Isolated/non-isolated analog inputs

[navyman]

Is there a rule of thumb for specifying isolated analog input PLC modules vs. non-isolated? I can see that isolation is needed if the analog signals are different (0-10 vs 4-20). And I understand the wiring diffences between the two.

I have a CompactLogix application where I need to bring in 2 4-20mA siganals. The signals are from a measuring light curtain controller that is in the same panel as the PLC.

1769-IF4 or 1769-IF4I?

 

[itsmoked]

Normally you want to use isolation if you have ground loop problems or potential ground loop problems. Another good reason is if, during a failure, high voltage could be applied to the normally low voltage signal line. Third would be if you have strong electric or magnetic fields co-located with your signal lines.

You application sounds like none of these are present. It sounds very benign.

I would not bother with "isolated" in your case.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[roydm]

It's more important to specify isolated outputs for your field instruments. There are still some out there with non-isolated sometimes referenced to ground, sometimes to PS positive, these can be a real headache at times.

Roy

 

[navyman]

Thanks for the info. We are going with non-isolated inputs on this application.