---

IEC 2613/10

^a Only for hospitals or other structures where failure of internal systems immediately endanger human life.

^b Only for properties where animals may be lost.

Figure 2 - Types of loss and corresponding risks resulting
from different types of damage

6. Need and economic justification for lightning protection

   1. Need for lightning protection

The need for the lightning protection of a structure to be protected in order to reduce the loss of social values L1, L2 and L3 shall be evaluated.

In order to evaluate whether or not lightning protection of a structure is needed, a risk assessment in accordance with the procedures contained in IEC 62305-2 shall be made. The following risks shall be taken into account, corresponding to the types of loss reported in 5.2:

• R^. risk of loss or permanent injury of human life;

• R₂: risk of loss of services to the public;

• R₃: risk of loss of cultural heritage.

NOTE 1 Risk R₄: risk of loss of economic values, should be assessed whenever the economic justification of lightning protection is considered (see 6.2).

Protection against lightning is needed if the risk R (R₁ to R₃) is higher than the tolerable level R_T

R > R_T

In this case, protection measures shall be adopted in order reduce the risk R (R₁ to R₃) to the tolerable level R_T

R < R_T lf more than one type of loss could appear, the condition R < R_T shall be satisfied for each type of loss (L1, L2 and L3).

The values of tolerable risk R_T where lightning could result in the loss of items of social value should be under the responsibility of a competent national body.

NOTE 2 An authority having jurisdiction may specify the need for lightning protection for specific applications without requiring a risk assessment. In these cases, the required lightning protection level will be specified by the authority having jurisdiction. In some cases, a risk assessment may be performed as a technique by which to justify a waiver to these requirements.

NOTE 3 Detailed information on risk assessment and on the procedure for selection of protection measures is reported in IEC 62305-2.

Besides the need for lightning protection for the structure to be protected, it may be useful to evaluate the economic benefits of providing protection measures in order to reduce the economic loss L4.

In this case, the risk R₄ of loss of economic values should be assessed. The assessment of risk R₄ allows for the evaluation of the cost of the economic loss with and without the adopted protection measures.

Lightning protection is cost effective if the sum of the cost C_RL of residual loss in the presence of protection measures and the cost C_PM of protection measures is lower than the cost Cl of total loss without protection measures:

Crl ^{+ c}pm ^<CL

NOTE Detailed information on the evaluation of economic justification of lightning protection is reported in IEC 62305-2.

6. Protection measures

   1. General

Protection measures may be adopted in order to reduce the risk according to the type of damage.

Possible protection measures include:

• adequate insulation of exposed conductive parts;

• equipotentialization by means of a meshed earthing system;

• physical restrictions and warning notices;

• lightning equipotential bonding (EB).

NOTE 1 Equipotentialization and an increase of the contact resistance of the ground surface inside and outside the structure may reduce the life hazard (see Clause 8 of IEC 62305-3:2010).

NOTE 2 Protection measures are effective only in structures protected by an LPS.

Protection is achieved by the lightning protection system (LPS) which includes the following features:

air-termination system;

• down-conductor system;

• earth-termination system;

lightning equipotential bonding (EB);

electrical insulation (and hence separation distance) against the external LPS.

NOTE 1 When an LPS is installed, equipotentialization is a very important measure to reduce fire and explosion danger and life hazard. For more details see IEC 62305-3.

NOTE 2 Provisions limiting the development and propagation of the fire such as fireproof compartments, extinguishers, hydrants, fire alarms and fire extinguishing installations may reduce physical damage.

NOTE 3 Protected escape routes provide protection for personnel.

Possible protection measures (SPM) include

• earthing and bonding measures,

• magnetic shielding,

• line routing,

• isolating interfaces,

• coordinated SPD system.

These measures may be used alone or in combination.

NOTE 1 When source of damage S1 is considered, protection measures are effective only in structures protected by an LPS.

NOTE 2 The use of storm detectors and the associated provision taken may reduce failures of electrical and electronic systems.

The protection measures listed in 7.2, 7.3 and 7.4 together form the overall lightning protection.

Selection of the most suitable protection measures shall be made by the designer of the protection measures and the owner of the structure to be protected according to the type and the amount of each kind of damage, the technical and economic aspects of the different protection measures and the results of risk assessment.

The criteria for risk assessment and for selection of the most suitable protection measures are given in IEC 62305-2.

Protection measures are effective provided that they comply with the requirements of relevant standards and are able to withstand the stress expected in the place of their installation.

