Answer Posted / sam

We normally use a clean separate power source to power all
the instrumentation on the plant including the DCS and
PLC's. This instrumentation power source is going through a
UPS and a inverter backup system to smooth and minimize
power failures. Each zone in the plant normally have its own
24VDC power supply unit, but they are all backed up by the
same UPS.
The UPS can normally keep the instrumentation and other
emergency and critical power for about 6 hours in case of a
total main power failure.

In your 3 wire system the supply to your TX will be from
this external power source, and the loop power for your 4 to
20 ma will be generated by the same TX and will not come
from the PLC or DCS as with a loop power 2-wire systems.
Since the external power to your TX is the same as the 24VDC
power it generates for the signal, you can use the negative
as a common. This common(OV) and the 4 to 20mA(24VDC) signal
is then connected to the PLC as for a normal 2-wire loop
powered TX.
This common from your TX is 0V and is the same as the 0V of
the PLC or DCS since they are all from the same UPS.

The PLC or DCS will only get the positive of your TX as the
input signal, since all the negatives of the inputs are all
connected together on a common 0V bar close to the the PLC
or DCS and only one wire from this negatives bar is then
daisy chained to each input. The negative from your TX you
therefore also need to connect to this common negative bar
of the PLC or DCS and only the positive goes to the input
connection.
The only thing you need to remember is that the 3 and 4 wire
systems are externally powered systems and are therefore
active inputs or source inputs and that the loop powered 2
wire system is a passive or sink type input. This you then
need to set up inside the PLC or DCS input configuration
parameters.
Good luck

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