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also known as transient overvoltage or spike, impulse. It can reach tens of kilovolts with a short duration (microseconds to milliseconds)

SPD-Surge Protective Device

A device that is intended to limit surge Voltages and divert surge currents. It contains at least one nonlinear component.
SPDs act by diverting surge current to earth Clamping output voltage to a safe level. Does NOT prevent lightning but protects against effects

one-port SPD
SPD having no intended series impedance

two-port SPD

SPD having a specific series impedance connected between separate input and output connections

voltage switching type SPD

SPD that has a high impedance when no surge is present, but can have a sudden change in impedance to a low value in response to a voltage surge.
Examples: Spark gaps, Gas tubes

voltage limiting type SPD

SPD that has a high impedance when no surge is present, but will reduce it continuously with increased surge current and voltage.
Examples: MOV, SAD

combination type SPD

SPD that incorporates both, voltage switching components and voltage limiting components. The SPD may exhibit voltage switching,limiting or both

mode of protection of an SPD

an intended current path, between terminals that contains protective components, e.g. line-to line, line-to-earth, line-to-neutral,neutral-to-earth.

1,2/50 voltage impulse

voltage impulse with a nominal virtual front time of 1,2μs and a nominal time to half value of 50μs

8/20 current impulse

current impulse with a nominal virtual front time of 8μs and a nominal time to half value of 20μs

combination wave

a wave characterized by defined voltage amplitude (UOC) and wave shape under open-circuit conditions and a defined current amplitude (ICW) and wave shape under short-circuit conditions

open circuit voltage (UOC)

open circuit voltage of the combination wave generator at the point of connection of the device under test

short-circuit current rating (SCCR for UL)

The suitability of an SPD for use on an AC power circuit that is capable of delivering not more than a declared rm symmetrical current at a declared voltage during a short circuit condition.

follow current interrupt rating (Ifi for IEC)

prospective short-circuit current that an SPD is able to interrupt without operation of a disconnector. This behavior does not concern low voltage surge protector using metal oxide varistor technology.

Class I SPD

Class I SPD should be tested with 10/350µs lightning impulse wave, simulating the effects of direct lightning discharges. They have to be installed where lightning currents and electromagnetic effects are unattenuated.

Class II SPD

Class II SPD should be tested with 8/20µs current impulse wave, simulating lightning secondary effects. They have to be installed where lightning currents and electromagnetic effects are already.


Class III SPD should be tested with combination impulse wave but with low values, simulating much attenuated overvoltages. Normally they are installed near the equipment and have low residual voltages.

In (nominaldischarge current for class II test)

crest value of the current through the SPD having a current waveshape of 8/20. Type 1 and Type 2 surge protective devices shall withstand 15current impulses 8/20 of

amplitude In, according to IEC 61643-11.

Imax (Maximum Discharge Current for class II test) 

crest value of a current through the SPD having an 8/20 waveshape and magnitude according to the manufacturers specification. Imax is equalto or greater than In

Iimp (impulse discharge current for class I test

crest value of a discharge current through the SPD with specified charge transfer Q and specified energy W/R in the specified time

Uc (maximum continuous operating voltage)

maximum r.m.s. voltage, which may be continuously applied to the SPD's mode of protection

Up (voltage protection level)

maximum voltage to be expected at the SPD terminals due to an impulse stress with defined voltage steepness and an impulse stress with a discharge current with given amplitude and waveshape.