0.125 1.47 58 0.48
MTL7766P ac a1 1.41 1.47 78 0.471 1.41 1.47 78 0.471
b 1.41 0.34 29 0.942 0.125 0.34 29 0.942c 0.125 1.15 39 0.942 0.125 0.34 29 0.942
MTL7767 + a1/ a2 0.58 1.45 66 0.56 0.58 1.45 66 0.56
b 0.58 0.32 22 1.125 0.58 0.32 22 1.125
MTL7778 ac a1/ a2 0.083 16 107 0.33 0.083 16 107 0.33
b 0.083 3.05 (4.2) 42 0.33 0.083 4.0 107 0.654
MTL7779 + a1/ a2 0.083 3.05 (4.2) 56 0.65 0.083 3.05 (4.2) 56 0.65
b NOT PERMITTED
MTL7787 +/ - a1 0.083 3.05 (4.2) 56 0.65 0.083 3.05 (4.2) 56 0.65
a2 0.083 0.083
b 0.083 3.05 (4.2) 56 0.65 0.011 4.2 56 0.65
MTL7787P + a1 0.083 1.82 (2.51) 44 0.835 0.083 1.82 (2.51) 44 0.83
a2 0.083 0.083
b 0.083 1.82 (2.51) 44 0.835 0.78 2.51 44 0.835
MTL7788 + a1 0.083 3.05 (4.2) 56 0.65 0.083 3.05 (4.2) 56 0.65
a2 3.0 0.91 74 0.5 3.0 0.91 74 0.5
b 0.083 0.33 25 0.92 0.083 0.33 25 0.92
MTL7788R + a1 0.083 3.05 (4.2) 56 0.65 0.083 3.05 (4.2) 56 0.65
a2 3.0 0.91 74 0.5 3.0 0.91 74 0.5
b 0.083 0.33 25 0.92 0.083 0.33 25 0.92
MTL7789 + C 0.083 16 106 0.33 0.083 16 106 0.33
MTL7796 +/ - a1 0.1 4.91 64 0.56 0.1 4.91 64 0.56
a2 0.22 13 136 0.26 0.22 13 136 0.26
b 0.1 1.94 34 0.81 0.096 1.94 34 0.81
Notes: 1 ac indicates a non-polarised star connected barrier configuration.
2 When the external circuit contains no lumped inductance greater than 10mH, the cable inductance may be increased to the values within
parenthesis.
For further barrier channel configuration data, please refer to the relevant certificate.
MAXIMUM CABLE PARAMETERS
Model No. ac1/dc Note 3 BASEEFA GroupIIB Matched FM (Grps C-G) Matched
Ref. C L L/ R Power C L L/ R Power
(F) (mH) (H/) (W) (F) (mH) (H/) (W)MTL7707P + a1 0.65 5.34 125 2.64 0.65 5.34 125 2.64
b 0.65 5.34 125 2.64 0.587 5.34 125 2.64
MTL7729P + a1 0.65 5.65 127 2.64 0.65 5.65 127 1.19
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June 2004
Module No. Bussed MTL7000 MTL7000 MTL7000 MTL700 MTL700 MTL700 TypicalPower Equivalent Original ATEX equivalent Original ATEX Application
Certificate Certificate Certificate CertificateNumber(s) Number(s) Number(s) Number(s)
MTL7710+ No Half of Ex95C2261 BAS99ATEX7285 MTL710+ Ex832452 BAS01ATEX7202 4/ 6VMTL7162+ Systems
MTL7715+ No N/ A N/ A N/ A MTL715+ Ex832452 BAS01ATEX7202 12V SystemsMTL7715P+ No N/ A N/ A N/ A MTL715P+ Ex92C2373 BAS01ATEX7202 12V Systems
MTL7722+ No MTL7122+ Ex95C2261 BAS99ATEX7285 MTL722+ Ex832452 BAS01ATEX7202 General Purpose
MTL7728+/- No MTL7028+/ - Ex95C2261 BAS99ATEX7285 MTL728+/ - Ex832452 BAS01ATEX7202 Analogue / DigitalMTL7128+/ -
MTL7728ac No N/ A N/ A N/ A MTL728ac Ex832452 BAS01ATEX7202 General PurposeMTL7728P+ No MTL7128P+ Ex95C2261 BAS99ATEX7285 MTL728P+ Ex92C2373 BAS01ATEX7202 Analogue / Digital
MTL7755ac No MTL7055ac Ex95C2261 BAS99ATEX7285 MTL755ac Ex832452 BAS01ATEX7202 RTD, Grounded
MTL7756ac No MTL7056ac Ex95C2261 BAS99ATEX7285 N/ A N/ A N/ A RTD, Grounded
MTL7758+/- No N/ A N/ A N/ A MTL758 Ex83453 BAS01ATEX7217 Active sensors,Thermocouples
MTL7760ac No N/ A N/ A N/ A MTL760ac Ex832452 BAS01ATEX7202 Active sensors,Thermocouples
MTL7761ac No MTL7261ac Ex95C2261 BAS99ATEX7285 MTL761ac Ex832452 BAS01ATEX7202 Strain Gauges
MTL7761Pac No MTL7061Pac Ex95C2261 BAS99ATEX7285 MTL761Pac Ex92C2373 BAS01ATEX7202 Load cellMTL7161Pac
MTL7764+ No MTL7164+ Ex95C2261 BAS99ATEX7285 MTL764+ Ex832452 BAS01ATEX7202 High resistance
MTL7764ac No MTL7264ac Ex95C2261 BAS99ATEX7285 MTL764ac Ex832452 BAS01ATEX7202 Strain / Level
GaugesMTL7765ac No N/ A N/ A N/ A MTL765ac Ex832452 BAS01ATEX7202 General Purpose
MTL7766ac No N/ A N/ A N/ A MTL766ac Ex832452 BAS01ATEX7202 Strain GaugesMTL7766Pac No MTL7066Pac Ex95C2261 BAS99ATEX7285 MTL766Pac Ex92C2373 BAS01ATEX7202 Strain Gauges
MTL7166Pac
MTL7767+ No MTL7167+ Ex95C2261 BAS99ATEX7285 MTL767+ Ex832452 BAS01ATEX7202 Dual MTL715
MTL7779+ No N/ A N/ A N/ A MTL779+ Ex832452 BAS01ATEX7202 Dual MTL728
MTL7787+/- Yes MTL7087+ Ex95C2261 BAS99ATEX7285 MTL787S+ Ex832452 BAS01ATEX7202 Analogue / DigitalMTL7187+
MTL7787P+ Yes MTL7087P+ Ex95C2261 BAS99ATEX7285 MTL787SP+ Ex92C2373 BAS01ATEX7202 Analogue / DigitalMTL7187P+
MTL7788+ Yes N/ A N/ A N/ A MTL788+ Ex832452 BAS01ATEX7202 Transmitters
MTL7788R+ Yes N/ A N/ A N/ A MTL788R+ Ex832452 BAS01ATEX7202 15V systemsMTL7796+/- No MTL7096- Ex95C2261 BAS99ATEX7285 MTL796+/ - Ex832452 BAS01ATEX7202 Vibration sensors
MTL7196-
CORRELATION BETWEEN MTL7700 MTL7000 MTL700 BARRIERS (IIC)
Note 3 :The circuit configuration for the output parameters given
in the table Maximum Cable Parameters are as follows:-
a - Single channel barrier.
