What’s the isolator unit used for?
Isolator is installed between a transmitter (i.e. sensor) and a receiver to galvanically isolate DC signals. Since field instruments (at field site) and control instruments (in control room) are isolated electrically through an isolator unit, we can minimize various influences coming from field site to control instruments (in control room).
In addition, with isolator unit installed, each instrument can choose own Ground point, each instrument can get stable operation by Ground connection if necessary.
Lastly, isolator unit can provide impedance conversion function and signal conversion function.
In case AC power is supplied to electric instruments, the power is supplied through transformer. One purpose of transformer is to convert the power voltage to meet each instrument's power requirement and to supply the power to it. Another purpose of this is the isolation. Isolation between primary coil and secondary coil inside transformer minimizes mutual influences.
As for AC signal, it is common to isolate between input signal and output signal through input and output transformer. These transformers provide not only impedance conversion but also circuit protection and noise elimination against various troubles coming from signal line.
As for DC signal, we can see many cases of non-isolation between transmitter (field site) and control instrument (control site) because it is difficult for us to find economical and reliable isolation method such as transformer. If your system is simple and is installed in a good electric environment, it is not necessary to install isolator units in your loops, however, you will meet an electric problem in case one signal is distributed to various instruments and your system is installed in noisy environment. Under this condition, if some electric trouble happens, the trouble may be spread out. By using isolator units, this kind of problem is solved.