Instrument Loop Checks and Commissioning Procedures
Key Takeaway
A loop check verifies that every instrument signal travels correctly from the field device through wiring, junction boxes, and marshalling cabinets to the PLC or DCS input — and that the displayed value matches the physical measurement. This guide covers the loop check procedure, test equipment, documentation, and common failures.
What a Loop Check Verifies
A loop check is the final proof that the signal path from a field instrument to the control system works correctly. It confirms:
- The field device is connected to the correct terminal at the PLC/DCS I/O card
- The signal type matches (4–20 mA, 0–10 V, discrete, thermocouple, RTD)
- The engineering units and scaling in the PLC/DCS match the instrument range
- Alarm setpoints trigger at the correct values
- Control outputs (if applicable) drive the final element correctly
Loop checks are performed after cable pulling and termination are complete, but before the process is introduced. They catch wiring errors, scaling mistakes, and configuration problems while correction is still cheap.
Loop Check Procedure — Analog Input
- Verify wiring — confirm cable tag matches the I/O list, check terminal assignments at both ends
- Inject signal at 0% — connect a milliamp source at the field device (or the junction box) and set to 4.00 mA. Verify the PLC reads 0% of range (e.g., 0 PSI).
- Inject signal at 50% — set to 12.00 mA. Verify the PLC reads 50% of range (e.g., 500 PSI on a 0–1000 PSI transmitter).
- Inject signal at 100% — set to 20.00 mA. Verify the PLC reads 100% of range.
- Check alarms — drive the signal to the high-alarm and high-high-alarm setpoints. Verify the SCADA/HMI displays the correct alarm state.
- Check wire-break/under-range — disconnect the signal wire. Verify the PLC detects the fault (reads below 4 mA or shows "bad quality").
- Document — record actual readings vs. expected at each test point on the loop check sheet. Note the instrument tag, cable number, I/O address, and any punch list items.
Loop Check Procedure — Discrete Input
For discrete (on/off) inputs like level switches or pressure switches:
- Simulate the switch — jumper the contacts or use a switch simulator
- Verify the PLC reads the correct state (open = 0, closed = 1 or vice versa per design)
- Verify the SCADA/HMI displays the correct status
- Test the alarm if the switch drives an alarm point
Loop Check Procedure — Analog Output
For control outputs driving valves or VFDs:
- Command 0% from the PLC or HMI — verify the valve is fully closed (or VFD is at minimum speed)
- Command 50% — verify the valve is at 50% open (or VFD is at 50% speed)
- Command 100% — verify fully open (or full speed)
- Verify fail-safe — remove the signal or simulate a PLC failure. Confirm the valve goes to its fail-safe position (fail-closed, fail-open, or fail-last per design).
Test Equipment
- mA loop calibrator (e.g., Fluke 773, 789) — sources and measures 4–20 mA
- Multimeter — for voltage, resistance, and continuity checks
- HART communicator — for configuring and troubleshooting smart transmitters
- Thermocouple/RTD simulator — for temperature loop checks
- Pressure hand pump — for checking pressure transmitters at actual pressure (not just mA injection)
Common Loop Check Failures
- Swapped wires — two instruments cross-wired at a junction box. Loop 1 reads Loop 2's value and vice versa.
- Wrong scaling — transmitter is 0–500 PSI but PLC is configured for 0–1000 PSI. Reads half the actual value.
- Reversed polarity — 4–20 mA reads negative or zero. Check + and − at the terminal block.
- Shield grounded at both ends — introduces a ground loop. Analog signal noise shows up as a jittery reading. Ground the shield at the marshalling cabinet end only.
- Missing IS barrier — intrinsically safe circuit wired without the barrier. The instrument works but the safety protection is absent.
Efficient loop checking requires an I&E technician at the field device and a controls engineer at the PLC cabinet, communicating by radio. NFM's I&E commissioning teams include both roles, so loop checks proceed without waiting for a separate contractor.
Frequently Asked Questions
A straightforward analog input loop check takes 15-30 minutes including documentation. Complex loops with control outputs, alarm testing, and fail-safe verification can take 45-60 minutes. Budget 20-25 loops per day for a two-person team (one at the field device, one at the PLC) on a well-organized project with good cable labeling.
You can do partial loop checks — verifying wiring continuity and that the raw mA signal reaches the correct I/O channel. However, verifying engineering units, scaling, and alarm setpoints requires the PLC program to be loaded and the HMI or SCADA configured. Full loop checks are typically done after the PLC and HMI are commissioned.
A calibration adjusts the instrument itself to read accurately (e.g., adjusting a pressure transmitter to output exactly 4.00 mA at 0 PSI and 20.00 mA at 500 PSI). A loop check verifies the entire signal path from the calibrated instrument through wiring, junction boxes, and the PLC/DCS to the operator display. Calibration comes first; loop check comes after wiring is complete.
A loop check sheet documents: instrument tag number, cable number, I/O address, signal type, engineering range, test points (0%, 50%, 100%), expected vs. actual readings at each point, alarm setpoints tested, pass/fail status, technician name, and date. Any discrepancies are noted as punch list items with a responsible party and resolution date.
Typically the I&E technician who performed the test, the controls engineer who verified the PLC readings, and the owner's representative or commissioning manager. All three signatures on the loop check sheet confirm that the signal path was verified from field device to SCADA display.