Troubleshooting

Noisy or Offset Analog Signal: Ground Loops and How to Break Them

The flow reading wanders by 3% and jumps when the pump starts. If the loop is fine on the bench, it is probably a ground loop — here is how to prove it.

· 8 min read · By TryPLC Team

  • Analog Input
  • Noise
  • Grounding
  • Shielding

The symptom

An analog value (flow, level, pressure) is unstable or carries an offset in the plant, yet the same transmitter reads perfectly when you test it on the bench. The reading gets worse when a pump or VFD starts.

What a ground loop is

Suppose the signal circuit is connected to earth at two places that sit at slightly different potentials: the transmitter body earthed at the pipe, say, and the PLC's 0 V earthed in the cabinet. A current then flows through the signal wiring between those two points, and it adds to or subtracts from the signal.

Step 1: prove it

  • Measure the loop current with a series mA meter while the reading is disturbed. If the current itself changes with the pump, the error is real current in the loop.
  • Measure AC and DC volts between the transmitter's negative terminal and local earth, and between the PLC card's 0 V and cabinet earth. More than a volt or two between earth points tells you there is a potential difference.
  • Temporarily lift the earth connection on one end of the signal circuit (never the protective earth of a mains device). If the reading settles, you have found the loop.

    • Step 2: shield rules

    Use twisted-pair shielded cable for analog signals, and never use the shield as a signal conductor.

    In most process plants the shield is grounded at one end, the cabinet, so that no current flows in the shield. Where high-frequency VFD noise is the problem, specifications often call instead for 360° shield clamps at both ends plus a good equipotential bonding conductor. Follow the plant standard and the device manual.

    Step 3: break the loop with isolation

    Isolation removes the second earth path from the signal circuit:

  • Fit galvanically isolated input cards or channel-to-channel isolated modules.
  • Put a signal isolator (4–20 mA in, 4–20 mA out) between the field and the card. Include the isolator's voltage drop in the loop budget.
  • If the inputs are not isolated, check the card's common-mode voltage limit. Exceed it and every channel on the card reads wrong.

    • Step 4: separation and filtering

    Run analog cables in separate trays from motor cables. VFD motor cables should be shielded, with their shields terminated properly. Only after that should you apply filtering, either the card's hardware filter or a software moving average for slow processes. A filter on a ground-loop error only hides it.

    Step 5: verify

    Log the value before and after the fix while the disturbing load cycles. A stable trace with the pump running is your proof. For a dead or overranged loop rather than a noisy one, see 4–20 mA loop troubleshooting.

    Frequently asked questions

    What causes a ground loop in an analog signal?

    The signal circuit is earthed at two points with different potentials, so a current flows through the signal wiring and adds error.

    Should a shield be grounded at one end or both?

    For analog process signals, one end (the cabinet) is common. For high-frequency VFD noise, both ends with a good bonding conductor is often specified. Follow the plant standard and device manual.