a1 - First channel of a dual/ triple channel barrier.
a2 - Second channel of a dual/ triple channel barrier.
a3 - Third channel of a dual/ triple channel barrier.
b - Both channels of a dual channel barrier connected in
parallel, with respect to earth.b1 - Two channels of a triple channel barrier connected in
parallel, with respect to earth.
b2 - Three channels of a triple channel barrier connected in
parallel, with respect to earth.
b3 - Both channels of each switch input connected together
c - Both channels of a dual channel barrier interconnected,
with no earth return.
c1 - Two channels of a triple channel barrier interconnected,
with no earth return.
c2 - Three channels of a triple channel barrier interconnected,
with no earth return. This assumes two of the channels are
in parallel.
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April 2006
for safe measurement andcontrol in hazardous areas
MTL700 SERIES
MTL700 Series shunt-diode safety barriersare 1- or 2-channel devices which pass an electricalsignal in either direction without shunting it, but limitthe transfer of energy to a level that cannot igniteexplosive atmospheres. Connected in series with thesignal transmission lines on a process plant, theyprotect hazardous-area wiring and equipmentagainst faults occurring in the safe area, and enablea wide range of measurement and controloperations to be carried out simply andinexpensively by intrinsically safe techniques.
Applications include the protection of installations
containing simple uncertified devices such asthermocouples, switches, and resistive sensors, orseparately certified energy storing or voltageproducing apparatus, for example ac sensors,transmitters, and current-to-pneumatic (I/P)converters.
Essential features of the MTL700 Series are theself checking as-you-mount-it earthing via two studsdirectly to nickel-plated brass or copper busbar. Theearth connection is on top of the unit, allowing easyinspection, installation and removal. The shape ofthe barrier has been designed for easy wiring, whilethe common (14.5 mm) space requirement of both1- and 2-channel units simplifies planning or
alteration of installations of all sizes. The busbar isinsulated for separate earthing, to eliminate thedanger of invasion by fault currents.
MTL700P shunt-diode safety barriers delivermore power into hazardous areas. Because of thehigher power levels available, it is important whenconsidering the use of MTL700P barriers to check thecompatibility of the electrical safety parameters ofthe field equipment (such as transmitters andsolenoid valves) with those of the barriers to makesure the combination is safe. In addition, with thebarriers designed for IIB gas group applications, theoverall gas classification of the system also needschecking
References. The following documents are
available for further information on MTL700 Seriesbarriers:
AN9007 A users guide to shunt-diode safety barriers
INM700 The MTL700 Series Instruction Manual
All models short-circuit proof
Fixed tagging & cable-screenearthing accessories
Certified to worldwide standards
1 or 2 channels in same package
Electronic protection preventsblown fuses
Higher-power barriers for groupIIC and IIB gases
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June 2004
MTL V mA
702 25 200 125 Transmitters 35 See706 28 300 93 Transmitters 35 'How
707 28 300 93 Switches 35 they
28 diode 50 work
707P 28 164 170 Transmitters, switches, 35 5015 diode controller outputs
708 28 300 93 Solenoids, alarms, LEDs, 35
710 10 50 200 6V dc & 4V ac systems 85 6.0 6.9 c 50710P 10 33 300 8V dc systems 42 8.0 9.2 200715 15 100 150 12V systems 155 12.0 13.0 100715P 15 50 291 12V dc systems 60 12.5 13.8 200722 22 150 147 18V dc systems 185 19.0 20.2 50722P 22 101 213 18V dc systems 121 18.5 20.0 100728 28 300 93 Controller outputs, solenoids 340 25.5 26.6 50
28 300 93 Transmitters 340 25.5b 26.6d 50728P 28 234 119 Controller outputs, solenoid valves 253 24.5 26.0 100729P 28 164 170 Controller outputs, solenoid valves 184 24.5 26.0 100
751 1 10 100 Active dc & ac sensors 20 0.3 2.0 2501 10 100 (low impedance receivers) 20 0.3 2.0 250
755 3 10 300 Resistance temperature 18.0a (0.6) 3.6 250
3 10 300 detectors 18.0a (0.6) 3.6 250
758 7.5 10 750 Gas detectors 18 6.0 7.0 2007.5 10 750 18 6.0 7.0 200
761 9 90 100 145 6.0 7.5 1009 90 100 145 6.0 7.5 100
761P 9 350 25 Strain-gauge 384 7.0 8.1 509 350 25 bridges 384 7.0 8.1 50
764 12 1k 12 Strain-gauge bridges 1075 10.0 10.7 e 5012 1k 12 1075 10.0 10.7 e 50
