Loop check — from field instrument to operator screen

A loop check proves that each instrument loop works end to end — device, wiring, marshalling, I/O channel, scaling, alarms and operator display — after construction and before cold commissioning (IEC 62382).

Prerequisites

• Installation and point-to-point wiring checks completed, insulation tests done.
• I/O list, loop diagrams and HMI available; control system powered with the commissioning software version.
• Calibrator / HART communicator, radios, a loop-check sheet per tag.

Analog input loop (4–20 mA)

1. Confirm tag, range and units against the I/O list and device configuration (HART).
2. Inject or simulate 4, 12 and 20 mA (0 %, 50 %, 100 %) at the device or junction box.
3. Check the value on the controller and HMI at each point: e.g. range 0–10 bar → 0, 5 and 10 bar.
4. Simulate alarm limits and check alarm text, priority and acknowledgement.
5. Check fault behaviour: open the loop (< 3.6 mA) — the channel must report a fault, not 0 %.

Analog output loop

Set 0/50/100 % from the HMI/manual mode, measure current at the actuator and observe the valve/damper position (stroke check).

Digital loops

Actuate the switch or simulate the contact; check state, alarm and interlocks. For outputs, command from HMI and watch the field device (with process precautions!).

Record

Tag, date, injected values and readings, deviations and fixes, signatures. Loops that fail go to the punch list; re-test after correction.