Commissioning levels, explained in plain language: L0 to L6

Before anything is built, the owner's commissioning authority asks one question: do the owner's requirements and the basis of design describe the same building? This rung costs a meeting. A comment raised here and never closed does not disappear, it resurfaces at L5 as a behaviour nobody expected, at the most expensive point on the programme. Candidates who can name L0 signal that they understand commissioning as assurance across the whole project rather than as testing at the end.

The equipment proves itself at the factory before it ships. Catching a defect here costs a factory visit; catching the same defect on site costs a slip to the program. L1 is the cheapest place to find a problem.

Delivered, installed, connected, and formally handed over. This is the seam with QA/QC: L2 hands commissioning an installation it can trust only if the quality dossier behind it is real. A weak handover here poisons every level above it.

Energized and exercised component by component. Does each device power on, move, read, and respond on its own? This is also where the concept of ready for start-up earns its meaning: it should be evidence-based, the pre-functional checklist closed line by line, not a date someone hoped to hit.

Does each system do what the design says, on its own, against written acceptance criteria? The classic L4 trap is a test that passes against the wrong setpoint: green on paper, wrong in reality. L4 is where acceptance criteria have to trace back to the actual design intent.

Everything together, under failure conditions: the black-building style test where the facility loses utility power and must survive on its own systems, in sequence, without dropping the load. L4 proves systems alone; L5 proves them together under stress, and interactions between systems are exactly where facilities die. That is why IST matters even when every system already passed L4.

Turnover to operations with the evidence trail: documentation, as-builts, training, and the seasonal testing that finishes after handover. A facility can pass every test and still fail here, because operations inherits a building they were never taught to run. If you have ever received a system with no O&M manual and no training, you already understand why L6 is a level rather than a formality.

RFSU, Ready For Start-Up, is the gate that closes L3: it declares a system safe to energize and begin start-up. Declaring RFSU early is a classic and costly mistake, because start-up then runs on an installation that was never finished, and the defects surface as functional failures at L4.

Ownership shifts partway up the ladder, and interviewers listen for it. The general contractor owns L1 to L3: proving its own work at the factory, on delivery and installation, and through pre-functional checks. The owner's commissioning authority owns L0 and then L4 to L6, and at L4 the question changes from built right to behaves right. If you can say where the handoff falls and why it falls there, you are answering as someone who has seen a project rather than a diagram.

Each level answers one question, and the questions get bigger: do the documents agree (L0), does the part work (L1), is it installed (L2), does it run (L3), does the system perform (L4), does the facility survive (L5), can operations run it (L6). Say that cleanly, add that you would confirm the site's own numbering, and an interviewer knows you understand commissioning as a sequence of proof rather than a checklist of tasks.