technical datasheet technical datasheet MTL700 Series for safe measurement and control in hazardous areas • 1 or 2 channels in same package • Electronic protection prevents blown fuses • Higher-power barriers for group IIC and IIB gases • All models short-circuit proof • Fixed tagging & cable-screen earthing accessories • Certified to worldwide standards MTL700 Series shunt-diode safety The shape of the barrier has been designed barriers are 1- or 2-channel devices which for easy wiring, while the common (14.5 mm) pass an electrical signal in either direction space requirement of both 1- and 2-channel without shunting it, but limit the transfer of units simplifies planning or alteration of energy to a level that cannot ignite explosive installations of all sizes. The busbar is atmospheres. Connected in series with insulated for separate earthing, to eliminate the signal transmission lines on a process the danger of invasion by fault currents. plant, they protect hazardous-area wiring MTL700P shunt-diode safety barriers and equipment against faults occurring in deliver more power into hazardous areas. the safe area, and enable a wide range of Because of the higher power levels available, measurement and control operations to it is important when considering the use of be carried out simply and inexpensively by MTL700P barriers to check the compatibility intrinsically safe techniques. of the electrical safety parameters of the field Applications include the protection of equipment (such as transmitters and solenoid installations containing ‘simple’ uncertified valves) with those of the barriers to make devices such as thermocouples, switches, sure the combination is safe. In addition, and resistive sensors, or separately certified with the barriers designed for IIB gas group ‘energy storing’ or ‘voltage producing’ applications, the overall gas classification of apparatus, for example ac sensors, the system also needs checking transmitters, and current-to-pneumatic (I/P) References. The following documents are converters. available for further information on MTL700 Essential features of the MTL700 Series Series barriers: are the self checking ‘as-you-mount-it’ AN9007 A user’s guide to shunt-diode safety earthing via two studs directly to nickel- barriers plated brass or copper busbar. The earth connection is on top of the unit, allowing INM700 The MTL700 Series Instruction easy inspection, installation and removal. Manual EPS700 Rev3 010213 www.mtl-inst.com enquiry@mtl-inst.com SpecificaTionS ‘Key’ barriers shown in blue Max. V at V fuse wkg max. Model polarities Safety description application Basic circuit end-to-end 10 (1) μa rating no. available resistance MTL Hazardous Safe Ω V V ma V Ω ma + – ac 706 28 300 93 3 – – 35 See 'How Transmitters See "HOW THEY WORK" they work’ 707 28 300 93 – – 35 3 Switches and 28 diode – See additional 50 Transmitters, switches, specification 707p† 28 164 170 "OVERVOLT-PROTECTED 35 50 3 controller outputs 15 diode – BARRIERS" – – Solenoids, alarms, LeDs, switches 708 28 300 93 3 – – 35 – c 710 10 50 200 3 3 3 85 6.0 6.9 50 6V dc & 4V ac systems 710p 10 33 300 3 3 42 8.0 9.2 200 8V dc systems 715 15 100 150 155 12.0 13.0 100 3 3 12V systems 3 1 715p 15 50 291 3 60 12.5 13.8 200 12V dc systems 722 22 150 147 3 3 18V dc systems 185 19.0 20.2 50 18V dc systems 722p 22 101 213 3 121 18.5 20.0 100 * controller outputs, solenoids 728 28 300 93 340 25.5 26.6 50 3 4 2 Transmitters b d 28 300 93 3 3 340 25.5 26.6 50 Controller outputs, solenoid valves 728p 28 234 119 3 253 24.5 26.0 100 Controller outputs, solenoid valves 729p† 28 164 170 3 184 24.5 26.0 100 3 1 751 1 10 100 3 20 0.3 2.0 250 Active dc & ac sensors 1 10 100 (low impedance receivers) 20 0.3 2.0 250 ac a Resistance temperature 755 3 10 300 18.0 (0.6) 3.6 250 3 detectors a 3 10 300 4 2 18.0 (0.6) 3.6 250 758 7.5 10 750 3 3 18 6.0 7.0 200 Gas detectors 7.5 10 750 18 6.0 7.0 200 761 9 90 100 3 145 6.0 7.5 100 3 (26V:796) 1 9 90 100 145 6.0 7.5 100 384 7.0 8.1 50 761p 9 350 25 3 Strain-gauge 9 350 25 bridges 384 7.0 8.1 50 * e 1075 10.0 10.7 50 764 12 1k 12 3 3 3 Strain-gauge bridges e 12 1k 12 1075 10.0 10.7 50 185 10.0 11.2 50 766 12 150 80 3 2 4 (20V:796) 12 150 80 185 10.0 11.2 50 766p 12 75 157 Strain-gauge 93 9.8 11.3 100 3 12 75 157 bridges 93 9.8 11.3 100 767 15 100 150 12V dc systems 155 12.0 13.0 100 3 3 15 100 150 155 12.0 13.0 100 768 22 150 147 3 3 18V dc systems 185 19.0 20.2 50 (768 & 769 require channels 22 150 147 185 19.0 20.2 50 } separate in IIC) 779 28 300 93 3 3 Controller outputs 340 25.5 26.6 50 28 300 93 340 25.5 26.6 50 796 26 300 