6. Basic criteria for protection of structures

   1. General

An ideal protection for structures would be to enclose the structure to be protected within an earthed and perfectly conducting continuous shield of adequate thickness, and to provide adequate bonding, at the entrance point into the shield, of the lines connected to the structure.

This would prevent the penetration of lightning current and related electromagnetic field into the structure to be protected and prevent dangerous thermal and electrodynamic effects of current, as well as dangerous sparkings and overvoltages for internal systems.

In practice, it is often neither possible nor cost effective to go to such measures to provide such full protection.

Lack of continuity of the shield and/or its inadequate thickness allows the lightning current to 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 which protection is required (lightning protection level).

For the purposes of IEC 62305, four lightning protection levels (I to IV) are introduced. For each 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 those relevant to LPL I 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 range of values defined for LPL I is less than 2 %.

The maximum values of lightning current parameters relevant to LPL I shall not be exceeded, with a probability of 99 %. According to the polarity ratio assumed (see Clause A.2), values taken from positive flashes will have probabilities below 10 %, while those from negative flashes will remain below 1 % (see Clause A.3).

The maximum values of lightning current parameters relevant to LPL I are reduced to 75 % for LPL II and to 50 % for LPL III and IV (linear for I, Q and d/7df, 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 to LPL IV allow one to consider values of probability of damage higher than those presented in Annex В of IEC 62305-2:2010, but not quantified and are useful for better tailoring of protection measures in order to avoid unjustified costs.

The maximum values of lightning current parameters for the different lightning protection levels 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 simulating the effects of lightning on such components (see Annex D).

The minimum values of lightning current amplitude for the different LPL are used to derive the rolling sphere radius (see Clause A.4) in order to define the lightning protection zone LPZ 0_Bwhich 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).

Table 3 - Maximum values of lightning parameters according to LPL

Table 4 - Minimum values of lightning parameters and related rolling sphere radius corresponding to LPL

From the statistical distributions given in Figure A.5, a weighted probability can be determined that the lightning current parameters are smaller than the maximum values and respectively greater than the minimum values defined for each protection level (see Table 5).

Table 5 - Probabilities for the limits of the lightning current parameters

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.

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.

With respect to the threat of lightning, the following LPZs are defined (see Figures 3 and 4):

LPZ 0_A zone where the threat is due to the direct lightning flash and the full lightning electromagnetic field. The internal systems may be subjected to full or partial lightning surge current;

LPZ 0_B zone protected against direct lightning flashes but where the threat is the full lightning electromagnetic field. The internal systems may be subjected to partial lightning surge currents;

LPZ 1 zone where the surge current is limited by current sharing and by isolating interfaces and/or SPDs at the boundary. Spatial shielding may attenuate the lightning electromagnetic field;

LPZ 2, .... n zone where the surge current may be further limited by current sharing and by isolating interfaces and/or additional SPDs at the boundary. Additional spatial shielding may be used to further attenuate the lightning electromagnetic field.

NOTE 1 In general, the higher the number of an individual zone, the lower the electromagnetic environment parameters.

As a general rule for protection, the structure to be protected shall be in an LPZ whose electromagnetic characteristics are compatible with the capability of the structure to withstand stress causing the damage to be reduced (physical damage, failure of electrical and electronic systems due to overvoltages).

NOTE 2 For most electrical and electronic systems and apparatus, information about withstand level can be supplied by manufacturer.

flash to the structure

flash near to the structure

flash to a line connected to the structure

flash near a line connected to the structure

rolling sphere radius

separation distance against dangerous sparking

^7 ground level

О lightning equipotential bonding by means of SPD

LPZ Од direct flash, full lightning current

LPZ 0_B no direct flash, partial lightning or induced current

LPZ 1 no direct flash, limited lightning or induced current

protected volume inside LPZ 1 must respect separation distance s

Figure 3 - LPZ defined by an LPS (IEC 62305-3)

/ ground level

О lightning equipotential bonding by means of SPD

LPZ 0_д direct flash, full lightning current, full magnetic field

LPZ 0_B no direct flash, partial lightning or induced current, full magnetic field

LPZ 1 no direct flash, limited lightning or induced current, damped magnetic field

LPZ 2 no direct flash, induced currents, further damped magnetic field

protected volumes inside LPZ 1 and LPZ 2 must respect safety distances d_s

Figure 4 - LPZ defined by an SPM (IEC 62305-4)

The structure to be protected shall be inside an LPZ 0_B or higher. This is achieved by means of a lightning protection system (LPS).