Loop Check Failures During Commissioning Explained

 During the commissioning phase of any industrial automation system, loop checking is a critical activity that ensures every signal path — from field instruments to the control system — is correctly 

wired, configured, and functional. When loop checks are not done properly, plants face operational delays, false readings, or even unsafe startup conditions. These issues are commonly known as loop check failures.

Understanding the real problems behind loop check failures requires experience from the field. Below are ten practical questions engineers frequently ask during commissioning, with detailed technical answers based on real site challenges.

1. What is a loop check and why is it important before commissioning?

A loop check is a verification process that ensures the integrity and accuracy of each instrument signal from the field device (such as transmitters, switches, or control valves) to the control system (PLC or DCS).

It confirms:

• Signal continuity (no open or short circuits).

• Correct polarity and grounding.

• Proper scaling and range configuration.

• Functional communication between field and control room.

Without loop checks, hidden wiring or configuration errors can cause serious faults during plant startup such as wrong readings, control failure, or nuisance alarms.

2. Why do we get signal mismatch between field and DCS during loop check?

Signal mismatches usually happen when the DCS scaling does not match the transmitter calibration. For example, a transmitter calibrated for 0–10 bar may be configured in the DCS for 0–100 bar, causing incorrect displayed values.

Other causes include:

1. Wrong engineering unit setup (psi, bar, or kPa).

2. Reversed polarity in the 4–20 mA loop.

3. Signal isolator not properly configured.

4. Incorrect I/O channel assignment in DCS.

To fix it, compare the transmitter range sheet with the DCS configuration, verify the input type, and check the polarity using a multimeter.

3. How do you identify open or short circuits in signal cables?

Cable faults are among the most frequent loop check failures during commissioning. The most efficient method is continuity testing using a digital multimeter or megger:

1. For open circuits, measure resistance between the two ends — if the reading is infinite, the cable is open.

2. For short circuits, measure resistance between conductors or between conductor and shield — any low resistance indicates a short.

3. Visual inspection of terminal blocks and marshalling cabinets often reveals loose or unconnected terminals.

Always verify labeling against loop drawings to ensure the correct cable pair is being tested.

4. Why does the transmitter show correct reading locally but wrong value in control room?

This is a classic symptom of signal distortion or incorrect scaling:

• If the local display reads correctly but the DCS value is higher or lower, check for ground loop interference or reversed polarity.

• If the signal fluctuates or drifts, inspect shielding and grounding — the cable shield should be connected only at one end (typically at the DCS side).

• Incorrect input type (voltage instead of current) can also distort readings.

Testing with a 4–20 mA simulator helps isolate whether the problem lies in the field transmitter, cable, or I/O card.

5. What are common documentation mistakes that lead to loop check failures?

Documentation errors cause confusion and wasted hours in commissioning. The most common issues include:

1. Outdated loop drawings not reflecting as-built wiring.

2. Incorrect I/O mapping between marshalling and DCS cabinets.

3. Missing or mismatched tag numbers between P&ID and loop folder.

4. Using obsolete calibration sheets with wrong range data.

Before starting the loop check, engineers should verify all drawings and loop folders are updated, cross-checked, and approved by QA/QC.

6. How to properly verify analog and digital signal loops?

For analog loops (4–20 mA):

1. Isolate the loop at the field terminal.

2. Inject a known current signal using a loop calibrator.

3. Verify the DCS or PLC displays the corresponding value.

For digital loops (dry/wet contacts):

1. Simulate contact open/close at the field.

2. Observe status change in DCS logic.

3. Confirm debounce or filtering time is configured correctly.

Each loop must be marked “pass” only after both field and control indications match precisely.

7. What is the correct procedure to simulate and verify signals during loop test?

A structured loop test involves:

1. Checking cable continuity and insulation.

2. Applying simulated 4–20 mA or voltage signals from the field end.

3. Observing signal reception and scaling in DCS/PLC.

4. Recording readings at 0%, 50%, and 100% of the range.

5. Verifying alarms, interlocks, and control outputs related to that loop.

All results should be documented in the loop check sheet with remarks on any deviations.

8. Why do some loops fail intermittently even after successful testing?

Intermittent failures are tricky and usually indicate physical or environmental problems:

1. Loose terminals at junction boxes or marshalling cabinets.

2. Moisture or corrosion inside terminals or connectors.

3. Poor cable routing close to high-voltage lines causing electromagnetic interference.

4. Vibration loosening connections over time.

Engineers should perform visual inspection, check tightness, and monitor the signal trend in DCS over time to detect instability.

9. How do you troubleshoot grounding and shielding issues during loop check?

Proper grounding is critical in instrumentation. To troubleshoot:

1. Inspect all cable shields — they must be grounded at one point only (preferably at DCS).

2. Measure potential difference between field and control room grounds — any voltage indicates a ground loop.

3. Ensure instrument enclosures are bonded to earth properly.

4. Avoid running signal cables parallel to power cables in the same tray.

Ground loop issues often cause fluctuating analog readings or noisy signals that fail loop testing.

10. What’s the best way to record and close punch list items for loop test failures?

When a loop fails, it must be logged immediately in a punch list to maintain traceability. The correct approach is:

1. Record loop number, instrument tags, issue description, and responsible discipline.

2. Assign corrective action (e.g., re-termination, recalibration, reconfiguration).

3. After correction, retest the loop and update status to “Closed.”

4. Keep all loop sheets signed and dated for final commissioning approval.

A properly managed punch list prevents repeated issues and ensures a clean handover to operations.

Conclusion

Loop check failures are not just technical inconveniences — they directly impact plant reliability, control accuracy, and safety during startup. Most failures arise from wiring mistakes, documentation errors, and grounding problems that could have been prevented with disciplined installation and verification practices.

A successful commissioning process depends on thorough loop testing, accurate documentation, and a systematic approach to troubleshooting. When each loop is verified with precision, the control system can operate exactly as designed, ensuring smooth and safe plant performance.