#instrumentloopcheck

One small loop check mistake can delay your entire plant start-up and cost days of commissioning time.

Many instrumentation problems happen because wiring, signal continuity, or I/O mapping were not fully verified before commissioning.

Instrument loop checking is one of the most critical activities during the commissioning phase of any industrial control system. A single mistake in loop checking can lead to false readings, wrong control actions, or serious safety issues.

This post gives you a clear technical overview of the instrument loop check procedure used by instrumentation and commissioning engineers in real projects.

What Is Instrument Loop Checking?

Instrument loop checking is the process of verifying that a signal travels correctly:

From the field instrument → through wiring & panels → to the PLC/DCS → and back to the control room indication or action.

The goal is simple: - Correct wiring - Correct signal type & range - Correct PLC/DCS reading - Correct alarms & interlocks

Instrument Loop Check Procedure

1- Field Inspection Verify the instrument installation, tag number, orientation, impulse lines, and power supply.

2- Wiring & Continuity Check Check cables from field device to junction box, marshalling panel, and I/O card. Ensure correct termination and polarity.

3- Signal Simulation Simulate input signals (4–20 mA, digital, RTD, thermocouple) and confirm accurate readings on the PLC/DCS.

4- PLC / DCS Verification Confirm correct scaling, engineering units, tag mapping, and logic response.

5- Alarm & Interlock Testing Test alarms, trips, and interlocks to ensure safe operation.

Common Loop Check Mistakes

Wrong I/O channel assignment

Incorrect signal range configuration

Loose terminations

Ignoring grounding and shielding

Testing without approved loop drawings

These mistakes are very common and can delay commissioning significantly.