87 3 3 Vibration probes 340 23.5 24.6 50 435 17.5 18.7 50 20 390 51 (MTL796 negative) 760 10 50 200 3 active dc & ac sensors 85 6.0 7.4 50 3 1 10 50 200 85 6.0 7.4 50 Thermocouples 765 15 100 150 3 135 12.0 13.2 50 15 100 150 135 12.0 13.2 50 ac 340 18.0 19.7 50 772 22 300 73 3 22 300 73 340 18.0 19.7 50 2-wire dc & ac systems 4 Star connected 665 24.0 25.7 50 778 28 600 47 3 2 28 600 47 665 24.0 25.7 50 2.2V+30Ω 25.5 26.6 50 786 28 diode – 3 3 3 1 28 diode – 2.2V+30Ω 25.5 26.6 50 Signal returns 4 2 3 787 28 300 93 3 3 1 340 25.5 26.6 50 Controller outputs, 2.2V+30Ω 25.5 26.6 50 28 diode – 787S switches 340 25.5 26.6 50 787S 28 300 93 3 Transmitters 28 diode – 0.9V+20Ω 25.5 26.6 50 4 2 controller outputs, switches 258 24.5 26.5 80 787Sp 28 234 119 3   28 diode – 0.9V+16Ω 24.5 26.5 80 Transmitters, controller outputs switches   3 (28V) 1 788 28 300 93 340 25.5 26.6 50 3 3 10 50 200 85 6.0 6.9 50 250Ω 788R 28 300 93 3 3 85 6.0 6.9 50 4 (10V) 2 10 50 200 3 1 791 11 51 216 31.25kbit/s fieldbus 62.6 10V (at 50uA) 10.5 100 3 installations 11 51 216 3 62.6 -10V (at 50uA) -10.5 100 Internal terminator 4 2 3 1 Terminal Dummy barrier for securing cables for future installations 1 & 2 799 open - takes hazardous-area circuits to earth. circuit 4 2 a: Tolerance ±0.15Ω at 20°C, channels track within 0.15Ω from –20 to +60°C. 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 for MTL787SP: UK Patent No. 2210522; USA Patent No. 4860151 EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213 3 4 HOW THEY WORK GENERAL SPECIFICATION All MTL700 Series barriers are based on the same simple principle. KeY BaRRieR Each channel contains two stages of pulse-tested Zener or forward- connected diodes and an ‘infallible’ terminating resistor. In the event ambient temperature and humidity limits of an electrical fault in the safe area, the diodes limit the voltage that –20 to +60°C continuous working can reach the hazardous area and the resistor limits the current. –40 to +80°C storage A fuse protects the diodes, and the two stages of voltage limitation 5–95% RH ensure continued safety if either stage should fail. No active output- Leakage current current limiting circuits are employed. All models are certified ‘ia’ for all For ‘basic’ barriers with a working voltage of 5V or more, the zones and ‘IIC’ for all explosive atmospheres (except MTL707P+ and leakage current decreases by at least one decade per volt MTL729P+, 'ia' 'IIB'). reduction in applied voltage below the working voltage, over two decades. For the MTL755 it decreases by at least one decade for TERMINOLOGY a 0.4V reduction in applied voltage. Terminations 1. Safety description Terminals accommodate conductors up to 4mm2 (12AWG) The safety description of a barrier, eg ‘10V 50Ω 200mA’, refers to the Hazardous-area terminals are identified by blue labels. maximum voltage of the terminating Zener or forward diode while the colour coding of barrier top fuse is blowing, the minimum value of the terminating resistor, and the Grey: non-polarised corresponding maximum short-circuit current. It is an indication of the Red: positive polarity fault energy that can be developed in the hazardous area, and not of Black: negative polarity the working voltage or end-to-end resistance. Black (red label for safe-area terminals): positive supply, negative to transmitter (MTL706) 2. Polarity White: dummy barrier, MTL799 Barriers may be polarised + or –, or non-polarised (‘ac’). Polarised Weight barriers accept and/or deliver safe-area voltages of the specified 125g approx polarity only. Non-polarised barriers support voltages of either polarity Mounting and earthing applied at either end. An exception to this is the MTL791 Fieldbus By two integral M4 x 9 tin-lead plated steel fixing studs and barrier which has one positive and one negative channel. stainless steel self-locking nuts (provided). 3. End-to-end resistance eMc compliance EN 61326-1:2006 The resistance between the two ends of a barrier channel at 20°C, ie (MTL707P+ is not CE marked. MTL706+ smart transmitter of the resistors and the fuse. If diodes or transistors are present, their communications may be disrupted at high levels of radiated interference). voltage drop (transistors ON) is quoted in addition. 4. Working voltage (Vwkg) DIMENSIONS (MM) The greatest steady voltage, of appropriate polarity, that can be applied between the safe-area terminal of a ‘basic’ barrier channel and earth at 20°C for the specified leakage current, with the hazardous- area terminal open circuit. 