Pre-Commissioning vs Commissioning: What’s the Difference?

Pre-commissioning is the set of checks and tests carried out on installed equipment and systems before process fluids, hydrocarbons or normal operating energy are introduced. Commissioning is the stage that follows: the system is brought live, run as designed and proven to perform its function. Put simply, pre-commissioning proves the system has been built correctly and is ready to be energised; commissioning proves it works.

The boundary between the two sounds obvious on paper and gets blurred on site. Different owners draw the line in different places, some contractors fold pre-commissioning into construction scope, and some projects use "cold" and "hot" commissioning instead. This guide sets out a typical split, what happens in each stage, and how the paperwork shows that one stage has finished and the next can start.

Where each stage sits in the project

Most industrial and energy projects move through the same broad sequence, even if the names differ:

1. Construction – install equipment, piping, cable, instruments and structures.
2. Mechanical completion – verify that each system is installed in accordance with drawings, specifications and vendor requirements.
3. Pre-commissioning – test and prepare systems in a de-energised or non-process state.
4. Commissioning – energise, introduce utilities and process media, and run systems functionally.
5. Start-up and handover – bring the facility into operation and transfer care, custody and control to the owner.

Mechanical completion and pre-commissioning often overlap in practice. A subsystem may be mechanically complete while pre-commissioning activities such as flushing are already under way on the same lines. What matters is that each activity is tied to the right system, has its prerequisites satisfied, and has a signed record.

What is pre-commissioning?

Pre-commissioning covers the activities that prove an installed system is fit to receive energy or process media. The work is mostly static or "dead": circuits are not energised under normal operating conditions, and process lines contain test media rather than product.

Typical pre-commissioning activities

• Piping: flushing, blowing, chemical cleaning, pickling, hydrostatic or pneumatic pressure tests (often part of construction test packs), drying and reinstatement.
• Mechanical: alignment checks, lubrication, rotation checks by hand, internal inspections of vessels before box-up, preservation removal.
• Electrical: insulation resistance testing, continuity, cable terminations, earthing checks, protection relay setting checks, panel and switchgear inspections.
• Instrumentation: calibration, loop checks from field device to control system, impulse line leak tests, valve stroke tests on air.
• Control systems: point-to-point checks, I/O verification, cause and effect preparation.
• Leak testing and inerting: service tests, nitrogen purging and oxygen-free confirmation where the system will see hydrocarbons.

The common thread is that each activity checks an individual tag or a short run of tags against an acceptance criterion. Results are recorded on inspection test records.

What is commissioning?

Commissioning is where the system first behaves as a system. Utilities are brought online, equipment is run under power, control logic is tested with real signals, and the system is proved against its functional and performance requirements.

Typical commissioning activities

• Energising motor control centres and switchboards, then running motors solo and coupled.
• Running pumps, compressors and fans on water, air or inert media.
• Function-testing control loops, interlocks, shutdown logic and alarms against the cause and effect.
• Proving fire and gas detection and firewater systems.
• Introducing utilities such as instrument air, cooling water, power and nitrogen to downstream systems.
• Performance and reliability runs, depending on contract.

Commissioning work is normally carried out under written commissioning test procedures, with operations staff increasingly involved as the stage progresses. The permit-to-work regime usually tightens here, because energy is live and systems begin to interact.

Side-by-side comparison

The labels vary. Some companies call pre-commissioning records "B" sheets and commissioning records "C" sheets; others use different prefixes entirely. Some owners have no separate pre-commissioning stage and treat everything after mechanical completion as commissioning. Check your contract's completions and commissioning philosophy before assuming either convention.

A and B ITRs: how the documentation splits

Most completions systems separate inspection test records by stage:

• "A" ITRs record construction and mechanical completion inspections: is the equipment installed, supported, terminated, labelled and correct to the drawing?
• "B" ITRs record pre-commissioning tests: insulation resistance, loop checks, flushing, alignment, calibration.

The relationship between them is a dependency. You should not be running a loop check on an instrument whose installation "A" ITR has not been completed and accepted, and you should not be doing insulation resistance tests on a cable whose termination has not been inspected. When this ordering is left to discipline and memory, it breaks down under schedule pressure, and you find B records signed against tags that later fail an A inspection.

