---

A.3.2 Grid-like spatial shields

In practice, the large volume shields of LPZs are usually created by natural components of the structure such as the metal reinforcement in the ceilings, walls and floors, the metal framework, the metal roofs and metal facades. These components together create a grid-like spatial shield. Effective shielding requires that the mesh width be typically less than 5 m.

NOTE 1 The shielding effect may be neglected if an LPZ 1 is created by a normal external LPS in accordance with IEC 62305-3 with mesh widths and typical distances greater than 5 m. Otherwise, a large steel frame building with many structural steel stanchions provides a significant shielding effect.

NOTE 2 Shielding in subsequent inner LPZ can be accomplished either by adopting spatial shielding measures, by using closed metal racks or cabinets, or by using the metal enclosure of the equipment.

Figure A.3 shows how in practice the metal reinforcement in concrete and metal frames (for metal doors and possibly shielded windows) can be used to create a large volume shield for a room or building.

Key

• welded or clamped at every rod and at the crossings

NOTE In practice, it is not possible for extended structures to be welded or clamped at every point. However, most of the points are naturally connected by direct contacts or by additional wiring. A practical approach therefore could be a connection at about every 1 m.

Figure A.3 - Large volume shield built by metal reinforcement and metal frames

Internal systems should be located inside a “safety volume” which respects a safety distance from the shield of the LPZ (see Figure A.4). This is because of the relatively high magnetic fields close to the shield, due to partial lightning currents flowing in the shield (particularly for LPZ 1).

NOTE The volume l/_s should keep a safety distance d_s/1 or d_s/2 from the shield of LPZ n - see Clause A.4.

Figure A.4 - Volume for electrical and electronic systems inside an inner LPZ n

A.3.3 Line routing and line shielding

Surges induced into the internal systems can be reduced by suitable line routing (minimizing the induction loop area) or by using shielded cables or metallic cable ducts (minimizing the induction effects inside), or a combination of both (see Figures A.5).

Кеу

1. equipment

2. signal wiring

3. power wiring

4. induction loop

/ЕС 2ПЫ10

Figure A.5a - Unprotected system

Key

1. equipment

2. signal wiring

3. power wiring

5 spatial shielding

Figure A.5b - Reducing the magnetic field inside an inner
LPZ by its spatial shield

Key

1. equipment

2. signal wiring

3. power wiring

6 line shielding

/EC 2787/10

Figure A.5c - Reducing the influence of the field on lines by
line shieldin

g

Key

1. equipment

2. signal wiring

3. power wiring

7 reduced loop area

/EC 2788/10

Figure A.5d - Reducing the induction loop area by suitable
line routing

Figure A.5 - Reducing induction effects by line routing and shielding measures

The conductive cables connected to internal systems should be routed as closely to the metal components of the bonding network as possible. It is beneficial to run these cables in metal enclosures of the bonding network, for example U-shaped conduits or metal trunking (see also IEC 61000-5-2 ^I6])'

Particular attention should be paid when installing cables close to the shield of an LPZ (especially LPZ 1) due to the substantial value of the magnetic fields at that location.

When cables, which run between separate structures, need to be protected, they should be run in metal cable ducts. These ducts should be bonded at both ends to the bonding bars of the separate structures. If the cable shields (bonded at both ends) are able to carry the anticipated partial lightning current, additional metal cable ducts are not required.

Voltages and currents induced into loops, formed by installations, result in common mode surges at the internal systems. Calculations of these induced voltages and currents are described in Clause A.5.

Figure A.6 provides an example of a large office building:

• Shielding is achieved by steel reinforcement and metal facades for LPZ 1, and by shielded enclosures for the sensitive internal systems in LPZ 2. To be able to install a narrow meshed bonding system, several bonding terminals are provided in each room.

LPZ 0 is extended into LPZ 1 to house a power supply of 20 kV, because the installation of SPDs on the high voltage power side immediately at the entrance was not possible in this special case

.

Metal component on the roof

LPZOa

Equipment on the roof

LPZ Ob

LPZ 1

LPZ 2

Shielded
cabinet

LPZ 1

LPZ 2

Steel
reinforcement
in concrete

Sensitive
electronic
equipment

LPZ 2

Key

Interception mesh

LPZ Ob

LPZ 1

LPZ 1

Bonding
terminals

LPZ Ob

Camera

-<i

Metal facade

LPZ 1

Steel reinforcement

LPZ 1

Ground level

'7///////////////

Extraneous metal services

Telecom lines

0,4 kV power line

LPZ Ob

xxxxs

20 kV power line

Car parking

equipotential bonding

surge protective device (SPD)

Metal cable conduit ’(extended LPZ Ob)

Foundation earthing electrode

/EC 2789/10

Figure A.6 - Example of SPM for an office building

А.4 Magnetic field inside LPZ

A.4.1 Approximation for the magnetic field inside LPZ

If a theoretical (A.4.2), or experimental (A.4.3), investigation of the shielding effectiveness is not performed, the attenuation should be evaluated as follows.