25.0 7.5 11.5 7.5 Hazardous-area Safe-area terminals terminals 5. Maximum voltage (Vmax) 9.0 2 1 The greatest steady voltage, of appropriate polarity, that can be applied continuously between the safe-area terminal of any barrier channel and earth at 20°C without blowing the fuse. For ‘basic’ barriers, it is specified with the hazardous-area terminal open circuit; if current is 61.5 drawn in the hazardous area, the maximum voltage for these barriers 49.2 is reduced. The ‘ac’ channels of ‘basic’ barriers and most channels of overvolt-protected barriers withstand voltages of the opposite polarity also – see circuit diagrams. 6. Fuse rating 85.0 14.2 The greatest current that can be passed continuously (for 1000 hours 93.5 at 35°C) through the fuse. 7. Star connection MTL700 SERIES KEY BARRIERS SUMMARISED TYPE APPLICATION KEY BARRIER In star-connected barriers, the two channels are interlocked such that the voltage between them cannot exceed the working voltage, Vwkg: Analogue Resistance temperature detectors 755ac this allows for higher cable capacitance or inductance. input (low-level) Thermocouples, ac sensors 760ac Controller outputs, one line earthed 728+ 8. Maximum safe-area voltage (Um) Analogue output Controller outputs, neither line earthed 787S+ The maximum permissible safe-area voltage (Um) for MTL700 Series dc power supply barriers is 250V ac/dc. 26.0V 20-35V Analogue 787S+ 706+ input (high-level) Digital (on/off) 787S+ 707+ input Digital (on/off) i 728+ 708+ output Patents for MTL706+, 707+, 708+, 787S+ The given data is only intended as a product description and should not be regarded as a legal warranty of proper- ties or guarantee. In the interest of further technical developments, we reserve the right to make design changes. EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213 2 1 aDDiTionaL SpecificaTion Supply voltage oVeRVoLT-pRoTecTeD 20 to 35V dc, positive w.r.t. earth output current BaRRieRS 4 to 20mA Voltage available for transmitter and lines 15V minimum at 20mA with 22V supply 15.5V typical at 20mA with 24V supply Note: voltages are negative w.r.t. earth oVeRVoLT–pRoTecTeD BaRRieRS Load resistance The 706+, 707+, 707P+ and 708+ have built-in overvolt protection, 250Ω ±5% (can be greater if reduced transmitter voltage is allowing their use with unregulated power supplies. In many acceptable) applications, e.g. sensor inputs or controller outputs, there is accuracy insufficient power available to blow the barrier fuse and this additional ±2µA under all conditions protection is not necessary. However, where the barrier is connected Supply current to a power supply, eg, for energising transmitters, switches, solenoids 35mA typical (at 20mA with 24V supply) or local alarms, overvolt protection allows the barriers to be used with 40mA maximum (at 20mA with 35V supply) unregulated supplies up to 35V dc and also gives protection against faulty wiring during commissioning. MTL707+ for switch inputs UK Patent Nos. 2245439, 2210521 MTL706+ for 'smart' 2-wire 4/20mA transmitters European Patent No. EP 0 310 280 B1 UK Patent No. 2205699 The MTL707+ is a 2-channel shunt-diode safety barrier similar to the USA Patent No. 4967302 MTL787+ but with built-in electronic overvolt protection. It is intended European Patent (Germany, France, Italy) No. EP 0 294 139 B1 primarily for safeguarding a hazardous-area switch controlling a relay, opto-coupler or other safe-area load from an unregulated dc supply in The MTL706+ is a 1-channel shunt-diode safety barrier, with built- the safe area. in electronic overvolt protection, for energising a 2-wire 4/20mA transmitter in a hazardous area. It is powered from a positive supply The outgoing channel accepts supply voltages up to +35V and is of 20–35V dc and delivers a 4/20mA signal into an earthed load in protected against reverse voltages: the return channel is unaffected by the safe area. It is proof against short circuits in the field and in the voltages up to +250V. safe area and is extremely accurate. The MTL706+ will pass incoming communication signals up to 10kHz from a ‘smart’ transmitter, while in In normal operation the protection circuit introduces only a small voltage the outgoing direction it will pass signals of any frequency likely to be drop and shunts less than 1mA to earth, so its overall effect is minimal. encountered. 