Surge arresters, or surge protective devices (SPDs), are devices designed to protect electronic devices and systems from surges. SPDs are classified according to their ability to handle different categories of surges. The main classes of SPDs are Class 1 and Class 2, each designed to deal with specific sources of surges. - Class I (Level 1 SPDs): These SPDs are designed to deal with direct surges caused by lightning. They are installed upstream of the main electrical installation, at the point where the power supply enters the building (entry point). Their main role is to protect against external overvoltages of atmospheric origin, such as direct lightning strikes. - Class II (Level 2 SPDs): Class 2 SPDs are designed to deal with indirect surges and voltage spikes of internal origin, such as those generated by interruptions or switching in the power grid. They are usually installed upstream of electronic devices or sensitive equipment, protecting against overvoltages that may originate from the internal electrical system or the public grid. The combined installation of Class 1 and Class 2 SPDs provides comprehensive protection against multiple sources of overvoltage, offering an effective defence for a building's entire electrical system. This layered approach to surge protection helps prevent damage to electronic devices and increase the reliability of electrical systems. It is important to note that surge protection should be approached in a comprehensive and integrated manner, considering the installation of Class 1, Class 2 and, if necessary, Class 3 SPDs (to protect individual devices).