Use shielded cable for 4-20 mA valve commands and position feedback when the route shares an industrial environment with a 20 kW or larger pump, motor switching, or other power wiring. Bond the shield at both ends only when the two bonding points are demonstrably equipotential; otherwise bond it at one end so ground-potential current cannot flow through the shield. For a run below 100 m, a 2 x 0.5 mm2 pair with about 8 ohms total loop resistance causes only 0.16 V drop at 20 mA, but cable routing, loop voltage margin, mechanical requirements, and the actual current of any 24 V circuit still decide suitability.
What do the symptoms say about the signal chain?
Look at the trend first. Compare the commanded current, measured loop current, valve position feedback, and process response on the same time base. A command that remains stable while feedback jumps points downstream toward cabling, grounding, the positioner, or its feedback circuit. Command and feedback moving together may indicate a controller-side reference or supply disturbance. Stable electrical signals with erratic process response shift attention to the actuator, valve mechanics, or process load.
| Signal | Source or measurement point | Wrong-value symptom |
|---|---|---|
| 4-20 mA command | Control output and valve input terminals | Noise or dropouts at both points indicate an output, supply, or common cable-path problem; a clean output but noisy valve input indicates the field circuit. |
| 4-20 mA position feedback | Positioner output and control input | A stable valve position with a fluctuating received value points to the feedback loop, termination, shielding, or input channel. |
| 0-24 V feedback | Field device output and input terminals | False transitions occur when induced voltage, conductor drop, or a reference-potential difference crosses the input threshold. |
| Dry contact | Contact terminals and receiving input | Chatter or intermittent state changes point to contact condition, field wiring, induced voltage, or the input supply/reference. |
| Shield current | Shield bond or drain conductor | Current flowing between two bonded ends identifies unequal ground potential or another unintended return path. |
Correlate disturbances with pump starts, stops, load changes, or switching events. A repeatable time relationship is stronger evidence than adjusting controller tuning until the trend looks quieter. Tuning does not fix wiring.
Why can a current loop still pick up interference?
A 4-20 mA receiver reads current through a finite-impedance circuit. Current transmission tolerates conductor resistance better than a voltage signal because the transmitter regulates loop current while sufficient compliance voltage remains. That does not make the loop immune to capacitive coupling, magnetic coupling, common-mode voltage, poor terminations, or supply disturbances.
The shield intercepts electric-field coupling and carries the coupled current to the bonding system. If both shield ends connect to points at different potentials, the shield also becomes a conductor for ground-potential current. That current can create a magnetic field and couple interference into the enclosed conductors, turning the attempted remedy into another noise source.
Bonding both ends gives the strongest high-frequency shielding only when the bonding system keeps the two ends equipotential and provides a suitable low-impedance path. When that condition has not been demonstrated, single-end bonding prevents circulating shield current. The unbonded end does not automatically make the shield an antenna; it reduces shielding performance compared with a correct two-end bond, but avoids the more damaging ground-loop mechanism.
A large pump can disturb the loop through its motor conductors and associated switching equipment. Coupling depends on cable separation, parallel-run length, switching edge energy, bonding quality, shield termination, and loop geometry—not only on the motor's 20 kW rating.
How should the cable and conductor size be selected?
For the stated runs below 100 m, the estimated total resistance of a 2 x 0.5 mm2 loop is about 8 ohms. At the maximum signal current:
Vdrop = I x R = 0.020 A x 8 ohms = 0.16 V
Add this drop to the valve or positioner burden, output-module requirements, barriers, terminals, and any other series devices. Compare the total with the loop supply's available voltage at the worst operating condition. The current loop is acceptable only when adequate compliance margin remains.
Moving from 0.5 to 0.75 mm2 changes little in a short, lightly loaded current loop unless voltage margin is already tight. Assuming the same conductor material, route, and temperature, resistance varies inversely with area: 8 ohms x 0.5 / 0.75 = 5.33 ohms. The corresponding drop at 20 mA is about 0.107 V, only 0.053 V below the 0.5 mm2 result.
Do not apply the 20 mA calculation to a 0-24 V circuit until its load current is known. Calculate Vdrop = Iload x Rloop, then compare the receiving voltage with the input's specified ON and OFF thresholds. Also check applicable installation rules for any minimum conductor section imposed for mechanical strength or the plant type; that requirement can govern even when voltage drop is negligible.