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 – Since the MTL706+ has no return channel for energising the load, the supply current is limited automatically to 50mA, protecting the fuse the entire output of the single ‘28V’ channel is available to power the and power supply and enabling the loop to continue working. transmitter, providing high output capability. This channel is negatively polarised, and the safe-area signal is in fact the very current that BaSic ciRcUiT returns through it from the hazardous area, the novel circuit being energised by a built-in floating dc supply derived from the external dc Hazardous area Safe area source of power. +35V max. To prevent any leakage through the Zener diodes and maximise 40mA max 1   the output voltage available at 20mA, the floating supply is given a  50mA Current limit   rising voltage/current characteristic. This is achieved by monitoring Regulate   Negative the 4/20mA current, an arrangement which allows all-frequency    4/20mA 4/20mA  communication in both directions. A separate circuit limits the current  3 to protect the fuse in the event of a short circuit in the hazardous area. 50mA 50mA 2 – Tx 250Ω + ±5% 4 With a 22V supply, the barrier will deliver 15V minimum at 20mA for 0V  the 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 aDDiTionaL SpecificaTion about 1kHz. Supply voltage (V ) s BaSic ciRcUiT 10 to 35V dc, positive w.r.t. earth output current (i ) Hazardous area Safe area out Up to 35mA available  +35V max. Maximum voltage drop (at 20°c, current not limited)   40mA max 1 I x 370Ω + 1.5V, terminal 1 to 3  50mA out Current limit   I x 50Ω + 2.1V, terminal 4 to 2 Regulate out Negative     Supply current 4/20mA 4/20mA   I + 1mA max, Vs <26V 3 out  50mA 50mA 2 Limited at 50mA, Vs >28V or low load resistance –    Tx  250Ω + ±5%   4   0V   EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213 MTL708+ for switched outputs UK Patent No. 2210521 oVeRVoLT-pRoTecTeD European Patent No. EP 0 310 280 B1 The MTL708+ is a 1-channel shunt-diode safety barrier similar to the BaRRieRS MTL728+ but with built-in electronic overvolt protection. It is intended primarily for safeguarding solenoids, alarms, light-emitting diodes or other hazardous-area loads controlled by a safe-area switch from an unregulated dc supply in the safe area. MTL707P+ for switch inputs, IIB gases The barrier accepts supply voltages up to +35V and is protected against reverse voltages. The MTL707P+ is a two-channel shunt-diode safety barrier similar to the MTL787SP+, but is designed for use with group IIB gases In normal operation the protection circuit introduces only a small and features built-in electronic overvolt protection allowing use with voltage drop and shunts less than 1mA to earth, so its overall effect unregulated power supplies up to 35V dc. It is intended primarily as is minimal. If the supply voltage exceeds about 27V, however, causing a low cost solution for driving IIB certified 2-wire 4/20mA transmitters, the Zener diodes to conduct – or if the hazardous-area load has a but can also be used with controller outputs with current monitoring, very low resistance – the supply current is limited automatically to solenoid valves and switches. To protect the fuse and enable the 50mA, protecting the fuse and power supply and enabling the loop to loop to continue working, the supply current is limited automatically at continue working. 50mA should the output be short-circuited or excess voltage applied. BASIC CIRCUIT BaSic ciRcUiT Hazardous area Safe area Hazardous area Safe area +35V max. 40mA max 1 +35V max.   40mA max 1 50mA  Current limit      50mA Regulate  Current limit   Negative  Regulate  Negative   4/20mA  4/20mA     3     4/20mA 4/20m  A      3 50mA 50mA 2 –   Tx 50mA 50mA 250Ω 2 – + ±5% Tx 250Ω 4 + ±5% 0V 4 0V    ADDITIONAL SPECIFICATION aDDiTionaL SpecificaTion Supply voltage (V ) s 10 to 35V dc, positive w.r.t. earth Supply voltage – channel 1 (V ) s 10 to 35V dc positive with respect to earth output current (i ) out Up to 35mA available output current – channel 1 (i out) Up to 35mA available Maximum voltage drop (at 20°c, current not limited) I x 370Ω + 1.5V, terminal 1 to 3 Maximum voltage drop (at 20°c, current not limited) out I x 200Ω + 0.2V, terminals 1 to 3 Supply current out I x 18Ω + 1.3V, terminals 4 to 2 I + 1mA max, V <26V out out s Limited at 50mA, V >28V or low load resistance Supply current s I + 2mA max, V <25V out s Limited at 50mA, V >28V or low load resistance s EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213         Hazardous area Safe area BaRRieRS foR SenSoRS  
 
 