A completions management system should hold both record types against the same tag, so progress by system can be reported for each stage separately and the dependency between them can be checked rather than assumed.

The certificates that close each stage

Certificates are how a stage is formally closed for a system or subsystem. A typical sequence looks like this:

1. Mechanical Completion Certificate (MCC) – all A ITRs complete, and no open punch items that would prevent safe pre-commissioning or commissioning.
2. Ready for Commissioning (RFC) – all B ITRs complete, pre-commissioning finished, and the system is fit to be energised or receive media.
3. Ready for Start-Up (RFSU) – commissioning complete and the system ready for introduction of process fluids and operation.
4. Handover or takeover certificate (often TOC) – care, custody and control pass to the owner or operator.

Owners and contractors name and order these differently. Some insert discipline acceptance or system handover certificates between MCC and RFC; some combine RFC and RFSU on simpler systems. The principle is consistent: each certificate relies on the one before it, and each defines which punch categories may remain open.

Where the transition goes wrong

The move from pre-commissioning to commissioning is where the most avoidable problems happen. Common failure modes include:

• Commissioning by area instead of by system. Construction finishes a module or a building, but the systems running through it are incomplete, so commissioning cannot start anything cleanly.
• Pre-commissioning skipped to recover schedule. Flushing gets shortened, or a loop check is "deferred to commissioning". The debris or the wiring error is then found with the pump running.
• Unclear boundaries. A subsystem is declared ready while a tie-in or a shared utility is still under construction, and nobody has marked the isolation point.
• Paper lagging the field. Tests are done but the records are sitting in a site cabin, so RFC cannot be issued and the commissioning team is told the system is not ready when it physically is.
• Punch items not categorised. Everything is marked "must fix before commissioning", or nothing is, and the RFC decision becomes a negotiation.

Checklist before declaring a system ready for commissioning

• All A ITRs for every tag in the subsystem complete and accepted
• All B ITRs complete, with witnessed tests signed by the required parties
• Pressure test packs closed and reinstated
• Flushing and cleaning records complete, with temporary strainers and blinds logged
• Preservation removed where required and recorded
• Open punch items reviewed and categorised; none that block commissioning
• System boundaries and isolation points marked on the P&IDs and single line diagrams
• Vendor pre-commissioning requirements signed off
• Commissioning procedures approved and available
• Permit-to-work arrangements agreed for the system going live

Managing this in Accord

Accord organises work by project, system, subsystem and tag, and separates "A" mechanical completion inspections from "B" pre-commissioning tests. Its dependency engine stops B records starting until their prerequisite A records are approved, and its certificate chain is enforced, so a Ready for Commissioning certificate cannot be issued without the certificates and records it depends on. Punch items carry Category A/B/C criticality and can block the relevant stage gate. Commissioning test procedures are authored, revisioned and executed step by step with hold points, and completion and readiness dashboards show where every system stands.

Frequently asked questions

Is pre-commissioning part of construction or commissioning?

It depends on the contract. On many EPC projects pre-commissioning is executed by the construction contractor under the commissioning team's direction; on others the commissioning team owns it outright. What matters is that the scope split is written into the commissioning plan so each test has a clear owner.

What is the difference between mechanical completion and pre-commissioning?

Mechanical completion confirms the system is installed correctly, using inspection records such as A ITRs. Pre-commissioning then tests that installation in a non-operational state, using records such as B ITRs. On site the two often run in parallel on different subsystems.

What are the commissioning stages?

A typical sequence is mechanical completion, pre-commissioning, commissioning, start-up and handover. Some projects split commissioning into cold (non-process) and hot (process) commissioning. The exact stage names and gates are set by the owner's or contractor's commissioning philosophy.

Can commissioning start before pre-commissioning is finished?

Commissioning can start on one subsystem while pre-commissioning continues on others, which is why work is organised by system. Within a single subsystem, starting commissioning with pre-commissioning tests still open is a recognised risk and should need formal approval with the open items recorded.

Who signs the Ready for Commissioning certificate?

Usually the contractor's completions or commissioning lead and the owner's representative, with discipline sign-offs behind it. The exact signatories are defined in the project's completions procedure.