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MTL Barriers

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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

    EUROPE (EMEA) Tel: +44 (0)1582 723633 Fax: +44 (0)1582 422283AMERICAS Tel: +1 603 926 0090 Fax: +1 603 926 1899ASIA PACIFIC Tel: +65 487 7887 Fax: +65 487 7997E-mail: [email protected] Web site: ww w.mtl-inst.com

    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


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