A surge protective device (SPD) is designed to protect electrical systems and equipment from surge events by limiting transient voltages and diverting surge currents.

The important technical characteristics of SPD:

-Maximum continuous operating voltage Uc

-Lightning impulse current limp

-Nominal discharge current In

-Voltage protection level Up

-Short circuit withstand capability Isccr

-Freewheeling interruption capability at UC (Ifi)

-Transient overvoltage TOV

SPD Categories or Types

The two main types of SPDs are voltage limiting and voltage switching components. Most systems today incorporate both component types together to aggregate the strengths and limit the weaknesses of each individual part.

Examples of voltage limiting components are metal oxide varistors (MOVs) and transient voltage suppression (TVS) diodes. Voltage switching components include gas discharge tubes (GDTs) and spark gaps.

According to ANSI/IEEE C62.41, IEC 61643-11 and VDE Classification, there are three different test classes corresponding to three types of SPDs.

SPD type 1: tested with the impulse discharge current Iimp (typically 10/350 µs) and with 8/20 µs current impulses.

SPD type 2: tested with the nominal discharge current In (8/20 µs) and optional with the maximum discharge current Imax (8/20µs). Imax should not be considered for choosing an SPD. When containing any voltage switching components SPDs type 1 and type 2 are additionally tested with 1,2/50 µs voltage impulses.

SPD type 3: tested with a combination wave generator providing an open circuit voltage Uoc (1,2/50 µs) and a defined short circuit current Icw (8/20 µs) with a fictive nominal output impedance of 2 Ω.