766 12 150 80 185 10.0 11.2 5012 150 80 185 10.0 11.2 50
766P 12 75 157 Strain-gauge 93 9.8 11.3 10012 75 157 bridges 93 9.8 11.3 100
767 15 100 150 12V dc systems 155 12.0 13.0 10015 100 150 155 12.0 13.0 100
768 22 150 147 18V dc systems 185 19.0 20.2 5022 150 147 185 19.0 20.2 50
779 28 300 93 Controller outputs 340 25.5 26.6 50
28 300 93 340 25.5 26.6 50796 26 300 87 Vibration probes 340 23.5 24.6 50
20 390 51 (MTL796 negative) 435 17.5 18.7 50
760 10 50 200 Active dc & ac sensors 85 6.0 7.4 50
10 50 200 Thermocouples 85 6.0 7.4 50
765 15 100 150 135 12.0 13.2 5015 100 150 135 12.0 13.2 50
772 22 300 73 2-wire dc & ac systems 340 18.0 19.7 5022 300 73 340 18.0 19.7 50
778 28 600 47 665 24.0 25.7 5028 600 47 665 24.0 25.7 50
786 28 diode Signal returns 2.2V+30 25.5 26.6 5028 diode 2.2V+30 25.5 26.6 50
787 28 300 93 Controller outputs, 340 25.5 26.6 5028 diode switches 2.2V+30 25.5 26.6 50
787S 28 300 93 Transmitters 340 25.5 26.6 50
28 diode Controller outputs, switches 0.9V+20 25.5 26.6 50
787SP 28 234 119 Transmitters, controller outputs 258 24.5 26.5 8028 diode switches 0.9V+16 24.5 26.5 80
788 28 300 93 340 25.5 26.6 5010 50 200 85 6.0 6.9 50
Transmitters 340 25.5 26.6 50788R 28 300 93 85 6.0 6.9 50
10 50 200
791 11 51 216 31.25kbit/s fieldbus 62.6 10V (at 50A) 10.5 10011 51 216 installations 62.6 10V (at 50A) 10.5 100
799 Dummy barrier for securing cables for future installations takes hazardous-area circuits to earth.
}
3
4
1
2
Internalterminator
Vmax
V
SPECIFICATIONS Key barriers shown in blue
Model
No.
Safety description Polarities
available
+ ac
Basic circuit
Hazardous Safe
Fuse
rating
mA
Vwkg at
10(1)A
V
Max. end-
to-end
resistance
Application
}
3
4
1
*2
(768 & 779 require channelsseparate in IIC)
}
3
4
1
2
ac
3 (26V:796)
4 (20V:796)
1
*
2
3
4
1
ac
2Star connected
3
4
1
23
4
1
2
787S
3 (28V)
4 (10V)
1
2
250
a: Tolerance 0.15 at 20C, channels track within 0.15 from 20 to +60C. d: ac version 26.1V.b: ac version 24.5V. e: ac version 11.2V.c: ac version 7.4V. : Gas group IIB (CENELEC), C (N America).*Diagrams show positive versions. All diodes reversed on negative versions. Additional diodes fitted on ac versions.Patents for MTL787S: UK Patent No. 2210522, USA Patent No. 4860151; Patents for MTL707P: UK Patent Nos. 2210521, 2210522; USA Patent No. 4860151; Patents forMTL787SP: UK Patent No. 2210522; USA Patent No. 4860151
3
4
1
2
Terminal1 & 2opencircuit
4 2
See 'HOW THEY WORK'and
'OVERVOLT-PROTECTED
BARRIERS'
See additional
specification
switches
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June 2004
Ambient temperature and humidity limits
20 to +60C continuous working40 to +80C storage595% RH
Leakage current
For basic barriers with a working voltage of 5V or more, theleakage current decreases by at least one decade per voltreduction in applied voltage below the working voltage, over twodecades. For the MTL755 it decreases by at least one decade fora 0.4V reduction in applied voltage.
Terminations
Terminals accommodate conductors up to 4mm2 (12AWG)Hazardous-area terminals are identified by blue labels.
Colour coding of barrier top
Grey: non-polarisedRed: positive polarityBlack: negative polarityBlack (red label for safe-area terminals):
positive supply, negative to transmitter (MTL706)White: dummy barrier, MTL799
Weight
125g approx
Mounting and earthingBy two integral M4 x 9 tin-lead plated steel fixing studs andstainless steel self-locking nuts (provided).
EMC compliance
EN 50 081-2/EN 50 082-2, generic emission/immunitystandards. These rtefer to appropriate IEC/CISPR standards.(MTL707P+ and MTL702+ are not CE marked)
GENERAL SPECIFICATIONHOW THEY WORK
All MTL700 Series 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 eventof an electrical fault in the safe area, the diodes limit the voltage thatcan reach the hazardous area and the resistor limits the current. Afuse protects the diodes, and the two stages of voltage limitationensure continued safety if either stage should fail. No active output-current limiting circuits are employed. All models are certified ia forall zones and IIC for all explosive atmospheres (except MTL707P+and MTL729P+, 'ia' 'IIB').
1. Safety descriptionThe safety description of a barrier, eg 10V 50 200mA, refers tothe maximum voltage of the terminating Zener or forward diode whilethe fuse is blowing, the minimum value of the terminating resistor, andthe corresponding maximum short-circuit current. It is an indication ofthe fault energy that can be developed in the hazardous area, andnot of the working voltage or end-to-end resistance.