       ANALOGUE INPUTS, LOW LEVEL   Thermocouples The preferred barrier for thermocouples is the MTL760ac, whose 2-channel non-polarised design retains the ‘earth-free’ nature of the signal. Provided that the receiver’s input circuit floats, the combination  rejects common-mode ac and dc interference up to at least 6V and is unaffected by earth faults on the primary element. Even if the receiver’s circuit is tied to its ‘0V’ rail, the use of a 2-channel barrier takes the worry out of earthing. To eliminate errors due to thermal  emfs, the compensating cable should be continued from the barrier to the receiver. For moving coil or other low resistance receivers, use the MTL751 (40Ω) or the MTL755ac (36Ω) if the resistance of the MTL760 (170Ω) is unacceptable. USA regulations permit the thermocouple to be earthed on the assumption that the barrier will not conduct, but Europe and other      ‘IEC countries’ assume that it may do so. In these countries either the          thermocouple 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 MTL4500 or MTL5500  series. photocells, ac sensors, flowmeters Similar arguments apply, and the MTL760ac is recommended, Fig. 3.   Any other 2-channel non-polarised barrier that will handle the voltage    would be suitable. All MTL barriers of this type transmit signals up  to a few kHz. At higher frequencies the self capacitance of the Zener diodes – around 1000pF – may attenuate the signal. No certification is   required for sensors generating less than 1.2V, 0.1A, 20µJ and 25mW. In practice this includes all photocells, but some ac sensors may have significant inductance and require to be designed and certified for use in hazardous areas.    Resistance temperature detectors    For 3-wire circuits with a floating bridge, the most economical    solution is provided by the MTL755ac 2-channel barrier,  Fig.4. The two leads from the bridge arms are protected by the barrier,   while the third (supply) lead is earthed at the busbar. The MTL755ac  has a low end-to-end resistance of only 18.0Ω per channel to minimise span changes, and its channels track within 0.15Ω (from –20 to +60°C) to minimise zero shift with temperature. Close tolerancing of each  channel to ±0.15Ω at 20°C facilitates barrier substitution.  If the bridge circuit is already earthed, a third barrier channel is  needed; 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 barrier  resistances, and can be protected more economically by two  MTL761ac 2-channel barriers, Fig.6. If the increase in loop resistance is too great, use two MTL755acs. Slidewire displacement transducers      There are many solutions. Perhaps the simplest is that shown in Fig. 7,      where an MTL760ac supplies power and brings back a unipolar signal. Other barriers that could be used include the MTL761ac, 765ac, 772ac,  778ac. Where polarity reversal or very high accuracy are required, use the techniques designed for strain-gauge bridges, overleaf. Note: voltage figures shown on busbar are safety description values. EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213      
  Hazardous area Safe area BaRRieRS foR SenSoRS   (continued)                  ANALOGUE INPUTS, LOW LEVEL (continued)       
Strain-gauge bridges Fig. 8 shows an arrangement using two or three barriers, which is safe   in IIC gases (system certificate no. Ex842125). With the MTL761ac, the      circuit is powered from a 12V, 290Ω source; if the bridge resistance is 290Ω, then the bridge voltage is 6V. If the MTL766ac is used, the source is 20V, 370Ω, and provides a bridge voltage of 10V when the  bridge resistance is 370Ω.             Quite frequently there is a demand to monitor three load cells, and a    possible circuit is shown in Fig. 9 (system certificate no. Ex842128).    The two channels of each MTL766ac barrier are connected in parallel  to reduce the source resistance, and provide 8V across the three 350Ω  bridges. However, the higher energy present means that the system is     safe in IIA and IIB gases only.                  MTL766Pac supply 12V 3 1 +10V 4 12V 2 –10V Weighing by load-cell is an application where the lower voltage drop of 0V the MTL766Pac, compared to the MTL766ac, is a great advantage. In 350Ω such applications, the MTL766Pac supplies power to the bridge while Bridge MTL761Pac an MTL761Pac interfaces with the sensing and pick-off circuits. Using 3 1 9V + the two barriers in combination (see Fig. 10), the minimum voltages Sense 4 2 9V – available in 350Ω bridge systems with a ±10V supply are as follows:- 350Ω Bridge 1 bridge: 13.0V MTL761Pac 9V 1 3 Output, mV 2 bridges: 9.7V 2 4 9V 3 bridges: 7.7V 350Ω Fig. 10 Bridge 4 bridges: 6.4V Gas detectors, logic systems MTL758+ Some devices require a high current at a low voltage, for example, 3 7.5V 1 300mA at 2.3V for a typical gas detector. The low end-to-end resistance +6V 4 2 7.5V (18Ω) of the 2-channel MTL758+, and its working voltage of 6V, make 