UPS systems are standardised according to the EU standard EN 50091-3 (IEC 62040-3). In this standard, UPS systems are classified according to the response time and the consideration of transient
disturbances. VI(Voltage Independent) systems are also known as Line Interactive. They permanently monitor the mains voltage and supply the connected devices and systems from the battery supply in the event of undervoltage or power failure. VI systems meet UPS protection class 2 and can be used economically up to outputs of 1 kVA.