I have a temperature transmitter that reads correct locally and has been reading correctly for a few years. But the signal to the PLC is receiving 45mA today and causing an input failure on our automation system while still reading fine locally. Any explanation that may be going on? I am not sure if there is a short on the cable but this is a 2-wire device or a problem with the transmitter inside the temperature instrument since the local display reads fine.

Anyone experience this?

 

Ground loops can cause a higher than expected current in a 4-20mA circuit. But what is the likelihood of a ground loop developing in a working system?

The fault is more likely a component failure on either the transmitter side or the PLC AI receiver side.

Disconnect the leads. Short the leads together and check the output of the transmitter with a mA meter. Value should match whatever the corresponding mA value is for the display reading. If it doesn't, it's a faulty transmitter.

If the transmitter checks out OK, connect a 4-20mA simulator to the input of the AI card and check the input card's performance.

 

Typically with a loop powered transmitter, you usually have a 24 VDC power source in the PLC location that is grounded to the signal ground connection of the mA inputs. The transmitter only controls the mA signal returned to the PLC. Such transmitters also include a current limiting resistor.

The PLC input will typically include a resistor connected from input to signal ground. This resistor converts the mA to voltage read by the PLC. Units taking voltage only, require an input resistor, to convert mA to voltage.

Make sure your transmitter signal wiring is not locally grounded and run in separate conduit or partitioned cable trays back to the PLC.

If you have a field mounted transmitter power supply, grounding issues can arise of not wired properly.