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The commissioning ladder, rung by rung
Commissioning runs as a ladder from L0 to L6, and each rung has an owner. The general contractor carries L1 to L3, factory acceptance through pre-functional checks. The owner's commissioning authority carries L4 and L5, where systems are proved alone and then together under failure. Knowing who signs is as much of the answer as knowing what is tested.
Who owns each commissioning level?
Ownership splits at L4. The general contractor runs L1 through L3: factory acceptance at the works, delivery and installation on site, then the pre-functional checks that prove the equipment is installed, terminated and pointed correctly. From L4 the owner's commissioning authority takes over, because from that point the question stops being "is it built right" and becomes "does it behave right", first system by system and then all together under a failure. That handover of ownership is the detail interviewers listen for, because a candidate who only describes the tests has usually read about commissioning rather than stood through it. Say who signs and the answer lands differently.
Scope. Every scenario here is fictional and generalized, set on the fictional Example Site EX1. This is career and interview preparation at awareness level, not a live operating procedure, engineering advice or any form of authorization. On a real site, follow your employer's procedures, your authorized training and local requirements, and if the two ever disagree, the site wins.
Which level does this belong to?
Reading the ladder and using it are different skills, and an interview asks for the second one. The useful move is to hear a finding, place it on a rung, name who signs that rung, and then say what the same finding would have cost if it had surfaced two levels later. That last sentence is the entire argument for the lower levels, and it is the one most candidates never make, because they learned the ladder as a list rather than as a set of places where a specific class of problem is meant to be caught.
Each of these is an activity or a finding on the fictional Example Site EX1. Place it on the ladder. The rung is the answer; who signs it is the half most candidates leave out, so say that out loud as well.
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01 The owner's requirements say the site keeps cooling through the loss of any single utility supply. The basis of design never says how, and nobody has raised it.
Choose the level
Answer
L0 · Design review
Design review is where the owner's requirements and the basis of design are reconciled, and a required outcome with no design decision behind it is a gap rather than a detail. Found here it costs a meeting. Carried forward, it is only discovered when everything runs together, which is the most expensive place to find it and the one with the largest audience. The owner's commissioning authority signs this level.
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02 A generator is run against a written expected result for every step, at the manufacturer's works, before it ships.
Choose the level
Answer
L1 · Factory acceptance
Factory acceptance proves performance while the manufacturer still has the unit and every means to fix it. The written expected result is the whole difference between a test and a demonstration. The general contractor owns this level, and will until the ownership changes at L4.
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03 The witness flew out, watched the unit run, and signed. Nobody wrote down what the unit was supposed to do first.
Choose the level
Answer
L1 · Factory acceptance
Still factory acceptance, and still the contractor's level, but the level has been spent rather than passed. Without an expected result agreed beforehand you have confirmed the unit switches on, not that it meets its specification, and by the time anyone notices, the equipment has left the works.
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04 A switchboard arrives with a crushed corner on its shipping crate. It is set in position and the crate is removed before anything is written down.
Choose the level
Answer
L2 · Delivery and installation
Delivery and installation is where transit and storage damage is found, recorded and attributed. Once the crate is gone the evidence is gone with it, and a fault that surfaces months later gets blamed on the equipment rather than on the journey it made.
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05 Terminations are complete and the panel is closed. The phase rotation check was done, but it is not recorded on any sheet.
Choose the level
Answer
L3 · Pre-functional
Pre-functional is static correctness with evidence attached. A check that happened and was not recorded cannot be relied on by the next level, and the next level is the one that energises. Unrecorded is treated as undone, which is uncomfortable and correct.
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06 Every control point is terminated. Nobody has stood at the device and confirmed that the value there matches the value on the graphic.
Choose the level
Answer
L3 · Pre-functional
Point-to-point proving belongs to pre-functional, and it is the row that carries all the others. Until somebody signs it, every later reading is the graphic's opinion rather than the plant's state, so a blank there stops the level instead of being carried into the functional test.
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07 One CRAH is driven through its sequence alone, with load applied, to see what the room does when the unit is failed deliberately.
Choose the level
Answer
L4 · Functional (FPT)
Functional testing proves one system alone against its sequence of operations, including how it fails and how the room responds. Ownership has changed here too: from L4 the owner's commissioning authority carries the level, because the question has changed from built right to behaves right.
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08 Load banks represent the heat load, the utility is dropped, and power and cooling are watched recovering together.
Choose the level
Answer
L5 · Integrated (IST)
Integrated testing is the first moment the whole plant runs hard enough for a design mismatch to show itself. It is deliberately the failure you choose, on a day you control, with the people who need to see it present, rather than the one that arrives with a customer on the floor.
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09 Six months after occupancy the plant is checked again in the opposite season, and the operators are trained on what changed.
Choose the level
Answer
L6 · Handover and soft landing
Handover and soft landing covers documentation, training and seasonal reopening. A season the building has never run through is a season it has not been proved in, which is why this level has a tail rather than a date.
Now say it out loud
Clicking the right option is the easy half.
In the room you will not be given the options. Cover them, and answer these three out loud, in full sentences, as if somebody had just asked. If a sentence stalls, that is the part to go back and read again.
- 01 Write out L0 to L6 from memory, what each proves, and who signs it.
- 02 Explain in one sentence why ownership of the ladder changes hands at L4, and what changes about the question being asked.
- 03 Take any finding from the drill and say what it would cost to fix at the level where it belongs, and at L5.
Where this shows up in an interview
Most commissioning questions are really ladder questions wearing a disguise. "Walk me through a functional test" is asking whether you can place it at L4 and name its prerequisites. "What happens if the client wants to skip a step" is asking whether you understand that a rung exists to catch a specific class of failure, and that skipping it moves the failure later, where it costs more. Anchor your answer to a level, say who signs it, and name the evidence it produces, and you have given a structured answer without needing to memorise a script. The pack below works each level with the documents that go with it.