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.

TT System
Technique for the protection of persons: the exposed conductive parts are earthed and residual current devices (RCDs) are used.

TN System
Technique for the protection of persons: interconnection and earthing of exposed conductive parts and the neutral are mandatory.

IT System
Technique for the protection of persons:

  • Interconnection and earthing of exposed conductive parts;
  • Indication of the first fault by an insulation monitoring device (IMD);
  • Interruption for the second fault using overcurrent protection (circuit-breakers or fuses).

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

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

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

Voltage switching type SPD (GAP)
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.

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

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.

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 I or type 1. The characteristic waveform of 10/350 μs.

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

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

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.

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)
A SPD design not causing any follow current.
SPDs with NFC-technology avoid any undesired current stress to disconnectors and protective devices upstream the SPD.

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

(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 (Ut)
SPDs shall: 

  • withstand without damage: withstand (W);
  • or fail in a safe way, maintaining its IP degree: safe (S). 

Status Indicator
Device that indicates the operational status of an SPD or a part of an SPD. Such indicator may be local visual and may have remote signalling and output contact capability. Intermediate stages of the status indicator may also be provided e.g. for preventive maintenance, before it has reached its end of life.

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

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