2. PolarityBarriers may be polarised + or , or non-polarised (ac). Polarisedbarriers accept and/or deliver safe-area voltages of the specifiedpolarity only. Non-polarised barriers support voltages of eitherpolarity applied at either end. An exception to this is the MTL791Fieldbus barrier which has one positive and one negative channel.
3. End-to-end resistanceThe resistance between the two ends of a barrier channel at 20C, ieof the resistors and the fuse. If diodes or transistors are present, theirvoltage drop (transistors ON) is quoted in addition.
4. Working voltage (Vwkg)The greatest steady voltage, of appropriate polarity, that can beapplied between the safe-area terminal of a basic barrier channeland earth at 20C for the specified leakage current, with thehazardous-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 barrierchannel and earth at 20C without blowing the fuse. For basicbarriers, it is specified with the hazardous-area terminal open circuit;if current is drawn in the hazardous area, the maximum voltage forthese barriers is reduced. The ac channels of basic barriers andmost channels of overvolt-protected barriers withstand voltages of theopposite polarity also see circuit diagrams.
6. Fuse ratingThe greatest current that can be passed continuously (for 1000 hoursat 35C) through the fuse.
7. Star connectionIn star-connected barriers, the two channels are interlocked such thatthe 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 MTL700 Seriesbarriers is 250V ac/dc.
TERMINOLOGY
APPLICATION KEY BARRIER
Resistance temperature detectorsThermocouples, ac sensors
Controller outputs, one line earthedController outputs, neither line earthed
Transmitters, 2-wire, 4/20mA
Switches
Solenoids, alarms, LEDs
755ac760ac
728+787S+
dc power supply
26.0V 2035V
787S+
787S+
728+
706+
707+
708+
Patents for MTL706+, 707+, 708+, 787S+
KEY MTL700 SERIES BARRIERS SUMMARISED
TYPE
Analogueinput (low-level)
Analogueoutput
Analogueinput (high-level)
Digital (on/off)input
Digital (on/off)output
49.2
85.093.5
9.0
25.0 11.57.5 7.5
14.2
61.5
3
4 2
1
2 1
Hazardous-area
terminals
DIMENSIONS (mm)
Safe-areaterminals
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April 2006
3
4
4/20mA
50mA
40mA max+35V max.
50mA
50mA
2505%
Current limit
RegulateNegative
4/20mA
0V
1
2
+Tx
MTL706+ for 'smart' 2-wire 4/20mA transmittersUK Patent No. 2205699USA Patent No. 4967302European Patent (Germany, France, Italy) No. EP 0 294 139 B1
The MTL706+ is a 1-channel shunt-diode safety barrier, with built-inelectronic overvolt protection, for energising a 2-wire 4/20mAtransmitter in a hazardous area. It is powered from a positive supplyof 2035V dc and delivers a 4/20mA signal into an earthed load inthe safe area. It is proof against short circuits in the field and in thesafe area and is extremely accurate. The MTL706+ will pass incomingcommunication signals up to 10kHz from a smart transmitter, whilein the outgoing direction it will pass signals of any frequency likely tobe encountered.
Since the MTL706+ has no return channel for energising the load, theentire output of the single 28V channel is available to power thetransmitter, providing high output capability. This channel is negativelypolarised, and the safe-area signal is in fact the very current thatreturns through it from the hazardous area, the novel circuit beingenergised by a built-in floating dc supply derived from the external dcsource of power.
To prevent any leakage through the Zener diodes and maximise theoutput voltage available at 20mA, the floating supply is given a risingvoltage/current characteristic. This is achieved by monitoring the4/20mA current, an arrangement which allows all-frequencycommunication in both directions. A separate circuit limits the currentto protect the fuse in the event of a short circuit in the hazardous area.
With a 22V supply, the barrier will deliver 15V minimum at 20mA forthe transmitter and lines and consumes less than 40mA in normal
operation.
Note: the MTL706+ supercedes the MTL705+, which was similar in basic
performance but did not pass outgoing communication signals below about
1kHz.
BASIC CIRCUIT
OVERVOLT-PROTECTED
BARRIERS
ADDITIONAL SPECIFICATIONSupply voltage20 to 35V dc, positive w.r.t. earth
Voltage available for transmitter and lines (at 20mA)(Vsupply 8V), limited at 16V
Voltage available for load (at 20mA)Vsupply 5V
Load resistance850 maximum
Output impedance to load>1M
Calibrated accuracy (at 20C with 250 load)0.05% of maximum output, including non-linearity andhysteresis
Zero temperature drift
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June 2004
OVERVOLT-PROTECTEDBARRIERS(continued)
MTL707+ for switch inputsUK Patent Nos. 2245439, 2210521European Patent No. EP 0 310 280 B1
The MTL707+ is a 2-channel shunt-diode safety barrier similar to theMTL787+ but with built-in electronic overvolt protection. It is intendedprimarily for safeguarding a hazardous-area switch controlling arelay, opto-coupler or other safe-area load from an unregulated dcsupply in the safe area.
The outgoing channel accepts supply voltages up to +35V and isprotected against reverse voltages: the return channel is unaffected byvoltages up to +250V.
In normal operation the protection circuit introduces only a smallvoltage drop and shunts less than 1mA to earth, so its overall effect isminimal. If the supply voltage exceeds about 27V, however, causingthe Zener diodes to conduct or if the safe-area load has a very lowresistance the supply current is limited automatically to 50mA,protecting the fuse and power supply and enabling the loop tocontinue working.
