Learn Automation with MINATA #50: Inspecting Panel Wiring — Commissioning Begins Before the First Energisation

Inspecting Panel Wiring: Commissioning Begins Before the First Energisation

Inspecting the wiring of a panel is the first part of commissioning, and it begins before the first energisation. A wiring error found with no power on costs a few minutes. The same error found after energisation can destroy equipment or hurt somebody.

Three stages of inspection

Split the electrical commissioning into three stages, and do not merge them:

Cold check, no power: compare the installed equipment against the drawings, check the labelling, tighten terminals to the specified torque, test continuity and insulation according to procedure, and verify PE.
Power-on check, energised but not running: check supply voltages, polarity, phase sequence, the 24VDC supply and the equipment power indicators — before anything is allowed to move.
Functional test, running in a safe state: test each input and output, jog each actuator, and verify the interlocks and safety functions.

Safety during the inspection

Apply lock-out tag-out and carry out the measurements under a procedure, by people competent to do so. A checklist must never become paperwork that legitimises skipping safety: every "pass" has to be the result of a real test. Insulation testing through electronic equipment such as drives and PLCs is done only as the equipment documentation allows, disconnecting where required.

Record it, so it becomes the as-built

For each stage, record who did it, the result, anything abnormal and how it was resolved. Changes made on site have to be carried back into the drawings so an as-built record exists — rather than being fixed in the panel while the drawing quietly drifts away from reality.

A worked engineering situation

Before the machine runs, the technician compares device labels against the drawings, tightens terminals to the specified torque, measures PE and insulation resistance, and then tests each input and output in a safe state, forcing outputs only where the actuator is isolated. Only once the cold check and the power-on check have passed does the machine move to functional testing.

The sequence, the measurements and the acceptance thresholds all follow the safety procedure, the applicable standards and the equipment manuals.

Common mistakes

Energising immediately "to see whether it runs", skipping the cold check.
Not tightening terminals to torque, leading to heat and loose contacts.
Insulation testing through drives or PLCs without disconnecting as the manual requires.
Treating the checklist as paperwork and marking "pass" without measuring.
Fixing something in the panel without updating the as-built drawing.

Wiring inspection checklist

[ ] Lock-out tag-out is applied and measurements follow a competent procedure.
[ ] Cold check: drawings, labels, torque, PE, insulation.
[ ] Power-on check: voltages, polarity, phase sequence, 24VDC.
[ ] Functional test of each I/O point and actuator in a safe state.
[ ] Results and anomalies recorded, and the as-built updated.

Disciplined commissioning is the cheapest way to avoid an incident on the day of energisation. #51 goes into fault-finding on the 24VDC supply, measuring in sequence.

Read more automation knowledge at MINATA: https://minatavn.com/en/blog/industrial-automation

Previous — #49: Mitigating noise on signals: https://minatavn.com/en/blog/automation-49-signal-noise-mitigation

Next — #51: Troubleshooting a 24VDC supply: https://minatavn.com/en/blog/automation-51-24vdc-troubleshooting

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