0V this barrier ideal for energising gas detectors, 5V logic systems, MTL761ac 3 1 9V certain displays and similar equipment. The two channels can be used Output 4 2 9V Gas detector separately or in parallel as required, and the system remains safe in IIC gases if an MTL761ac is added to bring back the measurement. Fig. 11 EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213 Hazardous area Safe area BaRRieRS foR TRanSMiTTeRS anD SWiTcHeS  ANALOGUE INPUTS, HIGH LEVEL     2-wire 4/20ma transmitters        If several transmitters are to be operated from a common dc supply,    and this can be closely regulated (at 26V max), the MTL787S+ now   beats the previously recommended MTL788+ by nearly half a volt, providing up to 12.9V at 20mA for a transmitter and its lines, as well as the usual 5V for the load, Fig. 12. Its return channel is more tolerant of errors during installation and fault finding, and it is safe with cables of much higher inductance. If the load requirement can be reduced, the voltage available for the transmitter will be greater.  If the supply can be closely regulated, and the transmitter is compatible      with the higher power levels available from this barrier in IIC gas groups,     the 2-channel MTL787SP+ is recommended. With a 26V supply it  provides 14.6V at 20mA for a transmitter and lines as well 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 increased by converting the return current into a 1–5V signal before it passes through an MTL788+ barrier. The MTL788R+ contains a 250Ω precision resistor for this purpose and makes 14.2V available, Fig. 13.     Where 2-way communication with ‘smart’ transmitters is required,     there are two solutions. If the supply is closely regulated, choose    the MTL787S+ (above). If it cannot be closely regulated, choose the   MTL706+, Fig. 14. This overvolt-protected barrier provides 15V at   20mA for the transmitter and lines from a 22–35V supply, plus 5V for the load. It is extremely accurate, has a standard safety description and consumes only 35mA. Note that the load resistor must be 250Ω ±5% and that terminal 3 is negative. If the supply is poorly regulated, the 2-channel MTL707P+ provides  a low cost solution for IIB applications, where its low end-to-end       resistance makes 13V available for the transmitter and field cabling    plus 5V for the load when powered from 24V dc, and its overvolt   protection allows supply variations up to 35V dc, Fig. 15.   

 ­ fire and gas detection  Designed primarily for fire and gas detection systems, the lower     maximum end-to-end resistance of the MTL722P+ (121Ω) compared       to the MTL722+ (185Ω) can be an advantage (see Fig. 16). In addition, it may prove useful in other 18V dc systems.   EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213 DIGITAL (ON/OFF) INPUTS Hazardous area Safe area Switches  For switch-status transfer, Fig. 17, the MTL707+ is preferred for two    reasons. 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 up to 35V from poorly regulated power supplies without blowing its fuse: if the supply is well regulated, the MTL787S+ can be used, see Fig. 18.   In both cases, optimum power transfer with relays is achieved if the resistance of the load is made about equal to the combined resistance  of the two channels. The relay coil should then be rated at about half     the supply voltage.        Switches, data-loggers, logic systems  The MTL710P+ can be used for supplying power to low-voltage       equipment in hazardous areas, such as data-loggers, switches    and logic systems. Its low maximum end-to-end resistance of 42Ω, compared to 85Ω for the MTL710+, means it can typically supply more   than 65mA current at 5V from an 8V supply (see Fig. 19). It also has the advantage of a higher working voltage (8V compared to 6V for the MTL710+). The MTL715P+ functions similarly for 12V dc systems and can also     be applied to multiple-switch and logic circuits where the additional       power proves useful, Fig. 20.   