ADDITIONAL SPECIFICATION
Supply voltage (Vs)
10 to 35V dc, positive w.r.t. earthOutput current (Iout)
Up to 35mA availableMaximum voltage drop (at 20C, current not limited)
Iout x 370 + 1.5V, terminal 1 to 3Iout x 50 + 2.1V, terminal 4 to 2
Supply current
Iout + 1mA max, Vs 28V or low load resistance
MTL707P+ for switch inputs, IIB gasesThe MTL707P+ is a two-channel shunt-diode safety barrier similar tothe MTL787SP+, but is designed for use with group IIB gases andfeatures built-in electronic overvolt protection allowing use withunregulated power supplies up to 35V dc. It is intended primarily asa low cost solution for driving IIB certified 2-wire 4/20mA
transmitters, but can also be used with controller outputs with currentmonitoring, solenoid valves and switches. To protect the fuse andenable the loop to continue working, the supply current is limitedautomatically at 50mA should the output be short-circuited or excessvoltage applied.
ADDITIONAL SPECIFICATIONSupply voltage channel 1 (Vs)
10 to 35V dc positive with respect to earthOutput current channel 1 (I out)
Up to 35mA availableMaximum voltage drop (at 20C, current not limited)
Iout x 200 + 0.2V, terminals 1 to 3Iout x 18 + 1.3V, terminals 4 to 2
Supply current
Iout + 2mA max, Vs28V or low load resistance
MTL708+ for switched outputsUK Patent No. 2210521
European Patent No. EP 0 310 280 B1
The MTL708+ is a 1-channel shunt-diode safety barrier similar to theMTL728+ but with built-in electronic overvolt protection. It is intendedprimarily for safeguarding solenoids, alarms, light-emitting diodes orother hazardous-area loads controlled by a safe-area switch from anunregulated dc supply in the safe area.
The barrier accepts supply voltages up to +35V and is protectedagainst reverse voltages.
In normal operation the protection circuit introduces only a smallvoltage drop and shunts less than 1mA to earth, so its overall effect isminimal. If the supply voltage exceeds about 27V, however, causingthe Zener diodes to conduct or if the hazardous-area load has avery low resistance the supply current is limited automatically to50mA, protecting the fuse and power supply and enabling the loopto continue working.
BASIC CIRCUIT
ADDITIONAL SPECIFICATION
Supply voltage (Vs)
10 to 35V dc, positive w.r.t. earthOutput current (Iout)
Up to 35mA available
Maximum voltage drop (at 20C, current not limited)Iout x 370 + 1.5V, terminal 1 to 3
Supply current
Iout + 1mA max, Vs 28V or low load resistance
Hazardous area Safe area
Currentlimit50mA
50mA
1 mA maxI out
+35V max1
2
3
4
Hazardous area Safe area
BASIC CIRCUIT
Hazardous area Safe area
BASIC CIRCUIT
Currentlimit
50mA
1 mA maxI out
3
2
+35V max1
LED,alarm,
solenoid,etc
4
Current
limit 50mA
50mA
1 mA maxI out
+35V max1
2
3
4
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June 2004
BARRIERS FOR SENSORS
Note: voltage figures shown on busbar are safety description values.
ANALOGUE INPUTS, LOW LEVEL
ThermocouplesThe preferred barrier for thermocouples is the MTL760ac, whose 2-channel non-polarised design retains the earth-free nature of thesignal. Provided that the receivers input circuit floats, the combinationrejects common-mode ac and dc interference up to at least 6V and isunaffected by earth faults on the primary element. Even if thereceivers circuit is tied to its 0V rail, the use of a 2-channel barriertakes the worry out of earthing. To eliminate errors due to thermalemfs, the compensating cable should be continued from the barrier tothe receiver. For moving coil or other low resistance receivers, use theMTL751 (40) or the MTL755ac (36) if the resistance of theMTL760 (170) is unacceptable.
USA regulations permit the thermocouple to be earthed on theassumption that the barrier will not conduct, but Europe and otherIEC countries assume that it may do so. In these countries either thethermocouple and its cables must be insulated to withstand 500V, Fig.1; or the earth loop must be broken by an isolating transmitter, Fig.2, or by one of the isolating interface devices in the MTL2000, 3000,4000 or 5000 Series.
Resistance temperature detectorsFor 3-wire circuits with a floating bridge, the most economical solutionis provided by the MTL755ac 2-channel barrier,Fig.4. The two leads from the bridge arms are protected by thebarrier, while the third (supply) lead is earthed at the busbar. TheMTL755ac has a low end-to-end resistance of only 18.0 perchannel to minimise span changes, and its channels track within0.15 (from 20 to +60C) to minimise zero shift with temperature.Close tolerancing of each channel to 0.15 at 20C facilitatesbarrier substitution.
If the bridge circuit is already earthed, a third barrier channel isneeded; in practice this can be one half of another MTL755ac, Fig.5. For extreme accuracy use three channels and an earth-free bridge,since the small errors due to barrier leakage tend to cancel.
4-wire constant-current circuits do not require matched barrierresistances, and can be protected more economically by twoMTL761ac 2-channel barriers, Fig.6. If the increase in loop resistanceis too great, use two MTL755acs.
Slidewire displacement transducersThere are many solutions. Perhaps the simplest is that shown in Fig.
7, where an MTL760ac supplies power and brings back a unipolarsignal. Other barriers that could be used include the MTL761ac,765ac, 772ac, 778ac. Where polarity reversal or very highaccuracy are required, use the techniques designed for strain-gaugebridges, overleaf.
Photocells, ac sensors, flowmetersSimilar arguments apply, and the MTL760ac is recommended, Fig. 3.Any other 2-channel non-polarised barrier that will handle the voltagewould be suitable. All MTL barriers of this type transmit signals up toa few kHz. At higher frequencies the self capacitance of the Zener
diodes around 1000pF may attenuate the signal. No certificationis required for sensors generating less than 1.2V, 0.1A, 20J and25mW. In practice this includes all photocells, but some ac sensorsmay have significant inductance and require to be designed andcertified for use in hazardous areas.
