MTL7755AC 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。

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