The ballast sets and holds the current the ultraviolet lamp arc runs at. A ballast drifting out of specification changes delivered UV-C output without failing outright, so the plant keeps running and quietly under-doses. Match the ballast to the lamp rating actually installed rather than to the reactor model alone. This is the D42-22-2-3 ballast for the Berson Maritime 3500. Marine installations run to their own approval regime, so substitute a ballast on a certified vessel only against the approved parts list.