Hazardous area Safe area
MTL760ac
0VVout+7V max.
Fig. 7
3
4
1
2
10V
10V
MTL760ac34 2
110V10V
Compensating cable
Insulatedfrom earth
Fig. 1
Recorder,controller,
data logger,computer
Fig. 2
Temperature transmitter34 2
1
10V10V
Fig. 3
MTL760acEarth-freesignal
Photocell,microphone,
turbineflowmeter, etc
34
21
Receiver
LNE
3V3V
Fig. 4
MTL755ac34
21
Receiver
LNE
3V3V
3V3V
Fig. 5
MTL755ac
1/2MTL755ac
34
21
3421
Receiver
LNE
MTL761ac
MTL755ac
MTL761ac
MTL755ac
Fig. 6
34 2
1
34 2
1
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June 2004
BARRIERSFOR SENSORS(continued)
ANALOGUE INPUTS, LOW LEVEL(continued)
Strain-gauge bridgesFig. 8 shows an arrangement using two or three barriers, which issafe in IIC gases (system certificate no. Ex842125). With theMTL761ac, the circuit is powered from a 12V, 290 source; if thebridge resistance is 290, then the bridge voltage is 6V. If theMTL766ac is used, the source is 20V, 370, and provides a bridgevoltage of 10V when the bridge resistance is 370.
Quite frequently there is a demand to monitor three load cells, and apossible circuit is shown in Fig. 9 (system certificate no. Ex842128).The two channels of each MTL766ac barrier are connected in parallelto reduce the source resistance, and provide 8V across the three350 bridges. However, the higher energy present means that thesystem is safe in IIA and IIB gases only.
Weighing by load-cell is an application where the lower voltage dropof the MTL766Pac, compared to the MTL766ac, is a great advantage.In such applications, the MTL766Pac supplies power to the bridgewhile an MTL761Pac interfaces with the sensing and pick-off circuits.Using the two barriers in combination (see Fig. 10), the minimumvoltages available in 350 bridge systems with a 10V supply are asfollows:-
1 bridge: 13.0V2 bridges: 9.7V3 bridges: 7.7V4 bridges: 6.4V
Hazardous area Safe area
Gas detectors, logic systemsSome devices require a high current at a low voltage, for example,300mA at 2.3V for a typical gas detector. The low end-to-endresistance (18) of the 2-channel MTL758+, and its working voltage
of 6V, make this barrier ideal for energising gas detectors, 5V logicsystems, certain displays and similar equipment. The two channelscan be used separately or in parallel as required, and the systemremains safe in IIC gases if an MTL761ac is added to bring back themeasurement.
Fig. 11
Gas detector
MTL758+
MTL761ac 0V
9V9V
7.5V7.5V +6V
Output
34
12
34
12
MTL766Pac
MTL761Pac
350Bridge
Fig. 10
12V12V
+ Sense
MTL761Pac
Output, mV
9V9V
9V9V
+10V
0V
10V
350
Bridge
350Bridge
supply34
12
34
12
34
12
Fig. 8
Bridge
MTL761acMTL766ac
MTL764ac
MTL761ac
0V
+ Sense
12V12V
9V9V
+10V (MTL766)+6V (MTL761)
6V (MTL761)10V (MTL766)
Optional
Output, mV
34
12
34
12
34
12
MTL764ac
MTL766ac
MTL766ac
Bridge
Bridge
Bridge
Fig. 9
12V12V
+ Sense
MTL764ac
Output, mV
12V12V
12V12V
12V12V
+10V
10V0V
34 12
34
12
34
12
34
12
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June 2004
BARRIERS FORTRANSMITTERS ANDSWITCHES
ANALOGUE INPUTS, HIGH LEVEL2-wire 4/ 20mA transmittersIf several transmitters are to be operated from a common dc supply,and this can be closely regulated (at 26V max), the MTL787S+ nowbeats the previously recommended MTL788+ by nearly half a volt,providing up to 12.9V at 20mA for a transmitter and its lines, as wellas the usual 5V for the load, Fig. 12. Its return channel is moretolerant of errors during installation and fault finding, and it is safewith cables of much higher inductance. If the load requirement canbe reduced, the voltage available for the transmitter will be greater.
If the supply can be closely regulated, and the transmitter iscompatible with the higher power levels available from this barrier inIIC gas groups, the 2-channel MTL787SP+ is recommended. With a
26V supply it provides 14.6V at 20mA for a transmitter and lines aswell as the usual 5V for the load, beating the MTL787S+ by 1.7V, Fig.12.
The voltage available for the transmitter and its lines can be increasedby converting the return current into a 15V signal before it passesthrough an MTL788+ barrier. The MTL788R+ contains a 250precision resistor for this purpose and makes 14.2V available, Fig.13.
If greater voltage capability is required for the transmitter or the load,or the supply is not closely regulated, then the MTL702+ is a provengood solution, Fig. 14. This overvolt-protected barrier delivers 14V at20mA for the transmitter and lines from a 2235V supply, plus a quiteexceptional 17V for the load, both voltages being increased by 2V if
the supply is at least 24V. It will pass incoming communication signalsof any frequency from a smart transmitter but inherently cannot passsignals in the outgoing direction. Accuracy is high and currentconsumption is less than 60mA.
Where 2-way communication with smart transmitters is required,there are two solutions. If the supply is closely regulated, choose theMTL787S+ (above). If it cannot be closely regulated, choose theMTL706+, Fig. 15. This overvolt-protected barrier derived from thepreviously recommended MTL705+ provides 15V at 20mA for thetransmitter and lines from a 2235V supply, plus 5V for the load. It islower in cost than the MTL702+, is extremely accurate, has a standardsafety description and consumes only 35mA. Note that the loadresistor must be 250 5% and that terminal 3 is negative.
