The answer: during engineering
Commissioning planning should start when the project is still defining systems, equipment, controls and handover requirements. Waiting until construction is almost finished is a common mistake.
Systemisation comes early
A system breakdown structure influences construction planning, test packs, handover sequence, procedures, flushing, utilities and startup logic.
PSSR is not a paperwork exercise
Pre-startup safety review only works if the project has controlled changes, open actions, punch lists, procedures, training and safety requirements throughout execution.
Lean projects need CSU discipline
Startups and scale-ups often run lean project teams. That makes early CSU thinking more important, not less.