BS EN 62305-1:2011 pdf download – Protection against lightning Part 1 : General principles
An ideal protection for structures would be to enclose the structure to be protected within anearthed and perfectly conducting continuous shield of adequate thickness,and to provideadequate bonding，at the entrance point into the shield, of the lines connected to thestructure.
This would prevent the penetration of lightning current and related electromagnetic fieldinto the structure to be protected and prevent dangerous thermal and electrodynamiceffects of current, as well as dangerous sparkings and overvoltages for internal systems.
ln practice, it is often neither possible nor cost effective to go to such measures to providesuch full protection.
Lack of continuity of the shield and/or its inadequate thickness allows the lightning currentto penetrate the shield causing:
-physical damage and life hazard;- failure of internal systems.
Protection measures，adopted to reduce such damages and relevant consequential loss,shall be designed for the defined set of lightning current parameters against whichprotection is required (lightning protection level).
8.2 Lightning protection levels (LPL)
For the purposes of lEC 62305,four lightning protection levels (l to lV) are introduced.Foreach LPL, a set of maximum and minimum lightning current parameters is fixed.
NOTE 1 Protection against lightning whose maximum and minimum lightning current parameters exceed thoserelevant to LPL l needs more efficient measures which should be selected and erected on an individual basis.
NOTE 2 The probability of occurrence of lightning with minimum or maximum current parameters outside the rangeof values defined for LPLl is less than 2 %.
The maximum values of lightning current parameters relevant to LPL l shall not beexceeded,with a probability of 99 %. According to the polarity ratio assumed (see ClauseA.2),values taken from positive flashes will have probabilities below 10 %，while those fromnegative flashes will remain below 1 % (see Clause A.3).
The maximum values of lightning current parameters relevant to LPL l are reduced to 75 %for LPL ll and to 50 % for LPL iil and lv(linear for l,Q and dildt, but quadratic for W/R).The time parameters are unchanged.
NOTE 3 Lightning protection levels whose maximum lightning current parameters are lower than those relevant toLPL V allow one to consider values of probability of damage higher than those presented in Annex B ofIEC 62305-2:2010， but not quantified and are useful for better tailoring of protection measures in order to avoidunjustified costs.
The maximum values of lightning current parameters for the different lightning protectionlevels are given in Table 3 and are used to design lightning protection components (e.g.cross-section of conductors，thickness of metal sheets, current capability of SPDs,separation distance against dangerous sparking) and to define test parameters simulatingthe effects of lightning on such components (see Annex D).
The minimum values of lightning current amplitude for the different LPL are used to derivethe rolling sphere radius (see Clause A.4) in order to define the lightning protection zoneLPz 0.which cannot be reached by direct strike (see 8.3 and Figures 3 and 4). The minimum values of lightning current parameters together with the related rolling sphere radius are given in Table 4. They are used for positioning of the air-termination system and to define the lightning protection zone LPZ 0 B (see 8.3).
The protection measures specified in IEC 62305-3 and IEC 62305-4 are effective against lightning whose current parameters are in the range defined by the LPL assumed for design. Therefore the efficiency of a protection measure is assumed equal to the probability with which lightning current parameters are inside such range. For parameters exceeding this range, a residual risk of damage remains.
8.3 Lightning protection zones (LPZ) Protection measures such as LPS, shielding wires, magnetic shields and SPD determine lightning protection zones (LPZ). LPZ downstream of the protection measure are characterized by significant reduction of LEMP than that upstream of the LPZ.BS EN 62305-1 pdf download.