A.4.1.1 Grid-like spatial shield of LPZ 1 in the case of a direct lightning strike

The shield of a building (shield surrounding LPZ 1) can be part of the external LPS; currents due to direct lightning strikes will flow along it. This situation is depicted by Figure A.7a assuming that the lightning hits the structure at an arbitrary point of the roof.

NOTE Distances d_w and d_r are determined for the point considered.

Figure A.7a - Magnetic field inside LPZ 1

NOTE Distances d_w and d_r are determined for the boundary of LPZ 2.

Figure A.7b - Magnetic field inside LPZ 2

Figure A.7 - Evaluation of the magnetic field values in case of a direct lightning strike

For the magnetic field strength at an arbitrary point inside LPZ 1, the following formula applies:

= k_h x/₀ x w_m/(d_w^x'd_r) (A/m) (A.1)

where

d_r (m) is the shortest distance between the point considered and the roof of shielded LPZ 1;

d_w (m) is the shortest distance between the point considered to the wall of shielded LPZ 1;

/₀ (A) is the lightning current in LPZ 0_A;

k_h (1/^m) is the configuration factor, typically k_h = 0,01;

w_m (m) is the mesh width of the grid-like shield of LPZ 1.

The result of this formula is the maximum value of the magnetic field in LPZ 1 (taking the note below into account):

• W_1/F/MAX = k_h x /_F/MAX x w_mI (d_w x d_r) (A/m) caused by the first positive stroke (A.2)

• ^H1/FN/MAX ^{= x ;}FN/MAX ^xz (^dw ^xxc/r) (A/m) caused by the first negative stroke (A.3) - W_1/S/MAX = k_h x /_S/MAX x w_mI (d_w x yld_r) (A/m) caused by the subsequent strokes (A.4)

where

'f/max (^A) ^{is the} maximum value of the first positive stroke current in accordance with the protection level;

^zfn/max (^A) ^{is the} maximum value of the first negative stroke current in accordance with the protection level;

^zs/max (^A) ^{is the} maximum value of the subsequent stroke currents in accordance with the protection level.

NOTE 1 The field is reduced by a factor of 2, if a meshed bonding network in accordance with 5.2 is installed.

These values of the magnetic field are valid only for a safety volume /_s inside the grid-like shield with a safety distance d_s/1 from the shield (see Figure A.4):

d_s/1 = w_m x SF/10 (m)forSF>10 (A.5)

(A.6)

(m) for SF< 10

d_s/1 =

where

SF (dB) is the shielding factor evaluated from the formulae of Table A.3;

w_m (m) is the mesh width of the grid-like shield.

NOTE 2 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.

EXAMPLE

As an example, three copper grid-like shields with dimensions given in Table A.2, and having an average mesh width of w_m = 2 m, are considered (see Figure A. 10). This results in a safety distance d_s/1 = 2,0 m defining the safety volume V_s. The values for H_1/MAX valid inside V_s are calculated for /_0/MAX - 100 kA and shown in Table A.2. The distance to the roof is half of the height: d_T- H/2. The distance to the wall is half of the length: d_w = LI2 (centre) or equal to: d_w = d_s/1 (worst case near the wall).

Table A.2 - Examples for /_0MAX = 100 kA and w_m = 2 m

A.4.1.2 Grid-like spatial shield of LPZ 1 in the case of a nearby lightning strike

The situation for a nearby lightning strike is shown in Figure A.8. The incident magnetic field around the shielded volume of LPZ 1 can be approximated as a plane wave.

Figure A.8 - Evaluation of the magnetic field values in case of a nearby lightning strike

The shielding factor SF of the grid-like spatial shields for a plane wave is given in Table A.3 below.

Table А.З - Magnetic attenuation of grid-like spatial shields for a plane wave

The incident magnetic field H_o is calculated using:

H_o= l₀ / (2 x л x s_a) (A/m) (A.7)

where

l₀ (A) is the lightning current in LPZ 0_A;

s_a (m) is the distance between the point of strike and the centre of the shielded volume.

From this, follows for the maximum value of the magnetic field in LPZ 0

• ^Wo/F/MAX ^{= Z}F/MAX ¹ (2 x 7t X s_a) (A/m) caused by the first positive stroke (A.8)

• ^ho/fn/max “ ^zfn/max / (2 x л x s_a) (A/m) caused by the first negative stroke (A.9)

• ^wo/s/max = ^Zs/MAX ! (2 X л X s_a) (A/m) caused by the subsequent strokes (A. 10)

where

/f/мах (^a) ^{is the} maximum value of the lightning current of the first positive stroke in accordance with the chosen protection level;

^zfn/max (^A) ^{is the} maximum value of the lightning current of the first negative stroke in accordance with the chosen protection level;

^zs/max (^A) ^{is the} maximum value of the lightning current of the subsequent strokes in accordance with the chosen protection level.

