danw
Lifetime Supporting Member
There are numerous 2 wire temperature transmitters, particularly low cost, low end transmitters (hockey puck style), that are non-isolated, input to output.
So, the installation uses a grounded thermocouple, and that ground potential is transferred through the transmitter to whatever is connected at the other end, in this case the PLC AI.
Add another grounding point from another non-isolated 2 wire transmitter, with who-knows-where and at-what-ground-potential with respect to the PLC AI and you've got a recipe for creating enormous potential (get the pun?) for ground loops and excessive common mode.
As a rule, industrial grade pressure transmitters are isolated, but temperature transmitters might or might not be, depends on the model.
Dan
So, the installation uses a grounded thermocouple, and that ground potential is transferred through the transmitter to whatever is connected at the other end, in this case the PLC AI.
Add another grounding point from another non-isolated 2 wire transmitter, with who-knows-where and at-what-ground-potential with respect to the PLC AI and you've got a recipe for creating enormous potential (get the pun?) for ground loops and excessive common mode.
As a rule, industrial grade pressure transmitters are isolated, but temperature transmitters might or might not be, depends on the model.
Dan