Because the electricity service is irregular in many countries. This means, the voltage feed can be above or below the nominal voltage equipments need, and this affects their performance and durability. The greatest danger of these variations is the fact that they’re not perceptible, and users may think their artifacts are working properly and are being fed with just the right voltage, when they are in risk. Another inconvenient which frequently occurs is the blackout which consists of a sudden power cut that considerably affects equipments.
