Pre-commissioning verifies, without power, that each device was properly installed and connected; commissioning energizes the equipment and systems and proves they work as designed; start-up is the beginning of real operation, with the process or the load. Contracts, industries and standards draw the boundaries differently, so the scope of each stage has to be written down.
Three words for three moments of the project
In an electrical project, between the end of installation and normal operation there is a stretch in which you prove that what was built works. That stretch has different names depending on the industry, the client and even the contract, and confusion is common. The stages applied to a complete substation are in substation commissioning.
- Pre-commissioning: checks with the equipment de-energized. It confirms that everything was installed and connected as the drawings say and that each device is sound.
- Commissioning: the equipment and systems are energized and tested in operation, first separately and then together, until they are shown to meet the design.
- Start-up: the beginning of real operation, with the load or the process. In an industrial plant it is when the feedstock comes in; in a substation, when it takes the load permanently.
IEC 62337, written for electrical, instrumentation and control systems in the process industry, sets out these phases and their milestones between the end of installation and acceptance of the plant by the owner. It also makes clear that installation and pre-commissioning can overlap: one part of the plant is verified while another is still being built.
- 01InstallationConstruction and installation of equipment and wiring
- 02Pre-commissioningDe-energized inspections and tests, device by device
- 03CommissioningEnergization and functional tests of equipment and systems
- 04Start-upBeginning of operation with real load or process
- 05AcceptanceThe owner takes over the installation with its documentation
How the terms are used
| Context | Usual terms | Comment |
|---|---|---|
| Power utilities and transmission | Acceptance testing, commissioning, energization | Field tests per ANSI/NETA ATS and ECS; protection testing per IEEE C37.233 |
| Process industry (oil and gas, mining, chemicals) | Pre-commissioning, commissioning, start-up | Phases and milestones per IEC 62337; see electrical commissioning of an industrial plant |
| Buildings and data centers | Commissioning levels 1 to 5 | Quality process from design onward; see commissioning levels |
| Factory and site integration | FAT, SAT, SIT | Acceptance tests per IEC 62381 |
| Contracts that split by power | Cold and hot commissioning | Without and with voltage on the equipment |
Whatever the words, what matters is that the contract says which tests each phase includes and who approves the move from one to the next.
Which test goes in each stage
| Stage | Examples in a substation or plant | Energized |
|---|---|---|
| Factory tests (FAT) | Routine tests of the transformer and switchgear, FAT of the protection and control panels | Not on site |
| Pre-commissioning | Installation inspection, torque checks, insulation resistance, CT and VT ratio and polarity, wiring continuity and point-to-point checks, ground resistance | No |
| Commissioning | Secondary injection with approved settings, real trips to breakers, commands and interlocks, signals to SCADA, end-to-end tests, first energization | Auxiliaries yes; power at the end |
| Start-up | Phasing and synchronism checks with voltage, directionality under load, motor starting, load pickup | Yes |
The boundary between pre-commissioning and commissioning is not always the power voltage: many commissioning tests are done with station service energized and the power circuit still off. That is why some contracts speak of cold and hot commissioning. The practical approach is to define the boundary by milestones, for example «mechanical completion» or «ready for energization», and not by the word alone.
The distinction has contractual consequences. While the installation is in pre-commissioning, the risk usually stays with the installation contractor; at energization, personnel safety, clearances and, if there is a grid connection, the utility come into play. That is why each change of phase should coincide with a clear change of who is responsible for operation and for the work permit.
Commissioning as a quality process
In buildings and data centers, commissioning is understood as a quality process that starts at design. ASHRAE Guideline 0 defines it as a process that verifies and documents that the facility meets the owner’s project requirements, from pre-design through operation.
In the power sector the idea is the same, although it is applied mainly in the field: ANSI/NETA ATS covers acceptance testing of each device before first energization and ANSI/NETA ECS the commissioning of the complete system. For protection and automation, IEEE C37.233 and CIGRE TB 637 describe acceptance, commissioning and maintenance tests. The complete framework of standards is in standards for commissioning, FAT and SAT.
Common mistakes when contracting
- Using the word without a scope. «Commissioning included» says nothing without a list of tests per device and per system.
- Leaving start-up without an owner. The electrical contractor hands over a tested installation, but someone has to accompany load pickup and the energized checks.
- Skipping pre-commissioning. Energizing without having verified point-to-point wiring and instrument transformer polarity turns the first energization into a test of errors.
- Not setting acceptance milestones. Each change of phase should have a record listing the open non-conformities and who is responsible for each.
Before comparing bids, review what substation commissioning includes and how to choose an electrical testing company.
From pre-commissioning to start-up with Tiza Group
Our team covers all three stages in medium, high and extra-high voltage substations and in industrial plants: primary equipment tests, protection, control and communications tests, and support during energization, in Colombia, or elsewhere on request. For the full scope, see testing and commissioning or write to us; we will propose a plan by stages.
Related guides
- What substation commissioning includes
- Standards for commissioning, FAT and SAT testing
- Substation primary equipment tests
- How to choose an electrical testing company
- Transformer factory tests
References
- IEC 62337:2012, Commissioning of electrical, instrumentation and control systems in the process industry – Specific phases and milestones.
- IEC 62381:2024, Automation systems in the process industry – Factory acceptance test (FAT), site acceptance test (SAT), and site integration test (SIT).
- ASHRAE Guideline 0-2019, The Commissioning Process.
- ANSI/NETA ATS-2025 and ANSI/NETA ECS-2024.
- IEEE C37.233-2023, IEEE Guide for Power System Protection Testing.
- CIGRE TB 637, Acceptance, commissioning and field testing techniques for protection and automation systems, 2015.
Frequently asked questions
Are commissioning and start-up the same thing?
Not exactly. Commissioning proves that equipment and systems work as designed; start-up is the beginning of operation with the real process or load. In many contracts start-up is the last phase of commissioning, and in power utility projects both are often grouped under commissioning and energization.
What other names are used for these stages?
Pre-commissioning is also called pre-energization checks or construction verification; commissioning on power systems is often split into acceptance testing (ANSI/NETA ATS) and system commissioning (ANSI/NETA ECS); start-up is also called initial operation. In buildings and data centers the stages are numbered as commissioning levels 1 to 5.
Are FAT tests part of commissioning?
They are performed earlier, at the factory, but commissioning frameworks usually include them as their first stage because they provide the reference against which site tests are compared. What does form part of on-site commissioning is the SAT.