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Isolated/non-isolated analog inputs

Isolated/non-isolated analog inputs

Isolated/non-isolated analog inputs

(OP)
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?

RE: Isolated/non-isolated analog inputs

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

RE: Isolated/non-isolated analog inputs

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

RE: Isolated/non-isolated analog inputs

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

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