Troubleshooting Measures for Bently Axial Displacement Measurement Point Fluctuations and Open-Circuit Faults

During night-shift operations at a plant, a disturbance in process conditions caused a sudden signal fluctuation in the Bently axial displacement measurement point for the air separation unit's booster compressor gearbox, followed immediately by an open-circuit fault alarm. As this measurement point operates on a "two-out-of-two" (2oo2) interlock logic, there was a high risk of triggering the interlock system. Upon receiving the report, the instrument team immediately contacted the process team, processed the maintenance and interlock bypass permits, ensured safety isolation on the process side, and proceeded to the site to troubleshoot and resolve the issue.

Initial Symptoms: The measurement point signal fluctuated before dropping to an open-circuit state; the proximity probe driver output was 0V, and the probe coil resistance was 8Ω (indicating the probe itself was functional).

I. On-Site Troubleshooting Process

1. Measurement at the Proximity Probe Driver (Field): The measured power supply voltage to the Bently proximity probe driver was approximately 12V, while the driver output voltage was 0V. The probe coil resistance measured 8Ω—within the acceptable range—ruling out damage to the probe itself. An abnormally low supply voltage was preliminarily identified as the cause of the fault.
2. Verification at the Safety Barrier (Cabinet Room): Working with the systems team, the output voltage of the safety barrier in the cabinet room was measured at approximately 12V. This was below the standard rated supply voltage of 23–24V for Bently proximity probe drivers, leading to a preliminary suspicion of a safety barrier fault. A power cycle (restart) was performed on the safety barrier; however, the output remained at 12V after power was restored. As the fault persisted, a temporary issue resolvable by a simple restart was ruled out.
3. Fault Localization via Segmented Isolation: The three cables connecting the safety barrier to the field (24V power supply, common/COM, and signal output) were disconnected. Measuring the safety barrier's output in isolation showed the voltage returning to 23.2V, confirming the safety barrier itself was functional and the fault lay within the cable circuit between the cabinet and the field.
4. Confirmation of Cable Fault: Insulation resistance measurements were taken for each cable core. An intermittent ground fault was detected on the common (COM) line. When the ground fault occurred, it pulled down the circuit supply voltage, resulting in insufficient power to the driver; this caused signal instability and fluctuations, eventually leading to an open-circuit fault as the grounding condition worsened. Intermittent grounding faults are influenced by factors such as vibration, cable compression, and fluctuations in ambient temperature and humidity; these faults are sporadic in nature and tend to manifest more frequently at night when equipment vibration is intense.
5. Temporary Remediation (Switching to Spare Cable): Retrieve the spare cable for the affected circuit, rewire both ends, and verify shielding and grounding specifications. After wiring, re-measure the power supply at the proximitor input (confirming 23.2V); verify that power is restored and the measurement signal is back online. Once the measurement reading stabilizes, coordinate with operations to restore the interlock and close out the work permit.
6. Post-Wiring Verification Items
   • Re-verify that the proximitor power supply voltage is stable within the 22.5–24V range;
   • Check the shielding layer: single-point grounding at the cabinet side and floating shield at the field side (grounding at both ends is prohibited);
   • Monitor real-time TSI values ​​and review 12-hour trend data to ensure there are no sudden spikes or fluctuations;
   • For "two-out-of-two" (redundant) measurement points, simultaneously verify the power supply, resistance, and trend data of the other channel, and document the comparison;
   • Restore the interlock only after confirming the measurement point is stable, then close out the work permit.

II. Root Cause Prevention and Optimization Measures

1. Cabling: The compressor gearbox area experiences high vibration; cables must not be routed in direct contact with the equipment casing and should be kept away from high-temperature zones and areas prone to vibration-induced friction. Use protective conduits for cables to prevent insulation wear caused by long-term vibration, which can lead to intermittent grounding. Conduct periodic insulation tests on TSI measurement cables and include cable insulation checks in major overhaul inspection protocols.
2. Proximitors and Safety Barriers: The Bently system's negative power supply is highly sensitive to voltage drops. A loose connection or grounding fault on the COM (common) line can directly pull the 24V supply down to approximately 12V, manifesting as signal fluctuations or open-circuit errors—typical symptoms of this type of fault. Include checks of the proximitor power supply voltage in daily inspections; do not rely solely on DCS display readings.
3. Interlock Management: For "two-out-of-two" (2oo2) logic interlock points, ensure close monitoring of unit operating conditions and minimize the duration of interlock bypass whenever a single-channel fault necessitates such action. Refine emergency response plans to clearly define the coordinated actions required between process and instrumentation personnel in the event of signal fluctuations or wire breaks at TSI measurement points.
4. Fault Log Management: Record this intermittent ground fault in the unit's instrumentation fault log. Mark the faulty cable; locate the point of damage and analyze the cause of wear during the next scheduled shutdown. Organize team training on the characteristic signs of common-terminal ground faults in Bently probes to prevent misdiagnosis involving safety barriers or proximitors.
5. Trend Monitoring: Adjust alarm thresholds within the TSI system to provide early warnings for minor jumps or fluctuations in displacement measurement points, rather than waiting for a wire break to occur before taking action.

III. Fault Summary

The root cause of this fault was insulation damage to the common (COM) core wire of the TSI cable running from the cabinet to the booster compressor gearbox. This caused an intermittent ground fault that pulled down the negative supply voltage to the proximitor, resulting in initial signal fluctuations followed by a wire break. A safety barrier fault was initially suspected because segmented cable isolation had not been performed. For Bently eddy-current probe systems, intermittent grounding of the common terminal is a frequent cause of fluctuations and wire breaks in displacement measurement points on large vibrating machinery. During troubleshooting, prioritize insulation testing with segmented cable isolation to avoid the unnecessary replacement of safety barriers, proximitors, or probes, thereby improving fault resolution efficiency.