If a 4 wire instrument is connected to an Analogue Input channel of a DCS system will the signal need a signal isolator.

the AI module at the DCS is powered by 24V derived from a power supply in the panel. The instrument is powered by 110 VAC and generated the 4-20mA signal that is connected back to the DCS. As the 0 volt reference is generated from 2 different sources can the signal be read correctly or is there likely to be interference as the grounds are different?

 

My crystal ball is cloudy.

You don't know until you try it and see.

Or you can be conservative and put an isolator in the loop.

 

You should be ok, most line powered instruments have an isolated power supply but just check Ohms to ground on each output terminal, switch the meter leads both ways to change the polarity. If it passes (showing open circuit) power up the instrument and check there is no current flow from either terminal to ground. (only 1 terminal and ground)you should get 0 mA

Hope this helps
Roy