SIS Diagnosing a Grounding Fault

Real Story: Diagnosing a Grounding Fault in Honeywell SIS During Commissioning

The commissioning phase of any industrial system is a critical time to identify and rectify potential issues before full-scale operations. This real-world story describes how a grounding fault in a Honeywell Safety Instrumented System (SIS) was detected during testing. The fault was traced to a torn cable near the cable gland, which caused intermittent grounding and disrupted the commissioning process.

1. The Incident

1.1 Project Overview

• Phase: Testing and commissioning of a new petrochemical plant.
• System Involved: Honeywell SIS controlling key safety loops.
• Equipment Affected: A critical pressure transmitter responsible for monitoring vessel pressure.

1.2 Fault Trigger

• During testing, the SIS flagged a grounding fault in the pressure transmitter loop.
• The fault caused incorrect signal readings, but since it was a test environment, it did not affect actual plant operations.

2. Initial Observations

2.1 SIS Behavior

• The SIS detected an anomaly in the loop and flagged a grounding fault alarm.
• The issue caused fluctuating readings from the pressure transmitter, preventing proper validation of the loop.

2.2 Related Loop Details

• Transmitter Type: Smart pressure transmitter (4–20 mA).
• Loop Function: Monitored vessel pressure and communicated with the SIS for safety control.
• Cable Route: Shielded instrument cable terminated at a junction box with a cable gland.

3. Troubleshooting Process

3.1 Systematic Investigation

1. Transmitter Inspection:
   • The transmitter was tested in isolation using a handheld communicator and was found to be functioning normally.
2. SIS Logic Verification:
   • The SIS logic and configuration were reviewed, confirming that the grounding fault alarm was triggered by actual electrical anomalies in the loop.
3. Cable Continuity Test:
   • The cable was tested for continuity using a multimeter. Initial readings were inconsistent, suggesting possible internal damage.

3.2 Visual Inspection

• A closer look at the cable termination point revealed no external signs of damage.
• The team initially suspected environmental interference or a loose connection.

3.3 Extended Diagnostics

• The cable was bent and flexed along its length to identify weak points.
• After significant effort, the team observed that flexing the cable near the external cable gland caused intermittent grounding.

4. Root Cause Discovery

4.1 Closer Examination of the Cable Gland Area

• The team inspected the area around the cable gland more closely and discovered the inner insulation of the cable had torn.
• The exposed conductor intermittently contacted the metallic cable gland body, causing a grounding fault.

4.2 Likely Cause of Damage

• The damage likely occurred during cable pulling or gland installation:
  • Excessive Bending: The cable may have been bent beyond its minimum bend radius.
  • Installation Stress: Over-tightening the gland may have compressed the cable, leading to internal damage.

5. Corrective Actions

5.1 Immediate Fix

• The damaged section of the cable was cut and re-terminated.
• A new cable gland with smooth edges was installed to prevent further wear.
• The installation team ensured the cable’s bend radius complied with manufacturer recommendations.

5.2 Testing the Fix

• The loop was reconnected to the SIS and tested under normal commissioning conditions.
• Signal stability was confirmed, and the grounding fault alarm cleared.

5.3 Final Validation

• The pressure transmitter loop was subjected to repeated tests to ensure consistent performance.
• The SIS logic and alarm handling were validated to confirm correct functionality.

6. Lessons Learned

6.1 Attention to Cable Installation

• Proper cable handling and gland installation are essential to prevent damage during commissioning.
• Ensure cables are not bent or compressed beyond their design tolerances.

6.2 Thorough Visual Inspections

• Grounding faults can be subtle and hard to diagnose. Revisiting seemingly minor details, such as gland terminations, is crucial.

6.3 Diagnostic Tools

• Use advanced cable testing tools, such as insulation testers, to identify internal faults more efficiently.

6.4 Commissioning as a Learning Phase

• The commissioning phase is the ideal time to identify and resolve such issues without impacting operations.

7. Recommendations for Future Projects

8. Conclusion

This incident during the testing and commissioning phase highlights the importance of attention to detail in installation practices. The grounding fault caused by a torn cable near the cable gland could have gone undetected until full operation, potentially triggering unnecessary shutdowns and safety alarms.

By systematically troubleshooting and implementing preventive measures, the commissioning team ensured a reliable system before plant startup. This case underscores the value of thorough inspections, proper installation practices, and proactive troubleshooting during the commissioning phase.