MTL7700 range 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。

　　ANALOGUE INPUTS (HIGH LEVEL) 2-wire transmitters, 4/20mA, conventional and smart The 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 250Ω load. This application and this barrier is suitable for use with 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 250Ω load. With the MTL7706+ terminal 3 is negative with respect to earth, so the connections to terminals 3 and 4 should be reversed。

　　Vibration probes The 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–.

　　ANALOGUE INPUTS (LOW-LEVEL)

　　Thermocouples and mV sources

　　The recommended barrier for thermocouples and mV sources is the MTL7760ac. This 2-channel non-polarised barrier retains the ‘earthfree’ nature of the signal and, providing the receiver’s input ‘floats’, rejects common-mode ac and dc interference up to at least 7V and is unaffected by earth faults on the primary element.

　　AC sensors, photocells, microphones and turbine flowmeters

　　The 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.

　　Slidewire displacement transducers

　　The simplest choice is the MTL7760ac. This barrier supplies power and brings back a unipolar signal.

　　RTDs

　　For 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-toend resistance of only 19Ω/channel to minimise span changes and its channels track within 0.15Ω (between –20°C and +60°C) to minimise zero shift with temperature

　　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 3 & 4 respectively) track to within 0.15Ω (between –20°C and 60°C).

　　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.

　　STRAIN-GAUGE BRIDGES

　　Single strain-gauge bridges

　　This shows an arrangement using two or three barriers, which is safe in IIC gases.

　　With the MTL7761ac, the circuit is powered from a 12V (±6V), 214Ω (107Ω + 107Ω) source. If the resistive bridge elements are 230Ω, then the voltage applied to the bridge will be 6.2V, and if the bridge resistances are 350Ω, then the bridge's applied voltage will be 7.4V.

　　An MTL7764ac can be used to sense the bridge supply voltage.

　　An MTL7761ac is used here for the mV output

　　STRAIN-GAUGE BRIDGES (cont)

　　Double strain-gauge bridges

　　Quite 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) while two MTL7761Pac barriers interface with the sense and pick-off circuits. Using 350Ω bridge systems, the following voltages are available from an MTL7766Pac with a ±10V supply:

　　1 bridge: 13.11V

　　2 bridges: 9.75V