Shunt Zeener Diode Isolators on a Floated Control System


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I'm looking at a system that uses a shunt diode type barrier to isolate RTDs and TCs. The outputs of the isolators then go into a Rockwell FlexNet IO system with a floated power supply.

My understanding of the isolator is that it shunts any fault voltage or current through the zener diode to your intrinsic ground, and hence ties one of the wires to intrinsic ground. Does this in effect un-float the control system, or do PLC I/Os somehow (optically?) isolate the measurement it takes at the bridge?