The reduction of H_o to inside LPZ 1 can be derived using the SF values given in Table A.3:

^hi/max ^{= h}o/max / 10^SF/2° (A/m) (A. 11)

where

SF (dB) is the shielding factor evaluated from the formulae of Table A.3;

Wq/max (A/m) is the magnetic field in LPZ 0.

From this follows for the maximum value of the magnetic field in LPZ 1

• ^H1/F/MAX ^{= H}o/F/MAX / 10^SF/20 (A/m) caused by the first positive stroke (A. 12)

” ^H1/FN/MAX ^{= W}o/FN/MAX /10^SF/20 (A/m) caused by the first negative stroke (A. 13)- ^Я1/s/мах “ Я_О/Э/МАХ / 10^SF/2° (A/m) caused by the subsequent strokes (A.14)

These magnetic field values are valid only for a safety volume V_s inside the grid-like shield with a safety distance d_s/2 from the shield (see Figure A.4).

(A. 15)

(A. 16)

for SF > 10

for SF < 10

• ^ds/2 = ^wm^SF/1° (m)

• ^ds/2 = ^wm (m)

where

SF (dB) is the shielding factor evaluated from the formulae of Table A.3;

w_m (m) is the mesh width of the grid-like shield.

For additional information concerning the calculation of the magnetic field strength inside grid­like shields in case of nearby lightning strikes, see A.4.3.

EXAMPLES

The magnetic field strength H_1/MAX inside LPZ 1 in the case of a nearby lightning strike depends on: the lightning current /_0/МАХ> the shielding factor SF of the shield of LPZ 1 and the distance s_a between the lightning channel and the centre of LPZ 1 (see Figure A.8).

The lightning current /_0/MAX depends on the LPL chosen (see IEC 62305-1). The shielding factor SF (see Table A.3) is mainly a function of the mesh width of the grid-like shield. The distance s_a is either:

• a given distance between the centre of LPZ 1 and an object nearby (e.g. a mast) in case of a lightning strike to this object; or

• the minimum distance between the centre of LPZ 1 and the lightning channel in case of a lightning strike to ground near LPZ 1.

The worst-case condition then is the highest current /_0/MAX combined with the closest distance s_a possible. As shown in Figure A.9, this minimum distance s_a is a function of height H and length L (or width W) of the structure (LPZ 1), and of the rolling sphere radius, r, corresponding to /_0/MAX (see Table A.4), defined from the electro-geometric model (see Clause A.4 of IEC 62305-1:2010).

Figure A.9 - Distance s_adepending on rolling sphere radius and structure dimensions

The distance can be calculated as:

s_a = уІ2хгхН-Н²+L/2

for H<r

(A. 17)

s_a = r + L I 2

for H > r

(A. 18)

NOTE For distances smaller than this minimum value the lightning strikes the structure directly.

Three typical shields may be defined, having the dimensions given in Table A.5. A grid-like shield of copper with an average mesh width of w_m = 2 m is assumed. This results in a shielding factor SF = 12,6 dB and in a safety distance d_sl2= 2,5 m defining the safety volume V_s. The values for H_0/MAX and H_1/MAX, which are assumed to be valid everywhere inside /_s, are calculated for /_0/MAX = 100 kA and shown in Table A.5.

Table A.4 - Rolling sphere radius corresponding to maximum lightning current

Table A.5 - Examples for /_0/MAX = 100 kA and w_m = 2 m corresponding to SF = 12,6 dB

A.4.1.3 Grid-like spatial shields for LPZ 2 and higher

In the grid-like shields of LPZ 2, and higher, no significant partial lightning currents will flow. Therefore, as a first approach, the reduction of H_n to H_n+1 inside LPZ n + 1 can be evaluated as given by A.4.1.2 for nearby lightning strikes:

F

(A. 19)

/_n+1 = ^Hn ¹ 10^SF/20 (A/m)

where

SF (dB) is the shielding factor from Table A.3;

/-/_n (A/m) is the magnetic field inside LPZ n (A/m).

If H_n= H<, this field strength can be evaluated as follows:

• In the case of lightning strikes direct to the grid-like shield of LPZ 1 see A.4.1.1 and Figure A.7b, while d_w and d_r are the distances between the shield of LPZ 2 and the wall respectively the roof.

• In the case of lightning strikes nearby LPZ 1 see A.4.1.2 and Figure A.8.

These magnetic field values are valid only for a safety volume inside the grid-like shield with a safety distance d_s/2 from the shield (as defined in A.4.1.2 and shown in Figure A.4)

.

A.4.2 Theoretical evaluation of the magnetic field due to direct lightning strikes

In A.4.1.1, the formulae for the assessment of the magnetic field strength H_1/MAX are based on numerical magnetic field calculations for three typical grid-like shields as shown in Figure A.10. For these calculations, a lightning strike to one of the edges of the roof is assumed. The lightning channel is simulated by a vertical conducting rod with a length of 100 m on top of the roof. An idealized conducting plate simulates the ground plane.