Surge Protection Devices (SPDs), also known as Transient Voltage Surge Suppressors (TVSSs) are non-linear voltage clamping devices. They are
installed across the lines to be protected (in parallel with the equipment) and thus do not need to be rated to the line current. The SPD is normally in a high impedance state under nominal supply voltage conditions. Under transient conditions, the line voltage is increased by the transient voltage to a point where the SPD clamping threshold is reached. The SPD then rapidly moves into a lower impedance state, diverting the excess transient energy (current) to ground/source and thus limiting the transient voltage to a safe level. The SPD automatically resets after the transient.
A Series Filtering SPD is an improvement on the standard SPD where a low pass and RFI/EMI filter is added. This secondary filtering stage reduces the rapid rate of pre clamped voltage raise, plus provides further attenuation to the clamped voltage as well as small signal noise that may be below the SPD clamping voltage. The Surge Filter is series-connected devices and must be rated to the maximum circuit current.