Conventional Wiring is one in which each individual instrument is wired separately to the marshalling rack.
The signal travel from each instrument to the IO card on separate wire /terminal dedicated for the same.
Typical type of Conventional wiring contains following steps for wiring.
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Generally a paired cable with shield wireruns from field instrument like transmitter, positioner etc connecting with + & - terminal of instrument to the terminals in Junction Box.
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Junction Box is generally metallic enclosure consisting long strip of terminal where wire from various field devices comes and terminate
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From Junction Box generally a multi-pair cable runs to the marshalling cabinet kept at safe shelter.
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Each pair of multi- pair cable carries signal of one device etc.
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Generally these multi pair cables thick cable in which each pair of cable has their individual shield and as well as overall shield wire.
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This multi- pair cable terminates at FTA( Field Terminal assembly) in Marshalling Cabinet.
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From these FTA arises cross cabling in which each signal is carried to their respective cards. i.e. Analog Input to Analogue Input Card , Digital Output to Digital Output card for Automation system to process.
Few things you should remember while performing conventional wiring instrument
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Instruments are not connected with ground wire and it is generally Cut & Taped at instrument.
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Ground wire is generally connected to grounded at marshalling end only to avoid forming the ground loop which completes the circuit and leads to flow of current and faulty signal
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Instruments , Junction Boxes and marshalling racks generally have Protective Body earthing which help protection from current discharges in body, lightning etc
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Generally it is advisable for longer length signal connection.
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It is more reliable but requires more cable and laying/maintenance of cables.