Knowledge of some basic technical terms and definitions associated with SPDs will facilitate an understanding of the contents of this catalogue. Please find below a selection of the most important.

SPD test class 1 (IEC) or Type 1 (EN): SPD tested with nominal discharge current In and with impulse current Iimp.

SPD test class 2 (IEC) or Type 2 (EN): SPD tested with nominal discharge current In and with max. discharge current Imax (optional).

SPD test class 3 (IEC) or Type 3 (EN): SPD tested with combination wave.

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. Common examples of components used in such SPDs are spark gaps, gas tubes and thyristors.

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. Common examples of components used in such SPDs are varistors and avalanche diodes.

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 a 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.

 N-PE SPD: SPD intended exclusively for application between N and PE conductors in an installation.

Multipole SPD: SPD with more than one mode of protection, or a combination of electrically interconnected SPDs offered as a unit.

Maximum Continuous Operating Voltage (Uc): Maximum r.m.s. voltage, which may be continuously applied to the SPD's mode of protection. This is comparable to the nominal voltage of other installation devices.

Impulse discharge current (Iimp): Crest value of a discharge current through the SPD with specified charge transfer Q and specified energy W/R in the specified time. This characterises an SPD as test class 1 or type 1. The characteristic waveform of 10/350μs.

Nominal discharge current (In): Crest value of the current through the SPD with a current waveshape of 8/20μs. This characterises an SPD as test class 2 or type 2.

Discharge current (Id): Presumed maximum crest value of the the current through the SPD when subjected to a combination wave with an open circuit voltage equal to Uoc. The real current through the SPD will always be lower than Isc.

Open circuit voltage (Uoc): Open circuit voltage of the combination wave generator at the point of connection of the device under test.

Maximum discharge current (Imax): Crest value of a current through the SPD having an 8/20μs waveshape and magnitude according to the manufacturers specification. Imax is an optional parameter. This parameter should not be considered for the selection of SPDs.

Total discharge current (Itotal): Current which flows through the PE or PEN terminal of a multipole SPD during the total discharge current test.

Short-circuit current rating (Isccr): Maximum prospective short-circuit current from the power system for which the SPD, in conjunction with the disconnector specified, is rated.

Follow current (If): Peak current supplied by the electrical power system and flowing through the SPD after a discharge current impulse.

Follow current interrupt rating (Ifi): Prospective short-circuit current that an SPD is able to interrupt without operation of a disconnector.

No Follow Current®(NFC): An SPD design not causing any follow current. SPDs with NFC-technology avoid any undesired current stress to disconnectors and protective devices upstream the SPD.

(Voltage) protection Level (Up): 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.

Noise level attenuation (dB): Reduction of the noise caused by electromagnetic interferences, both in common and differential mode.

Temporary Overvoltage (TOV): Power frequency overvoltage of relatively long duration. A temporary overvoltage is undamped or weakly damped.

• SPD behaviour in case of temporary overvoltages (TOV): SPDs shall: 

- withstand when subjected to overvoltages caused by faults or disturbances in the low voltage system and

- either withstand or fail in manner not creating a hazard when subjected to overvoltages caused by faults or disturbances in the high voltage system.

Pollution Degree (PD): Numeral characterizing the expected pollution of the relevant environment.

PD 1: No pollution or only dry, non-conductive pollution.
PD 2: Only non-conductive pollution, except an occasionally temporary conductivity caused by condensation.
PD 3: Conductive pollution or dry non-conductive pollution which becomes conductive due to expected condensation.


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