4–20 mA signal isolation: stopping ground loops and VFD noise

In a healthy loop the transmitter pushes 4 to 20 mA through the receiver's input resistor and the voltage across that resistor is the only thing the PLC reads. Problems start when the two ends of the loop sit at different ground potentials, or when a nearby variable-frequency drive couples high-frequency current into the cable. Both add a current that has nothing to do with the process value, and the PLC cannot tell the difference.

1. Three symptoms and what they tell you

Symptom	Likely cause	Quick test
Reading is offset by a fixed amount, calibration does not remove it	Ground loop: the transmitter ground and the PLC ground differ by a few hundred millivolts, which drives extra current through the shield or the 0 V conductor	Measure DC voltage between the two 0 V points with the loop disconnected; anything above 50 mV is suspect
Reading jumps or flickers when a motor starts, then settles	Common-mode noise from a VFD or contactor coupled into the cable	Note whether the jump coincides with a drive run signal; check if the shield is grounded at both ends
Reading drifts with time of day or production load	Ground potential changes with plant load; long cable runs between buildings	Log the error against plant load; loops crossing buildings or going to an outdoor pump are the usual victims

2. Why isolation works

A galvanic isolator breaks the metallic path between input and output. The input side measures the 4–20 mA, converts it to a digital value, passes it across an optical or transformer barrier, and reproduces a clean current on the output side. Since there is no copper between the two sides, a difference of a few volts between the two grounds cannot drive any current through the loop. SENECA's K-line and Z-line isolators are rated for 1.5 kVac between input, output and supply, which is far beyond any ground offset you will meet on a plant floor.

3. Which SENECA isolator for which case

Situation	Model	Why
2-wire transmitter in the field, PLC has a passive (non-powered) input, you need to feed the transmitter	K109S	Three-way isolation (input, output, supply) and a 17–20 V auxiliary supply for the transmitter; 0.1 % accuracy, 14-bit; square-root extraction for DP flow and linearisation for horizontal cylindrical tanks built in
Simple mA-to-mA or mA-to-V isolation, configured in the field without a PC	K109UI	Everything set by DIP switch: 0/4–20 mA or 0–5/1–5/0–10/2–10/0–15/0–30 V in, mA or V out, three-way 1.5 kVac isolation
No 24 V available in the field, the PLC or DCS card powers the loop	K121	2-wire loop-powered: it takes 7–30 Vdc from the loop itself, outputs a passive 4–20 mA, and is ATEX II 3G/3D rated for Zone 2 and Zone 22
One sensor must feed the PLC and a local indicator or a second system	Z170REG-1	One input, two isolated outputs; each output can be a different range or type
Mixed sensors (thermocouple, Pt100, mA) and you want one spare part plus a limit alarm	Z109REG2-1	Universal input, isolated analog output, one SPST alarm relay; 10–40 Vdc or 19–28 Vac supply

Ten K-line units occupy 62 mm of DIN rail, and they can share supply through the K-BUS connector instead of individual wiring, which matters when you retrofit forty loops into an existing cabinet.

4. Wiring rules that make the isolator work

1. Ground the cable shield at one end only, normally the control-room end. A shield grounded at both ends is itself a ground loop.
2. Keep the 4–20 mA pair in its own screened cable, at least 30 cm from motor cables, and cross power cables at 90 degrees.
3. Run the isolator's supply from the instrument 24 V, not from a supply shared with solenoids or drive cooling fans.
4. Use the isolator's square-root or tank function instead of PLC code when the PLC input card is already scaled; it removes one place where scaling errors hide.
5. Set the output to 4–20 mA rather than 0–20 mA so that a broken wire reads 0 mA and is detected as a fault, not as a valid low value.

5. When isolation is not the answer

If the reading is unstable even with the cable disconnected from the field, the fault is in the panel: a loose terminal, a PLC input card with a failed channel, or a 24 V supply with excessive ripple. If the error scales with the measured value rather than being a constant offset, check the range settings on both ends first. And if the loop runs through a hazardous area with a Zone 0 or Zone 1 classification, you need an intrinsically safe barrier, not a Zone 2 isolator; ask us and we quote the correct GM International barrier instead.

Quick answers

Can I use a K109S to power a 2-wire transmitter?

Yes. The K109S provides a 17–20 V auxiliary supply on its input side, so a 2-wire transmitter can be fed from the isolator while the PLC side stays fully isolated.

Does the isolator change the accuracy of the loop?

The K-line adds a 0.1 % class error with 14-bit (K109S) or 16-bit (K121) conversion. On a loop whose transmitter is typically 0.1–0.25 %, the added error is below what you can see on the HMI.

Is isolation needed when the PLC input card is already isolated?

Most PLC cards isolate the group of channels from the backplane, not channel from channel. If two loops on the same card have different field grounds, you still need an isolator on at least one of them.

How many isolators fit in a standard cabinet?

At 6.2 mm each, 16 K-line units fit in 100 mm of rail. The same count of 22.5 mm converters would need 360 mm.