Commissioning is the product

Every data centre is designed to a topology, and every topology implies a set of failures the facility is supposed to survive. The design proves those on paper. Only commissioning proves them in the building.

The distinction matters because the failures that actually occur are rarely the single component failures the topology anticipates. They are interactions: a transfer that works but takes long enough for a chiller to trip, a generator that starts but cannot accept the step load once the cooling restarts, a battery string that meets its rating at 25 °C and not at the temperature of the room it is actually in.

Integrated systems testing under simulated load is the only stage of a project where those interactions are visible before a client is depending on them. It is also the stage most often compressed, because it sits at the end of the programme where the schedule pressure has accumulated.

The practical consequence is a rule: the commissioning scripts are written during design, not after installation, and the failure modes to be demonstrated are agreed with the client at the same time as the topology. A commissioning plan written after the fact tests what was built. A commissioning plan written during design tests what was promised.