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 electronic ballast rated for 65–80 W lamps on a 220 V supply. Confirm both the lamp wattage and the supply voltage against the existing unit before ordering.