---

(with mesh width 2 m)

Mesh width

Figure A.10 - Types of grid-like large volume shields

In the calculation, the magnetic field coupling of every rod within the grid-like shield, including all other rods and the simulated lightning channel, is considered and results in a set of equations to calculate the lightning current distribution in the grid. From this current distribution, the magnetic field strength inside the shield is derived. It is assumed that the resistance of the rods can be neglected. Therefore, the current distribution in the grid-like shield and the magnetic field strength are independent of the frequency. Also, capacitive coupling is neglected to avoid transient effects.

For the case of a type 1 shield (see Figure A.10), some results are presented in Figures A.11 and A.12.

Figure А.11 - Magnetic field strength H_1/MAX inside a grid-like shield type 1

Figure A.12 - Magnetic field strength H_1/MAXinside a grid-like
shield type 1 according to mesh width

NOTE 1 Experimental results of the magnetic field inside an LPZ 1 with a grid-like shield indicate that the increase of the magnetic field close to the shield is less than that resulting from the equations above.

NOTE 2 The calculated results are valid only for distances d_s/1 > w_m to the grid-like shield.

In all cases, a maximum lightning current /_O/MAX “ ^{100 kA is} assumed. In both Figure A.11 and Figure A.12, H_1/MAX is the maximum magnetic field strength at a point, derived from its components H_x, H_y and Hz:

H_1/Max =Jh²₊H²+H² (A.20)

In Figure A.11, H_1/MAX is calculated along a straight line starting from the point of strike (x - у - 0, z = 10 m) and ending at the centre of the volume (x = у - 5 m, z = 5 m). H_1/MAX is plotted as a function of the x-coordinate for each point on this line, where the parameter is the mesh width w_m of the grid-like shield.

In Figure A.12, H_1/MAX is calculated for two points inside the shield (point A: x = у = 5 m, z - 5 m; point В: x - у = 3 m, z = 7 m). The result is plotted as a function of the mesh width w_m.

Both figures show the effects of the main parameters governing the magnetic field distribution inside a grid-like shield: the distance from the wall or roof, and the mesh width.

In Figure A.11 it should be observed that along other lines through the volume of the shield, there may be zero-axis crossings and sign changes of the components of the magnetic field strength H_1/MAX. The formulae in A.4.1.1 are therefore first-order approximations of the real, and more complicated, magnetic field distribution inside a grid-like shield.

A.4.3 Experimental evaluation of the magnetic field due to a direct lightning strike

The magnetic fields inside shielded structures can also be determined by taking experimental measurements. Figure A.13 shows a proposal for the simulation of a direct lightning strike to an arbitrary point of a shielded structure, using a lightning current generator. Such tests can be performed using a simulated lightning current source of lower current level but with the same representative waveshape as the actual lightning discharge.

Figure A.13a - Test arrangement

Key

U typically some 10 kV

C typically some 10 nF

Figure A.13b - Lightning current generator

Figure A.13 - Low-level test to evaluate the magnetic field inside a shielded structure

A.5 Calculation of induced voltages and currents

A.5.1 General

Only rectangular loops in accordance with Figure A.14 are considered. Loops with other shapes should be transformed into rectangular configurations having the same loop area.

Figure А.14 - Voltages and currents induced into a loop formed by lines

A.5.2 Situation inside LPZ 1 in the case of a direct lightning strike

For the magnetic field H₁ inside the volume V_s of an LPZ 1, the following applies (see A.4.1.1):

= k_h x/₀ x w_m/(d_w x <d_r) (A/m) (A.21)

The open circuit voltage U_oc is given by:

^UOC = V_o^xb* ^ln(^{1 + //d}l/w) ^xkh ^x ( ^Wm /^l/r) ^{x d/}0 ^/df (^V) (^A-²²)

The peak value С/_ос/МАХ occurs during the front time

^ос/мах - /^о ^x b ln(1 + l/dy_w) x x (w_m / >/d|/_r) x /₀/мах (V) (A.23)

where

p._o is equal to 4 x л x 10~⁷ (Vs)/(Am);

b (m) is the width of the loop;

i

c'l/w (^m) ^dlr (^m) l₀ (A)

Ad/max (A) k_h (lMn) /(m)

T, (s)

(m)

s the distance of the loop from the wall of the shield, where ^l/w - ^s/1^:

is the average distance of the loop from the roof of the shield;

is the lightning current in LPZ 0_A;

is the maximum value of the lightning current stroke in LPZ 0_A;

is the configuration factor k_h = 0,01;

is the length of the loop;

is the front time of the lightning current stroke in LPZ 0_A;

is the mesh width of the grid-like shield.