If the supply is poorly regulated, the 2-channel MTL707P+ provides alow cost solution for IIB applications, where its low end-to-endresistance makes 13V available for the transmitter and field cablingplus 5V for the load when powered from 24V dc, and its overvoltprotection allows supply variations up to 35V dc, Fig. 16.
Fire and gas detectionDesigned primarily for fire and gas detection systems, the lowermaximum end-to-end resistance of the MTL722P+ (121) comparedto the MTL722+ (185) can be an advantage (see Fig. 17). Inaddition, it may prove useful in other 18V dc systems.
MTL707P+*28VDiode15V
+35V max.15V
0VFig. 16
4/20mA
34
12
* CENELEC gas group IIB (C & D N. America)
IIB
Hazardous area Safe area
+18V
0V
MTL722P+
22VEarth18V dcsystems
Fig. 17
34 12
MTL787S+MTL787SP+
28VDiode
Fig. 12
4/20mA 250
+26V max.15V
0V
34
12
MTL788R+28V10V
Fig. 13
4/20mA
+26V max.15V
0V
34
12
MTL702+
25VE
+35V max.15V
0VFig. 144/20/mA 250
34
12
MTL706+28V
E+35V max.15V
0VFig. 15
Negative
4/20/mA 250
34
12
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June 2004
DIGITAL (ON/ OFF) INPUTS
SwitchesFor switch-status transfer, Fig. 18, the MTL707+ is preferred for tworeasons. First, the circuit fails safe if there is an earth fault on either line ie the safe-area load de-energises. Second, the MTL707+ accepts upto 35V from poorly regulated power supplies without blowing its fuse:if the supply is well regulated, the MTL787S+ can be used, see Fig. 19.
In both cases, optimum power transfer with relays is achieved if the
resistance of the load is made about equal to the combined resistanceof the two channels. The relay coil should then be rated at about halfthe supply voltage.
Switches, data-loggers, logic systemsThe MTL710P+ can be used for supplying power to low-voltageequipment in hazardous areas, such as data-loggers, switches andlogic systems. Its low maximum end-to-end resistance of 42,compared to 85 for the MTL710+, means it can typically supplymore than 65mA current at 5V from an 8V supply (see Fig. 20). It alsohas the advantage of a higher working voltage (8V compared to 6Vfor the MTL710+).
The MTL715P+ functions similarly for 12V dc systems and can also beapplied to multiple-switch and logic circuits where the additionalpower proves useful, Fig. 21.
Hazardous area Safe area
28VDiode
+35V max.Logic signal
0VFig. 18
MTL707+
34
12
28VDiode
+26V max.Logic signal
0VFig. 19
MTL787S+
34
12
+8V
0V
MTL710P+10VEarth
8V dcsystems
Fig. 20
34
12
+12V
0V
MTL715P+15VEarth
12V dcsystems
Fig. 21
34
12
ANALOGUE OUTPUTS
Controller outputs 4 to 20mAIn most cases the output current of a controller flows directly to its 0Vrail and the rail can be earthed at the busbar, Fig.22. Occasionally theoutput circuit may be fully floating. In either event the 1-channelMTL728+ or half an MTL779+ provides the solution. Overvoltprotection is not required, since current limiting in the controller protectsthe fuse in the barrier. The voltage drop introduced by the barrier is6.8V at 20mA.
The maximum voltage drop introduced by the MTL728P+ is only 5.1Vat 20mA (compared to 6.8V for the MTL728+).
For IIB applications, the MTL728P+ can be replaced with the similarlydesigned 1-channel MTL729P+.
If the output circuit of the controller is separated from the 0V rail by thecontrol transistor, Fig.23, a 2-channel barrier is necessary. TheMTL787S+ is recommended, since its return channel can handle up to25.5V, allowing the control signal to be turned off completely. Thevoltage drop introduced by the barrier is 8.1V at 20mA.
The maximum voltage drop introduced by the MTL787SP+ is only
6.4V at 20mA (compared to 8.1V for the MTL787S+). The barrierreturn channel can handle up to 24.5V, allowing the control signal tobe turned off completely.
BARRIERS FOR
CONTROL ELEMENTS
Hazardous area Safe area
MTL728+MTL728P+MTL729P+
+
0V rail28V
E
I/P
Fig. 22
4/20mA34
12
+4/20mA
0V rail
MTL787S+MTL787SP+
28VDiodeI/P
Fig. 23
34
12
Controller
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June 2004
Hazardous area Safe area
MTL787S+MTL787SP+
Monitor
0V rail
+
28V
DiodeI/P
4/20mA
Fig. 24
34
12
Controller
The MTL787S+ and MTL787SP+ are also suitable for controllerscontaining a resistor which enables the return current to be monitoredfor high-integrity operation, Fig.24.
For IIB applications where a 2-channel barrier is needed (see above),the MTL707P+ can be used as, although the overvolt protectionprovided by this barrier is not necessary, the diode return channel andlow voltage drop makes this an ideal replacement for the MTL787S+or MTL787SP+ for IIB gas groups, Fig.25.
DIGITAL (ON/ OFF) OUTPUTS
Solenoids, alarms, LEDsIf the supply is well regulated, an MTL728+ (or half an MTL779+) canbe used.
For IIC applications, the lower end-to-end resistance of the MTL728P+can make a big difference to the overall loop design when drivinghigher-power solenoid valves, provided the power supply is wellregulated, Fig.26.
Should greater power be required for heavy-duty solenoids in IIB or IIAgases only (IEC countries; C & D in USA and Canada), use the twochannels of the MTL779+ connected in parallel or consider theMTL3022 or MTL5022.
