Answer / sam

There are only three ways to wire any instrument in the
field. We called them two, three or four wire systems.
Loop powered is the most common and widely used everywhere
to connect the field instruments to PLC's or DCS.

2 wire = 1 wire Pos 24VDC (4 - 20mA signal)(loop Powered)
1 wire 0V

3 wire = 1 wire Pos 24VDC (Instrument supply V)
1 wire 0V (Common for supply and signal)
1 wire Pos 24VDC (4 - 20mA signal)

4 wire = 1 wire Pos 24VDC (Instrument supply V)
1 wire 0V (for supply)
1 wire Pos 24VDC (4 - 20mA signal)
1 wire 0V (for signal)

Also in 4 wire system:

4 wire = 1 wire Live 110VAC (Instrument supply V)
1 wire Neutral 0V (for supply)
1 wire Pos 24VDC (4 - 20mA signal)
1 wire 0V (for signal)

Good Luck

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