In short
- L1 tests each device at the factory, L2 checks it on delivery and installation, L3 starts it up on site.
- L4 tests each system end to end; L5 tests all systems together under load with failures simulated.
- A level should not start until the previous one is complete for the equipment involved.
- Every level leaves a signed record: FAT report, checklist, start-up sheet, test script.
The five levels at a glance
| Level | Proves | Led by | Record |
|---|---|---|---|
| L1 Factory acceptance | The device meets its specification | Manufacturer, witnessed | FAT report |
| L2 Installation checks | It arrived undamaged and is installed correctly | Contractor, CxA spot checks | Pre-functional checklist |
| L3 Start-up | It runs on its own in its real location | OEM and contractor, CxA witness | Start-up report |
| L4 System tests | Each system works end to end | CxA, trades support | Functional test scripts |
| L5 Integrated systems test | All systems together survive failures under load | CxA, owner, operations | IST scripts and report |
Some programmes mark each completed level with a coloured tag on the equipment, so anyone on site can see how far it has got.
L1 · Factory acceptance tests (FAT)
Generators, UPS modules, switchgear, transformers, busbar, chillers and CDUs are tested at the manufacturer before shipment. Typical FATs: generator load runs, UPS efficiency and transfer tests, switchgear insulation and interlocks, chiller performance. Witnessing the critical ones catches problems while they are still cheap to fix and before they hold up site work.
L2 · Delivery and installation checks
Before anything is energised, each item is checked against a pre-functional checklist: transport damage, nameplate data against the submittal, clearances, supports, terminations, torque marks, labelling, pipe flushing and pressure tests. The contractor completes the checklists; the CxA spot-checks a sample.
L3 · Equipment start-up
Each device is powered and run on its own for the first time, to the OEM procedure: rotation, protection settings, alarms, set points and basic performance. The OEM's start-up engineer signs the report; the CxA witnesses the critical items such as generators, UPS and chillers.
L4 · Functional system tests
Each system is tested end to end, still without interaction between systems: a power train from utility to rack, a chilled-water loop with all its pumps and valves, the BMS and EPMS points, the fire alarm. Tests follow scripts with pass criteria written in advance. Most of the defects that would otherwise surface at L5 are found here.
L5 · Integrated systems test (IST)
Everything runs together with load banks at design IT load while failures are simulated: utility loss, a generator that does not start, a UPS module fault, a chiller trip, loss of controls, a fire alarm, a black start. A pass means the IT load never drops and temperatures stay inside limits. The IST guide covers scenarios and load banks in detail.
Common mistakes
- Starting L4 with L3 open: functional tests stop on start-up defects and the schedule slips anyway.
- Scripts written too late: pass criteria written during the test are negotiated, not tested.
- Controls left to the end: BMS and EPMS integration is often the last thing ready and the first thing to fail in IST.
- No operations staff at L4 and L5: the people who will run the site learn it during the tests, or not at all.
- Issues log not closed: open items at handover become operating risks.
Questions about commissioning levels
What is the difference between L4 and L5?
L4 tests each system on its own. L5 runs them all together under load and fails things on purpose to prove the systems react together as designed.
Can L1 be skipped for standard equipment?
Witnessing can be limited to critical items, but every device should still have a factory test record. A missing FAT moves the risk to site, where fixing is slower.
Who signs off each level?
The party that performs it signs the record, and the CxA verifies it. L5 is usually signed off by the CxA and the owner together.
Are the levels the same everywhere?
The five-level structure is common industry practice, though names and the split between L2 and L3 vary between owners and standards.
Sources
- ASHRAE Guideline 0, The Commissioning Process.
- Level definitions follow common data center industry practice; owners' own specifications take precedence.