The short-circuit current /_sc is given by:

'sc = _Ao x Ь In( 1 + //d_l/w) x k_h x ( w_m / <d_l/r) x /₀ / L_s (A) (A.24)

where the ohmic resistance of the wire is neglected (worst case).

The maximum value /_SC/max ^is 9^iven by:

^SC/МАХ ~ Ao ^xbln(1 +//d_l/w)xk_hx( w_m/<d_l/r)x

^0/MAX / L-S (A) (A.25)

where L_s (H) is the self-inductance of the loop.

For rectangular loops, the self-inductance L_s can be calculated from:

L

+ 0,4 x b x In I 2 // r_c

)/f ¹ + /Ї

}x10’⁶

(H)

(A.26)

_s ={0,8хд//² + b²- 0,8x(/ + b) + 0,4x/ x In (2b/r_c)/f 1 + + (b//)² where r_c (m) is the radius of the loop conductor.

The voltage and current induced by the magnetic field of the first positive stroke (^ = 10 ps) is given by:

^oc/f/max - 1,26 x b x ln(1 + l/d_i/w) x ( w_m />/d|_/r) x /_F/MAX (V) (A.27)

^sc/f/max = 12,6 x 10^-6 x b x ln(1 + l/d_]hN) * (^w_m /^i/r) ^x /f/max/ ^ls (A) (A.28)

The voltage and current induced by the magnetic field of the first negative stroke = 1 ps) is given by:

^OC/FN/MAX ~ 12,6 x b x ln(1 + l/d_[lvJ) x (w_m / >/d|/_r) X ^FN/MAX (V) (A.29)

^SC/FN/MAX ⁼¹²,6 X 10-6 xb x ln(1 + l/d_Uw) x (w_m / л/Ь,_/г) x /_FN/MAX/ L_s (A) (A.30)

The voltage and current induced by the magnetic field of the subsequent strokes (7^ = 0,25 ps) is given by:

^OC/S/MAX ^{= 50}’^{4 x b x ln}(^{1 +} ^l/w) ^x ( ^{x Z}S/MAX (^V) (A.31)

^sc/s/max “ 12,6 x 10-6 x b x ln(1 + l/dy_VJ) x (w_m / >ld|_/r) x /g/M_AX/^-s (A) (A.32)

where

/_F/max (^kA) ^{is the} maximum value of the current of the first positive stroke;

^zfn/max (^kA) ^{is the} maximum value of the current of the first negative stroke;

^zs/max (^kA) ^{is the} maximum value of the current of the subsequent strokes.

A.5.3 Situation inside LPZ 1 in the case of a nearby lightning strike

The magnetic field inside volume V_s of LPZ 1 is assumed to be homogeneous (see A.4.1.2).

The open circuit voltage U_oc is given by:

iy_oc = _Ju_oxbx/xdH₁/df (V) (A.33)

The peak value L/_QC/MAX occurs during the front time Тў

^oc/max = Ao ^x b x / x H_1/MAX / T-i (V) (A.34)

where

A_o is equal to 4л 10^-7 (Vs)/(Am);

b (m) is the width of the loop;

A/₁ (A/m) is the time dependent magnetic field inside LPZ 1;

^H1/MAX (A/m) is the maximum value of the magnetic field inside LPZ 1;

/ (m) is the length of the loop;

(s) is the front time of the magnetic field, identical with the front time of the lightning current stroke.

The short circuit current /_sc is given by:

/_sc = p₀ x b x / x / L_s (A) (A.35)

where the ohmic resistance of the wire is neglected (worst case).

The maximum value /_SC/MAX, is given by:

^zsc/max “ A_o^x x I x H_1/MAX / L_s(A) (A.36)

where L_s (H) is the self-inductance of the loop (for the calculation of L_ssee A.5.2).

The voltage and current induced by the magnetic field H_1/F of the first positive stroke (^ = 10 ps) is given by:

^oc/f/max “ 0.126 x b x I x H_1/F/MAX (V) (A.37)

^sc/f/max ~ 1 ’26 x 10-6 x ь x I x H_1/F/MAX / L_s(A) (A.38)

The voltage and current induced by the magnetic field H_1/FN of the first negative stroke (T^ = 1 ps) is given by:

^OC/FN/MAX = 1,26 x b x / x H_1/FN/MAx (V) (A.39)

^SC/FN/MAX = 1,26 10^-6 x b X / x H_1/FN/MAX / L_s(A) (A.40)T

(A.41)

he voltage and current induced by the magnetic field /7_1/s of the subsequent strokes (^ - 0,25 ps) are given by:

^OC/S/MAX - ⁵’^{04 x} b x / x H_1/S/MAX (V)

^SC/S/MAX - "1.26^x10 ⁶ X b x I x H_1/S/MAX/ L_s

(A)

(A.42)

where

^H1/FN/MAX (A/m)

^wi/s/max (A/m)

is the maximum stroke;

is the maximum stroke;

is the maximum strokes.

of the

of the

magnetic field inside LPZ

magnetic field inside LPZ

of the

1 due to the first positive

1 due to the first negative

magnetic field inside LPZ 1 due to the subsequent

A.5.4 Situation inside LPZ 2 and higher

The magnetic field H_n inside LPZ n for n > 2 is assumed to be homogeneous (see A.4.1.3).

