Loop Checking & Commissioning

What a loop check proves

Each instrument is usually calibrated on the bench and the wiring is tested by the electrical team. The loop check is the step that ties it all together. It confirms that:

• the right device is wired to the right channel (tag numbers match end to end);
• the range, units and direction configured in the DCS/PLC match the device and the datasheet;
• alarms, trips and interlocks act at the correct values;
• outputs move the right final element the right way (fail-safe position included).

A cold loop check is done before process fluids are introduced, by simulating signals. A hot or live check confirms the reading against the real process during start-up.

Before you start

• Permits and isolation: work permit, and lockout of anything a test signal could start or open.
• Documents: loop diagram, instrument datasheet or index, I/O list, cause & effect chart, and a blank loop check sheet for each tag.
• Completed earlier steps: installation inspected, cables insulation-tested and continuity-checked, instrument calibrated, power available, control system configured.
• Tools: a calibrated loop calibrator (mA source/measure), multimeter, HART communicator or laptop, pressure or temperature source if checking from the sensor, and two radios: one person in the field, one at the operator station.

The signal path

Analog input (transmitter → DCS)

1. Confirm the tag on the device nameplate matches the loop sheet.
2. With a HART communicator, check the transmitter's tag, range (LRV/URV), units and transfer function (linear or square root).
3. Drive the loop with the transmitter's loop test function or a calibrator, or apply real pressure/temperature at the sensor for a full check.
4. Step through 0, 25, 50, 75 and 100 % (4, 8, 12, 16, 20 mA). The operator reads the value on the screen at each step.
5. Check each alarm (LL, L, H, HH) and trip point activates at the right value, and resets.
6. Check failure handling: open the loop or drive below 3.6 mA / above 21 mA; the DCS should show bad quality or a fault alarm (NAMUR NE 43).

Quick maths for the expected values: 4–20 mA Signal Scaler. Loop supply and resistance: Loop Power Check.

Analog output and control valves

1. Put the controller in manual and set the output to 0, 25, 50, 75, 100 %.
2. At each step, the field person confirms the valve position (local indicator or positioner display) and the mA at the positioner.
3. Check the action: does 100 % output mean open (air-to-open) or closed (air-to-close), as shown on the datasheet?
4. Check the fail-safe position: remove the signal and the air; the valve must go to its fail position (FC, FO or FL).
5. Check limit switches and position feedback against the DCS, and time the full stroke if the specification gives a stroke time.

Discrete inputs and outputs

• Digital inputs (switches, limit switches, push buttons): operate the actual device (or jumper it at the field end) and confirm the correct state and alarm on screen. Check normally-open vs normally-closed and the "healthy" state: shutdown inputs are usually closed (energised) when healthy.
• Digital outputs (solenoids, motor start/stop): force the output from the DCS with permission and confirm the field device responds, then confirm feedback returns to the DCS.
• For trips, check the full cause and effect: one cause must produce every listed effect.

Practise the logic with the Logic Gate Simulator, including 2oo3 voting.

Records and sign-off

• Fill the loop check sheet as you go: tag, test points, readings, alarms, trips, valve action and fail position, defects found and the fix.
• Mark the loop diagram in a different colour for any change (a red-line or as-built mark-up).
• Both the field technician and the control-room engineer sign; the client or commissioning lead witnesses critical and safety loops.
• Keep a punch list: items that do not pass are tracked until fixed and re-tested.

Common faults found during loop checks

Q&A: test yourself

Related: Transmitters & Signals · HART Communication · 4–20 mA loop animation