BaRRieRS foR conTRoL eLeMenTS ANALOGUE OUTPUTS controller outputs – 4 to 20ma In most cases the output current of a controller flows directly to its ‘0V’    rail and the rail can be earthed at the busbar - Fig. 21. Occasionally   the output circuit may be fully floating. In either event the 1-channel     MTL728+ or half an MTL779+ provides the solution. Overvolt protection   is not required, since current limiting in the controller protects the fuse in the barrier. The voltage drop introduced by the barrier is 6.8V at  20mA. The maximum voltage drop introduced by the MTL728P+ is only 5.1V at 20mA (compared to 6.8V for the MTL728+). For IIB applications, the MTL728P+ can be replaced with the similarly designed 1-channel MTL729P+. If the output circuit of the controller is separated from the ‘0V’ rail by  the control transistor - Fig. 22 - a 2-channel barrier is necessary. The   MTL787S+ is recommended, since its return channel can handle up     to 25.5V, allowing the control signal to be turned off completely. The     voltage 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 barrier return  channel can handle up to 24.5V, allowing the control signal to be turned off completely. EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213 Hazardous area Safe area              The MTL787S+ and MTL787SP+ are also suitable for controllers  containing a resistor which enables the return current to be monitored for high-integrity operation, Fig.23.         For IIB applications where a 2-channel barrier is needed (see above),     the MTL707P+ can be used as, although the overvolt protection    provided by this barrier is not necessary, the diode return channel and low voltage drop makes this an ideal replacement for the MTL787S+ or  MTL787SP+ for IIB gas groups, Fig.24.        DIGITAL (ON/OFF) OUTPUTS    Solenoids, alarms, LeDs        If the supply is well regulated, an MTL728+ (or half an MTL779+) can       be used.  For IIC applications, the lower end-to-end resistance of the MTL728P+ can make a big difference to the overall loop design when driving  higher-power solenoid valves, provided the power supply is well regulated, Fig.25.     Should greater power be required for heavy-duty solenoids in IIB or IIA gases only (‘IEC countries’; C & D in USA and Canada), use the two channels of the MTL779+ connected in parallel or consider the MTL5522. Similarly the MTL729P+ is ideal for powering high-power heavy-duty  solenoid valves, in IIB gas group applications, providing a maximum    usable output power of 0.78W at 24V, Fig.25.     Most solenoid valves, alarms, light-emitting diodes (LEDs) and other on/off hazardous-area loads are best driven via an MTL708+ 1-channel   barrier, with built-in overvolt protection, Fig.26. The circuit fails safe with an earth fault on the live line and is unaffected by an earth fault on the earth return, while the barrier will accept up to 35V without blowing    its fuse.         If the control switch is on earth, then a 2-channel barrier has to be      used, Fig.27, but be aware that an earth fault on the return line will    energise the solenoid, whereas one on the live line does the opposite.    If the supply is poorly regulated use the MTL707+ (or MTL707P+ for IIB gases). If it is well regulated use the MTL787S+ (or MTL787SP+).   Alternatively, for operational safety, use an MTL4500 or MTL5500 series isolating interface unit.     Fieldbus applications The MTL791 (Fig. 28) is a 2-channel barrier designed specifically for     31.25kbit/s fieldbus applications. Used with a suitable floating fieldbus           power supply of up to 20V (such as that provided by the MTL5995 unit)    
it extends the range of fieldbus applications into hazardous areas. It is  provided with an internal integral safe-area terminator*.      * ‘Terminators’ are used to terminate a fieldbus. EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213 MTL700 SeRieS acceSSoRieS • Mounting kits for up to 20 barriers • Busbar, earth terminals and insulating mounting blocks for separate connection to IS earth • Earth terminal rail for cable screens and earth returns • Integral tagging system to display barrier loop data DO-IT-YOURSELF MOUNTING ARRANGEMENT Barriers are carried on lightweight plated busbar, which can be mounted on ‘top hat’ or G-profile rail or any flat surface with the busbar insulated for separate earthing. It is recommended that twin End elevations with recommended spacing. Shaded portions earth cables should be used for maximum security and easy testing. show areas swept by barrier during installation and removal. Each barrier is clamped to the busbar and thereby earthed in a single operation, making it virtually impossible to forget the earth connection, MOUNTING KITS so easily left untightened or untested if there are scores of individual Mounting kits provide all the necessary parts for installing up to earth wires. Robust soft-plated double fixing studs on the top of a specific number of MTL700 or MTL700P Series barriers. The kits each barrier ensure permanent ‘gas-tight’ joints, which are immune available are the MK02 (2 barriers), MK05 (5 barriers), MK12 (12 barriers), to vibration and corrosion yet allow barriers