How to Diagnose a Ground Loop

Procedure

1. 1

   Characterize the symptom

   A ground loop produces a signal offset that changes with load, a 60 Hz hum on the signal, or noise that appears when a specific motor or drive runs. Trend the signal against equipment run status. A reading that is offset by a constant amount that does not change with load is more likely a scaling or calibration problem.

2. 2

   Identify the reference points

   Find where the signal circuit is referenced to ground at the panel end: the negative of the loop power supply, the input card common, or the instrument ground bar. Then find every other place the circuit could touch ground: the transmitter case, a grounded sensor element, the cable shield at the field end, a junction box, a conduit body where a cable is damaged.

3. 3

   Measure the voltage between grounds

   With the circuit intact, measure AC and DC volts between the panel signal ground and the field ground, using long leads if necessary. A few tens of millivolts is normal. Several hundred millivolts, or volts, is a potential difference that will drive current through any path between them.

4. 4

   Measure current in the shield

   Clamp the shield drain wire alone. Any measurable AC current in a shield means it is grounded at both ends or is touching ground somewhere. A shield should carry no current.

5. 5

   Lift the suspected second ground

   With the loop in manual and operations informed, disconnect the field end of the shield, or lift the transmitter from its grounded mount with an insulating adapter, or disconnect whatever the drawing shows as the second reference. Watch the signal. If the noise or offset vanishes, the diagnosis is made. Reconnect and confirm it returns.

6. 6

   Substitute the transmitter

   If lifting the shield changes nothing, disconnect the transmitter and inject a fixed current with the calibrator at the field end. A clean signal now means the transmitter or its mounting provides the second ground: a case-grounded transmitter, a grounded sensor, or moisture in the head.

7. 7

   Check the panel end

   Confirm the shield is landed on the instrument ground bar and not on a terminal that is also the equipment ground for a drive or a starter. Confirm the loop supply negative is grounded at one point. Look for a second supply feeding the same loop from another panel.

8. 8

   Fix it

   Remove the extra ground where it is not needed: re-terminate the shield at one end only, insulate the transmitter mount, repair a damaged cable. Where the second ground is inherent, install a loop isolator between the field device and the input. Where the shield must be grounded at both ends for high-frequency noise, ground one end solidly and the other through a capacitor, as the platform grounding guide describes.

9. 9

   Verify under load

   Reproduce the condition that showed the symptom: run the drive, start the motor, wait for the load that correlated with the offset. A fix tested with the plant quiet has not been tested.

10. 10

    Record it

    Note the second ground that was found, the fix, and the measured voltages before and after, on the loop sheet. Ground loops recur when the next person re-grounds the shield to be safe.

Reading the measurements

Verification

• The symptom is reproduced, then eliminated by the fix, then confirmed absent under the load that produced it.
• Voltage between the grounds is measured before and after and recorded.
• The shield is grounded at exactly one documented end, or the two-end scheme is deliberate and recorded.
• Equipment grounds were never lifted.
• The loop sheet records the fix.