SPDs (Surge Protective Devices) are devices designed to protect electrical systems and equipment against transient and impulse overvoltages, such as those caused by atmospheric discharges and electrical switching. The function is to divert the discharge or impulse current generated by an overvoltage to ground (PE), thus protecting the equipment in the network. SPDs are installed in parallel with the power line to be protected. At the nominal voltage of the network they are comparable to an open circuit and have a high impedance at their ends. In the presence of an overvoltage, this impedance falls to very low values, closing the circuit to Ground (PE). Their impedance quickly rises again to the initial (very high) value, returning to open loop conditions.