I'm using a CLICK PLC to read three 4-20mA signals and then output the highest. The three signals I'm sending to the PLC can be read in series and parallel with a multimeter. The PLC output can only be read in series. Reading the output in parallel gives me a reading that's double what the series reading is. I am able read the PLC output with a Current/Voltage Calibrator in parallel and get a correct signal reading though. Why is there a difference in with reading these signals?

 

When there is a voltage difference between AO (-) and AI (-) that voltage difference will drive a current that is called a common mode ground loop.

If an active load powered by a line voltage (an analog input powered by a line powered device) is put across an active, powered analog output, then there's a very high likelihood of a ground loop occurring. The ground loop current added to the output current is more or less than the output current itself (polarity of the ground loop can add or subtract current from the output current). When a battery powered, floating current/voltage calibrator is used to source or read a signal, there's no ground connection so there's no ground loop and the calibrator 'sees' only the output current. I suspect your current/voltage calibrator is battery powered.

Sometimes PLC's have an isolated AI card available. The other approach is to use a 4-20mA repeater/isolator that eliminates the ground loop and passes only the intended current signal.

 

I'm using a CLICK PLC to read three 4-20mA signals and then output the highest. The three signals I'm sending to the PLC can be read in series and parallel with a multimeter. The PLC output can only be read in series.

A multimeter set to current mode (or an ammeter) must be used in series. It is used to measure current flowing through a circuit without changing the circuit (at least, very minimally impacting the circuit). It has a very low resistance, so by connecting it in parallel, you are essentially applying a short circuit to the circuit you're trying to measure.

A 4-20mA output modulates voltage, given a load resistance, in order to achieve constant current (according to Ohm's Law, V = IR). It's possible that the Click PLC's precision of modulating its voltage is (understandably) too coarse for the low-resistance load you've connected by connecting the multimeter in parallel. As a quantitative example, say the resistance of the multimeter is 1 milliohm (i.e. 0.001 ohm). Modulating between the entire 4 - 20mA range would be a change in voltage of only 16 microvolts (i.e. 0.000016 V).

 

The calibrator I'm using is battery powered. Your saying since the calibrator has its own separated ground we can just read the signal when connected parallel? The multimeter in parallel is affected by a ground loop at low resistance and thus has to be read in series so the current can just flow over/through it? So, I'm safe to assume then that the calibrator is giving me an accurate enough reading?

 

Without a diagram, I am not sure what you mean by 'connected parallel'.

No, a milliampmeter cannot read current correctly when connected in parallel because some of the current will not flow through the meter. A milliampmeter always needs to be in series to produce a valid reading.

When you put a milliampmeter across an active analog output (active = powered by the AO itself) with nothing else connected to the AO, then the meter is connected in series. All the current runs through the meter. It might appear that the meter is in parallel, {(+) to (+), (-) to (-)], but in reality, the meter is in series because the meter is the load, there is no other path for the current. If this is what you mean by "connected in parallel", it is not a parallel connection it is a "connected in series" connection.

When another device or load is connected to an analog output, the milliampmeter has to be in series, somewhere in the current loop, to get a valid reading. The meter has to be inserted into the circuit (a wire disconnected and the meter probes connected to those two points).

There is a special case where due to difference in ground potential and the resulting common mode voltage, that the current flow in the loop is more or less than the current output of the analog output. A meter connected in series will 'see' the sum of the AO current and ground loop current.

The ground loop will disappear when the field device that is at a different ground potential is disconnected and the ground loop will re-appear when the field device is reconnected.