Similarly the MTL729P+ is ideal for powering high-power heavy-duty
solenoid valves, in IIB gas group applications, providing a maximumusable output power of 0.78W at 24V, Fig.26.
Most solenoid valves, alarms, light-emitting diodes (LEDs) and otheron/off hazardous-area loads are best driven via an MTL708+ 1-channel barrier, with built-in overvolt protection, Fig.27. The circuit failssafe with an earth fault on the live line and is unaffected by an earthfault on the earth return, while the barrier will accept up to 35V withoutblowing its fuse.
If the control switch is on earth, then a 2-channel barrier has to be used,Fig.28, but be aware that an earth fault on the return line will energisethe solenoid, whereas one on the live line does the opposite. If thesupply is poorly regulated use the MTL707+ (or MTL707P+ for IIBgases). If it is well regulated use the MTL787S+ (or MTL787SP+).
Alternatively, for operational safety, use an MTL2000, 3000, 4000 or5000 Series isolating interface unit.
Fieldbus applicationsThe MTL791 (Fig.29) is a 2-channel barrier designed specifically for31.25kbit/s fieldbus applications. Used with a suitable floatingfieldbus power supply of up to 20V (such as that provided by theMTL5995 unit) it extends the range of fieldbus applications intohazardous areas. It is provided with an internal integral safe-areaterminator*.
* Terminators are used to terminate a fieldbus.
MTL707P+
0V rail
+
28V
DiodeI/P
4/20mA
Fig. 25Controller
34
12
*
* CENELEC gas group IIB (C & D N. America)
MTL708+28VEarth
+35V max
0V
Fig. 27
34
12
28VDiode
+26.6Vmax (MTL787S+)+26.5Vmax (MTL787SP+)+35V max (MTL707+)+35Vmax (MTL707P+)
0VFig. 28
MTL787S+
MTL787SP+
MTL707+
MTL707P+
34
12
* CENELEC gas group IIB (C & D N. America)
*
+11V11V
T
Fig. 29
MTL791+
*FBT1Fieldbusterminator
Fieldbus devices Fieldbus devicesFieldbuspowersupply
34
12
MTL728+MTL728P+
MTL729P+*28VEarth
+26.6Vmax (MTL728+)+26V max (MTL728P+)+26Vmax (MTL729P+)
0V
Fig. 26
34
12
* CENELEC gas group IIB (C & D N. America)
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June 2004
702+ 1 Yes 0.110 2.39 47 0.17 2.2 0.782706+ 1 Yes 0.083 3.05 56 0.12 4.0 0.65707+ Both Yes 0.083 3.05 56 0.12 4.0 0.65708+ 1 Yes 0.083 3.05 56 0.12 4.0 0.65710 1 Yes 3.0 0.91 74 3.0 1.0 0.50
710P 1 Yes 3.0 0.38 44 4.89 0.22 0.75715 1 Yes 0.580 1.45 66 0.7 1.4 0.56715P 1 Yes 0.580 0.33 28 1.04 0.23 1.09722 1 Yes 0.165 1.45 45 0.2 1.4 0.81722P 1 Yes 0.165 0.30 32 0.33 0.53 1.18728 1 Yes 0.083 3.05 56 0.12 4.0 0.65728P 1 Yes 0.083 1.82 44 0.16 2.86 0.83751ac 1 Yes 100 3.72 1464 1000 4.5 0.025
2 Yes 100 0.96 558 1000 1.2 0.05No 100 3.72 732 1000 4.5 0.05
755ac 1 Yes 100 0.46 145 1000 0.4 0.2252 Yes 100 0.13 69 150 0.1 0.45
No 40 0.41 73 150 0.1 0.453 No 40 0.125 48 0.684 Yes 40 0.035 31.25 0.92
No 40 0.06 42 0.92758 1 Yes 11.1 0.070 26 6.0 0.05 1.40
2 Yes 11.1 0.02 10 6.0 0.02 2.80760ac 1 Yes 3.0 0.91 74 3.0 0.9 0.50
2 Yes 3.0 0.20 27 3.0 0.2 1.00761ac 1 Yes 4.9 3.72 163 3.1 3.5 0.225
2 Yes 4.9 0.91 62 0.4 1.0 0.45No 0.31 3.72 81 0.4 1.0 0.45
4 Yes 0.42 0.20 26.39 0.90No 0.42 0.37 37.78 0.90
6 Yes 0.42 0.085 14.39 1.35No 0.42 0.13 18.67 1.35
761Pac 2 Yes 0.31 56 306 0.43 14.4 0.115764 1 Yes 1.41 240 1000 1.5 200 0.036
2 Yes 1.41 61 360 1.0 60 0.072764ac 1 Yes 1.41 240 1000 1.5 200 0.036
2 Yes 1.41 61 360 0.18 60 0.072No 0.125 240 500 0.18 60 0.072
765ac 1 Yes 0.58 1.45 66 0.7 1.3 0.562 Yes 0.58 0.32 22 0.7 1.4 1.125
766ac 1 Yes 1.41 5.8 151 1.5 5.6 0.242 Yes 1.41 1.47 58 0.18 1.5 0.48
No 0.125 5.8 75 0.18 1.5 0.48766Pac 2 Yes 1.41 0.34 29 0.22 0.20 0.942
767 1 Yes 0.58 1.45 66 0.7 1.7 0.562 Yes 0.58 0.32 22 0.5 0.4 1.125
768 1 Yes 0.165 1.45 45 0.2 1.7 0.81772ac 1 Yes 0.165 6.77 89 0.2 6.0 0.404
2 Yes 0.165 1.45 34 0.2 1.8 0.808778ac 1 Yes 0.083 16 107 0.12 14 0.327
2 Yes 0.083 3.05 42 0.12 4.2 0.654779 1 Yes 0.083 3.05 56 0.12 4.0 0.65786 1 or