When using mA on analog devices, does the other wire go to 0V or Earth?

Question

On the Schneider analog input module (link) we have IC, IV, IS, 0V and Earth terminals for a single channel. I've seen people connecting the signal wire from a transducer to the IC terminal and the other wire to the earth terminal. Also, I've heard that you should actually be connecting the other (reference) wire to 0V instead since the cable screen will be connected to earth too.

When using mA, where should your reference wire be connected, 0V or Earth and why? Also, is it wrong to connect the reference wire to earth, because I've seen it many times in the plant?

Any information would be really appreciated

Answer 1

It's usually fairly simple.

• Most PLC analog inputs are voltage inputs with an optional shunt resistor to convert 0 - 20 mA to 0 - 5 V or 0 - 10 V. A 250 Ω or 500 Ω resistor will do that job.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. a) A two-wire sensor connection. b) An externally powered sensor connection.

In Figure 1a the sensor has no power supply so it uses the PLC's PSU and regulates the current back to the input. There is no sensor ground connection. If a screen is used it could be connected at the PLC end and left unterminated at the sensor end.

In Figure 1b the sensor is externally powered so it is capable of sourcing current which must return on to its GND terminal. The screen can be used as the ground connection in this case.