What installation procedure prevents recurring noise?
- Classify every circuit as a 4-20 mA command, 4-20 mA feedback, powered 0-24 V signal, or dry contact. Record the source, receiver, supply, expected current, and grounding reference.
- Calculate each loop's resistance and voltage burden. Use the actual cable length and device data rather than selecting conductor area from distance alone.
- Route signal cables separately from power cables in accordance with the applicable EMC and installation requirements. Minimize long parallel paths near pump and motor conductors.
- Install each signal cable as one continuous run from the control cabinet to the field device. Avoid intermediate joints, which add failure points, shield discontinuities, and uncertain bonding.
- Test conductor continuity, conductor-to-conductor insulation, and conductor-to-shield insulation before connecting electronics. Record the results by cable identifier.
- Determine whether the field actuator bonding point and control-panel
PEare equipotential. Measure voltage between the intended shield-bonding points under normal operation and during pump switching; use a method and instrument appropriate to the installation's electrical safety category. - If equipotential bonding is verified, terminate the shield at both designated bonding points using the installation's approved method. If it is not verified, bond the shield at one end, commonly the control-system end when project documentation does not designate another point, and insulate the remote end against accidental contact.
- Keep the shield separate from signal return conductors unless the device documentation explicitly combines those functions. Label the termination choice so later maintenance does not create an unintended second bond.
How is the completed installation verified?
Inject or command several points across the operating range and measure current at the source and receiver. Confirm that valve travel and position feedback follow the command without steps, oscillation, or dropout. At 20 mA, compare measured cable drop with the value calculated from measured loop resistance.
Repeat the test while the pump starts, stops, and changes operating state. Trend the command, feedback, and process variable together. A clean loop should not show repeatable electrical excursions synchronized with those events.
For 0-24 V feedback and dry contacts, measure the receiver-terminal voltage in both states while the power equipment operates. Judge the result against the input module's documented thresholds, not against the nominal 24 V label. Where both shield ends are bonded, measure shield current and investigate any unexpected value before accepting the installation.
Which installation mistakes recur on these circuits?
Connecting both shield ends because two enclosures each have a PE bar is a common error. Separate bars are not proof of equal potential. Another is bonding one end intentionally, then creating a second bond through a connector shell, cable gland, junction enclosure, or exposed drain wire.
Running shielded signal cable beside power conductors for most of a 100 m route asks the shield to compensate for poor segregation. Shielding and separation solve different coupling paths; use both where the installation requires them.
Extra conductors can provide useful spares when many signals follow the same route, but a larger multicore cable is not automatically better. It increases copper, installation effort, and the number of conductors that must be identified and insulated. Select multicore construction when routing and maintenance strategy justify it, then test every installed spare.
Finally, increasing conductor area from 0.5 to 0.75 mm2 does not correct a ground loop, an accidental shield bond, a bad terminal, or an EMC routing problem. Size for electrical burden and mechanical requirements; diagnose interference from measurements.
FAQ
How do I ground a 4-20 mA cable shield?
Bond both ends only when the panel and actuator bonding points are demonstrably equipotential. Otherwise bond one end and insulate the remote shield termination so it cannot form an accidental second connection.
How do I check whether two PE points are equipotential?
Measure between the intended shield-bonding points during normal operation and pump switching with equipment suitable for the installation. Also inspect the protective and equipotential bonding path; two separate PE bars alone do not prove equal potential.
How do I calculate voltage drop on a 100 m 4-20 mA loop?
Use Vdrop = I x Rloop. With the stated estimate of 8 ohms for a 2 x 0.5 mm2 loop, the drop at 20 mA is 0.16 V.
How do I test whether a 20 kW pump is disturbing the loop?
Trend command current, received current, position feedback, and process response while the pump starts, stops, and changes state. Repeatable excursions aligned with those events justify checking separation, bonding, shield current, and terminal voltage before changing control tuning.
When should I stop troubleshooting and contact official support?
Stop when measured loop current or terminal voltage remains abnormal after continuity, insulation, routing, burden, bonding, and shield termination have been verified, or when safe equipotential measurements require equipment or access outside your authorization. Escalate to the control-system or field-device manufacturer's official support channel with the wiring diagram, device data, resistance and voltage measurements, termination details, and synchronized trends.