Mechanical completion

What the certificate asserts, and what it does not

The wording varies by contract form, but the substance is consistent: the works, or a defined part of them, are installed and complete in accordance with the contract, all pre-energisation tests have been performed with acceptable results, and the plant is safe to energise.

The evidence behind that is conventional power-plant commissioning discipline applied at container scale — insulation resistance and hipot on cables and windings, point-to-point verification of wiring against the drawings, protection relay testing by secondary injection, CT and VT polarity and ratio checks, grounding continuity, and functional proof of every safety interlock and emergency stop. What that installation-conformity evidence is called, and which standard actually defines it, belongs to the site acceptance test entry.

What the certificate does not assert is performance. At mechanical completion the plant has no measured deliverable energy, no measured round-trip efficiency at any boundary, and no availability history at all, because it has never been dispatched. Every number the contracts pay against is generated afterwards. Mechanical completion is a gate to testing, not a result of it.

There is normally a punch list at mechanical completion too, but of a different kind from the one carried into substantial completion: items that do not prevent safe energisation. Where the line sits between "does not prevent energisation" and "does not prevent use" is precisely the difference between the two milestones, and it is worth agreeing in the contract rather than at the certificate meeting.

What it unlocks

Energisation. Backfeeding the site from the grid is a formal, permitted event coordinated with the utility, and mechanical completion of the relevant electrical scope is what makes it possible to request. Once the main power transformer and the auxiliary supply are live, the plant starts running itself — thermal management, controls, communications, fire detection — which matters more on a battery plant than on most, because the cooling has to be working before cells are cycled and because auxiliary consumption becomes a metered cost from that moment.

Then hot commissioning: first charge and discharge on each block, control-loop tuning, and verification that the EMS, PPC and SCADA actually do what the point list promised. Only after that come the tests that carry money — the energy capacity test, and the reliability run that gates COD — each of which has its own entry. Commercially, mechanical completion is usually a payment milestone in the EPC ladder, and in US tax-equity structures it appears as a funding condition under the ECCA, certified by the EPC contractor and an independent engineer.

On a BESS it is not one date

A ten-block site does not finish at once, and there is no reason to make it. Blocks are built in sequence, so mechanical completion arrives as a series of partial certificates — block by block, feeder by feeder, often with the substation and the collection system certified separately from the blocks they serve.

That is a schedule decision as much as a construction reality: commissioning the early blocks while the last ones are still being installed is what compresses the window between mechanical completion and COD on a large site, and it is why phased or partial COD mechanics exist in the offtake at all.

The consequence is a site where live medium-voltage equipment and active construction coexist for months. That has to be designed rather than improvised — physical segregation and barriers between energised and construction areas, a permit-to-work and lockout/tagout regime that both the EPC's crews and the OEM's commissioning engineers work under, unambiguous marking of which feeders are live, and one named authority deciding who may energise what.

It also has to be written into the certificate structure, because care, custody and control begin moving across the site block by block. An unclear boundary is a safety problem and an insurance problem at the same time.

Little of the gap to COD is optional

The plant is physically finished and it earns nothing. For a 100 MW-class plant, commissioning plus capacity testing plus market qualification commonly runs about two to four months after mechanical completion, and longer where grid outages or ISO test slots are scarce. Little of that window is optional, and most of it depends on parties the contractor does not control: the utility for permission to operate, the market operator for registration and test slots, the authority having jurisdiction for its inspections.

The failure mode is predictable. A team that reads mechanical completion as "done" plans the revenue start from it, discovers the window, then compresses the tests to recover — truncating the capacity test whose result will define the warranted beginning-of-life baseline for the next fifteen years. The cheaper discipline is to schedule backwards from the COD prerequisites and treat mechanical completion as the day the second half of the project starts.