to be installed, inspected and removed easily. and MK20 (20 barriers). Each kit provides facilities for mounting and earthing the barriers, connecting the IS earth cable, terminating cable The accessories system also provides a convenient row of terminals screens and noting tagging information (except the MK02 kit which for terminating earth returns and cable screens, which remains in does not provide tagging facilities). Instruction Sheet INS701 includes place to keep these safely earthed should any barrier be extracted: full assembly instructions. a dummy barrier is available as an alternative for the same purpose. With similar attention to users’ needs, a unique lift-up tagging facility  provides permanent identification of circuits and barrier types to   guide installation, fault-finding and inspection, and to ensure correct   replacement of any barrier for long-term safety.     ACCESSORIES   EBB7 earth busbar, nickel-plated brass and ready drilled in one   metre lengths. IMB7 & SMB7 insulating mounting blocks are a convenient  method of supporting the busbar. They are supplied complete with  fixing screws and are ready for mounting on any flat surface. IMB7 will  also mount on top hat or G-profile DIN-rail. T-section DIN-rail THR2 is available in 1 metre lengths. MK02 Assembly diagram TAG7 tagging strips clip onto the mounting blocks to positively identify each location and provide space for the user to note details of barrier type, loop identification etc. Access to barrier mounting studs    is not impaired; just unclip one edge and swing the strip ‘open’ or, if the optional plastic seal (TGS7) has not been fitted, unclip both edges  and lift it right off. TGL7 replacement labels for the tagging strips are  also available.  ERL7 earth rail is a nickel-plated 3 x 10mm rail that attaches to  the mounting blocks via an ERB7 earth rail mounting bracket. It will  accommodate up to 2.5 ETM7 earth terminals per barrier location for  terminating earth returns and cable screens from the hazardous area.     ERB7 earth rail mounting bracket mounts directly over either type     of mounting block; for a rigid earth rail an ERB7 on each mounting block is recommended. One end carries a bolt-down fitting for the rail   – enabling easy removal for adding extra ETM7 terminals – the other  end carries a 16mm2 terminal. When installed these 16mm2 terminals provide connections for the high-integrity IS earth, for linking between  sections of busbar, and for a common earth return from the safe area.     Other accessories available are the TQS7 torque spanner for safe tightening of the vibration-proof self-locking nuts on the barrier earthing studs; SMC7 surface mounting clips for mounting a single MK05, MK12, MK20 Assembly diagram barrier on a flat surface; and ISL3 or ISL7 self adhesive ‘Take Care’ Note: MK05 has no earth rail, but does have terminal clamps on earth rail mounting intrinsic safety warning labels. brackets EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213 DIMENSIONS (mm)  TAG7 tagging strip with label and 6 ‘clic’ rivets – 1 metre lengths TGS7 tagging strip seal – sold in bags of 10 ETM7 earth terminal – sold in 25  Klippon Electricals Ltd. SMC7 surface mounting clip – sold in bags of 10. Two clips needed per barrier TGL7 tagging strip label only – packs of 10 x 0.5 metre lengths  TQS7 torque spanner – set to 2.3Nm torque. 2 bags of 50. For cable 4mm . ERL7 earth rail – 10 x 3mm - sold in 1 metre lengths. Unplated rail Also available as ‘ZB4’ from available as ‘SSch 10 x 3mm brass busbar', from Klippon Electricals Ltd.  95  ERB7 earth rail mounting bracket – with earth rail bolt-down fitting 2 and terminal for cable 16mm 14.5    4.2    Complete with 7mm A/F socket. ISL3 or ISL7 ‘Take Care’ intrinsic safety label – 175 EBB7 earth busbar – 1 metre lengths. Mounts up to 64 barriers.     ISL3 adhesive back, metal *ISL7 adhesive front, plastic IMB7 insulating mounting block mounts on a flat surface or top 140 (65*) DRK700 adaptor kit allows MTL700 Series barriers to be hat rail (to EN 50 022 – 35 x 7.5; BS 5584; 35 x 27 x 7.3 DIN 46277)  or G-profile rail (to EN 50 035 – G32; BS 5825; 32 DIN 46277).  Recommended maximum number of barriers between blocks is 25. SMB7 insulating mounting block mounts on a flat surface and 100 (40*) provides minimum overall installation height. Recommended maximum number of barriers between blocks is 25. mounted directly on DIN-Rail. EUROPE (EMEA): +44 (0)1582 723633 THE AMERICAS: +1 800 835 7075 ASIA-PACIFIC: +65 6 487 7887 enquiry@mtl-inst.com csinfo@mtl-inst.com sales@mtlsing.com.sg EPS700 Rev3 010213