Electronic apparatus protection against LEMP: Surge threat for the SPD selection

Abstract

The paper intends to give a contribution to the investigation on the expected surge current, peak value and shape, due to lightning electromagnetic impulse (LEMP) by flashes to the structure for the surge protective device (SPD) proper selection in order to assure electronic apparatus operation. The obtained results are discussed with the SPD requirements of international standard [1]. © 2012 IEEE

Stress to surge protective devices system due to direct flashes to low voltage lines

“…The wave shape 0.25/100 s, due to high value of current steepness, is responsible for highest voltage drop on connection leads of SPD and for the reflection in the circuit between apparatus and the closest upstream SPD and then affects the SPD dimensioning in terms of protection level (U p ) [17][18][19]. The lightning current striking the line is represented by Heidler function [14][15][16]. The investigation includes both switching and limiting SPD types simulated to achieve proper characteristic voltage-ampere (U-I) and voltage-time (U-t) according to previous methodology presented in [20].…”

“…In previous contributions [14][15][16] the problem of selection and installation of SPD in the case of direct lightning flash to a structure, (source of damage S1) has been analyzed. The investigation has been performed by means of computer simulations with models similar to the real circuits tested in the high voltage laboratory of University of Rome "La Sapienza" and Warsaw University of Technology.…”

Impact of an extended grounding system on the factors affecting selection of an SPD system for apparatus safety

“…Following the intervention of SPD1 and SPD2, the voltage drop ΔU Z is responsible of flowing through SPD2 of a current I SPD2z (and the associated charge Q SPD2z ) which combines with: -the current I SPD2f (and associated charge Q SPD2f ) transmitted by the SPD1(feeding effect) [13] and -the current I SPD2i (and associated charge Q SPD2i ) induced by lightning in the loop circuit formed by the phase conductor and the two SPDs (inducing effect) [14]. The latter two currents (and associated charges) would still be present even if the earth-termination system was equipotential.…”

Procedure for selection of the SPD system according to the probability of damage

“…For evaluation of induced overvoltages U i , approximate methods can be applied, as reported in [3]. If spatial shielding of the structure (or of the rooms) and/or line shielding (use of shielded cables or metallic cable ducts) are provided, the induced overvoltage U i is usually negligible and may be disregarded in most cases.…”

“…In the previous contributions [2,3,4], the expected current I exp (and the related charge Q exp ) for typical SPD system have been assessed, according to typical characteristics of the upstream line and of the downstream protected circuit. For this reason several computer simulations by means of commercial transient software EMTP-RV have been executed.…”