Therefore, the same formulae for the calculation of induced voltages and currents apply (A.4.1.2), where H₁ is substituted by H_n

.Annex В
(informative)

Implementation of SPM for an existing structure

For equipment within existing structures it is not always possible to follow the SPM outlined in this standard. This annex attempts to describe the main points for consideration and provides information on protection measures which are not mandatory but may help to improve the overall protection provided.

In existing structures, suitable protection measures need to take into account the given construction, conditions of the structure, and the existing electrical and electronic systems.

A set of checklists facilitates risk analysis and selection of the most suitable protection measures.

For existing structures in particular, a systematic layout should be established for the zoning concept and for earthing, bonding, line routing and shielding.

The checklists given in Tables B.1 to B.4 should be used to collect the required data of the existing structure and its installations. Based on these data, a risk assessment in accordance with IEC 62305-2 should be performed to determine the need for protection and, if so, to identify the most cost-effective protection measures to be used.

NOTE 1 For further information on protection against electromagnetic interference (EMI) in building installations, see IEC 60364-4-44 ^[1].

The data collected by means of the checklists are also useful in the design process.

Table B.1 - Structural characteristics and surroundings

The first step in the design process is to work through the checklist in accordance with Clause B.2 and to conduct the risk assessment.

If this analysis shows that SPM is required, then this should be implemented following the steps outlined in Figure B.1.

Assign suitable LPZs to all locations where equipment to be protected is located (see 4.3).

The basis of the SPM shall be an internal screening and bonding network. This network should have mesh widths not exceeding 5 m in any direction. If the lay-out of the structure does not permit this screening and bonding network at least a ring conductor inside the outer wall of the structure on each floor should be installed. This ring conductor should be bonded to each down-conductor of the external LPS.

NOTE Retrofitting screening measures to an existing building is often impractical and uneconomic. Where this is the case, the use of SPDs provides an effective alternative.

The protection measures should be based on the internal screening and bonding network or the ring conductor inside the outer wall, which is normally the boundary of LPZ 1. If the outer wall is not the boundary of LPZ 1 and an internal screening and bonding network is not possible, a ring conductor should be installed at the boundary of LPZ 1. The ring conductor has to be connected to the ring conductor of the outer wall at least at two locations as far apart as possible.

The protection measures are based on the internal screening and bonding network or the ring conductor inside the outer wall. If an internal screening and bonding network is not possible, a ring conductor should be installed at the boundary of every LPZ 2. If an LPZ 2 is larger than 5 m x 5 m a subdivision has to be made creating meshes not exceeding 5 m x 5 m. The ring conductor has to be connected to the ring conductor of the surrounding LPZ 1 at two locations at least, and as far apart as possible.

The protection measures are based on the internal screening and bonding network or the ring conductor inside the LPZ 2. If an internal screening and bonding network is not possible, a ring conductor should be installed at the boundary of every LPZ 3. If an LPZ 3 is larger than 5 m x 5 m a subdivision has to be made creating meshes not exceeding 5 m x 5 m. The ring conductor has to be connected to the ring conductor of the surrounding LPZ 2 at two locations at least, and as far apart as possible.

A coordinated SPD system should be designed to protect the cables crossing borders of the different LPZs.

Designing additional measures will greatly improve the protection by bonding and SPD systems.

The design of cable trays, cable ladders and the like has to be improved to make them proper screens for the cables running in and/or over them.

If possible, additional measures such as screening of walls, floors, ceilings etc should be considered to provide additional protection to that already applied (see Clause 6).

Design measures to improve interconnections between the structure under consideration and other structures (see Clause B.11).

In the case where new internal systems are installed in a structure already equipped with protection measures, the design process should be repeated for the location of those internal systems.

The complete design process is illustrated in the flow chart (see Figure B.1).

Figure B.1 - SPM design steps for an existing structure

An existing LPS (in accordance with IEC 62305-3) around LPZ 1 can be improved by

• integrating existing metal facades and metal roofs into the external LPS,

• using such structural reinforcing bars as are electrically continuous from the upper roof to the earth termination system,

• reducing the spacing of the down-conductors and reducing the mesh size of the air­termination system to typically below 5 m,

• installation of flexible bonding conductors across the expansion joints between adjacent, but structurally separated, reinforced blocks.