Combined SPD

Combination type SPD

SPD that incorporates both voltage switching type components and voltage limiting type components may exhibit voltage switching, voltage limiting or both voltage switching and voltage limiting behaviour depending upon the characteristics of the applied voltage.
1.2/50μs voltage impulse– a voltage impulse with a virtual front time (time to rise from 10% to 90% of the peak value) of 1.2μs and a time to half-value of 50μs; it is used for classification of the SPD class I and II.

Combination wave– the combination wave is delivered by a generator that applies a 1.2/50μs voltage impulse across an open circuit, and an 8/20 current impulse into a short circuit; the voltage, current amplitude and waveforms that are delivered to the SPD are determined by the generator and the impedance of the SPD to which the surge is applied. The ratio of peak open-circuit voltage to peak short-circuit current is 2Ω; this is defined as the fictive impedance Zf; the short-circuit current is symbolized by ISC ; the open-circuit voltage is symbolized by UOC.
Note: In practice when it comes to the arresters (class III) there are data Imax (8/20) and In (8/20) often mentioned in place of UOC because of the commercial reasons. They are always mentioned in relation to the stated voltage protection level Up (the data concerned are always derived from the tests by the standardized combined impulse with the amplitude UOC; the test is carried out by use of a hybrid generator with inside resistance 2Ω).
Sparkover voltage of a voltage switching SPD– maximum voltage value before disruptive discharge between the electrodes of the gap of a SPD; it is used for classification of the SPD class I and II by testing impulse with waveshape 1.2/50μs.
Nominal discharge current In (8/20)– the crest value of the current through the SPD having a current waveshape of 8/20; this is used for the classification of the SPD for class II test and also for preconditioning of the SPD for class I and II tests; arrester must discharge this current at least 15 times without any essential changes in its qualities.
Impulse current Iimp – it is defined by a current peak value Ipeak and the charge Q; tested according to the test sequence of the operating duty test; this is used for the classification of the SPD for class I test; arrester must safely discharge this current without any obvious damage or aberration from the temperature stability