Some components fail on a schedule that can be predicted. Cooling fans degrade mechanically: airflow falls before the fan stops. Electrolytic capacitors change characteristics through internal chemical ageing, and a failing capacitor can interrupt the load and damage the battery system.

Both are strongly temperature dependent. Chloride guidance for its CP-series UPS: fans at around 4–5 years at 25°C and 3 years at 50°C ambient; power capacitors at around 6–8 years at 25°C and 4 years at 50°C.

It is more efficient to replace these together in one planned intervention than to take separate outages, so EDPN groups wear-component replacement into a single remedial visit wherever the ages allow.