Earthing and Lightning Protection Guide

8/22/2019 Earthing and Lightning Protection Guide

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CTBTO/IMS Earthing andLightning Protection Minimum

Standard


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TABLE OF CONTENT

1 Introduction ......................................................................................... 4

2

General Part ......................................................................................... 5

2.1 Lightning activity and exposure.......................................................... 5

2.1.1 Frequency of lightning strikes ................................................................. 5

2.1.2 Direct lightning strikes to a structure....................................................... 6

2.1.3 Assessment of the average annual number of dangerousevents due to flashes near a structure NM .............................................. 7

2.1.4 Direct and indirect lightning strikes to an incoming overhead lineor cable ................................................................................................... 8

2.2 Lightning Protection System (LPS) .................................................. 10

2.2.1 Air-termination system .......................................................................... 11

2.2.2 Down-conductors.................................................................................. 13

2.2.3 Earth-termination system ...................................................................... 14

2.2.4 Equipotential bonding ........................................................................... 23

2.2.5 Surge Protection ................................................................................... 28

3 CTBTO/IMS Specific Part ...................................................................42

3.1 Classification of CTBTO/IMS stations .............................................. 42

3.1.1 Classification in terms of lightning exposure ......................................... 42

3.1.2 Classification in terms of Lightning Protection Zones ........................... 42

3.2 Protection of the Central Recording Facility .................................... 43

3.2.1 Air Termination system and down-conductors ...................................... 43

3.2.2 Earth termination system ...................................................................... 43

3.2.3 Equipotential bonding system ............................................................... 44

3.2.4 Surge protection ................................................................................... 44

3.3 Protection of the remote elements.................................................... 45

3.3.1 Air termination system .......................................................................... 46

3.3.2 Down-conductors.................................................................................. 473.3.3 Earth-termination system ...................................................................... 47

3.3.4 Equipotential Bonding system .............................................................. 50

3.3.5 Surge Protection ................................................................................... 51

3.4 Technology Specific Situations ........................................................ 51

3.4.1 Seismic Monitoring Stations ................................................................. 51

3.4.2 Infrasound Monitoring Stations ............................................................. 52

3.4.3 Hydroacoustic Monitoring Stations ....................................................... 52

3.4.4

Radionuclide Monitoring Stations ......................................................... 53


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Annex A Maintenance and inspection of the Lightning ProtectionSystem ................................................................................................. 54

A.1 General ................................................................................................. 54

A.2 Recommended frequency of inspection ............................................... 54

A.3

Visual inspection................................................................................... 54

A.4 Testing .................................................................................................. 55

Annex B Soil resistivity measurements ........................................................... 56

B.1 Specific earth resistance E.................................................................. 56

B.2 Seasonal fluctuations ........................................................................... 57

B.3 Measurement of specific earth resistance E [8], [10] .......................... 58

Annex C Earthing Electrode System Testing/Verification .............................. 60

C.1 Overview .............................................................................................. 60

C.2 3-pole/4-pole Measurement of Earthing Resistance ............................. 60

C.3 Clamp-on Ohmmeter ............................................................................ 62

C.4 Prerequisites for Testing ....................................................................... 63

C.5 Required Test Equipment and Supplies ............................................... 65

Annex D Dissimilar Metals and Corrosion Control ......................................... 68

D.1 Choice of earth electrode materials ...................................................... 69

D.1.1 Hot-dip galvanized steel ....................................................................... 69

D.1.2 Bare steel ............................................................................................. 69

D.1.3 Steel with copper sheath ...................................................................... 69

D.1.4 Bare copper .......................................................................................... 69D.1.5 Stainless steels .................................................................................... 69

D.2 Combination of earth electrodes made of different materials ............... 70

D.3 Methods to help reduce Corrosion ....................................................... 71

D.3.1 Galvanized steel connecting cables from foundation earthelectrodes to down conductors ............................................................. 71

D.3.2 Earth entries ......................................................................................... 71

D.3.3 Underground terminals and connections .............................................. 71

Annex E Lightning Protection System Compliance Matrix ............................ 72

Annex F List of Abbreviations .......................................................................... 78

Annex G References .......................................................................................... 79


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1 Introduction

The International Monitoring System (IMS) consists of a worldwide network of 321stations, including primary and auxiliary seismic, hydroacoustic, infrasound andradionuclide monitoring stations supported by 16 radionuclide laboratories. The IMSfacilities transmit data using a closed and secure satellite communications networkvia the Global Communications Infrastructure to the International Data Centre (IDC)in Vienna using very small aperture terminal technology.

The IMS network is required to be in continuous operation with high dataavailability requirements in the IDC, which can be summarized as 98% for seismic,hydroacoustic and infrasound stations and 95% for radionuclide monitoring stations.

Many stations are located in areas with high annual ground flash density andunreliable mains power. At those locations, the equipment is subject to damagecaused by lightning, both direct strikes and surges from indirect flashes, and byelectrical surges. This seriously affects the data availability requirements.

This document provides information about the lightning protection and installationof earth-termination system, lightning equipotential bonding and surge protectiondesignated to minimize damage caused by these events.

In general, the cases which require lightning protection are the following:

(1) Existence of large crowds(2) Necessity of service continuity(3) Very high lightning flash frequency(4) Tall isolated structures(5) Buildings containing explosive or flammable materials(6) Buildings containing irreplaceable cultural heritage

For CTBTO/IMS stations cases (2), (3) and (4) are applicable and hence properlightning protection is needed in order to achieve the expected performance.

The main and most effective measure for protection of structures against physicaldamage is considered to be the Lightning Protection System (LPS). It usuallyconsists of both external and internal lightning protection systems.

The external LPS is intended to intercept direct lightning flashes to the structure,including flashes to the side of the structure, and to conduct the lightning current fromthe point of strike to the ground. The external LPS is also intended to disperse thiscurrent into the earth without causing thermal or mechanical damage, nor dangeroussparking which may trigger fire or explosions.

An internal LPS prevents dangerous sparking within the structure using eitherequipotential bonding or a separation distance (and hence electrical insulation)between the external LPS components and other electrically conducting elementsinternal to the structure.

IEC Standard 62305, made up of four parts [1], [2], [3] and [4] provides all basicinformation for a proper lightning protection of CTBTO/IMS stations.


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2 General Part

2.1 Lightning activity and exposure

The probability that a structure or object will be struck by lightning is the product ofthe equivalent collection area of the structure or object and the flash density for thearea that the structure is located.

Lightning Flash Density (Ng), sometimes also called Ground Flash Density, is theaverage yearly number of flashes to ground per square kilometer. This value isavailable from ground flash location networks in some areas of the world. If a map ofNg is not available, in temperate regions Ngmay be estimated by:

dg TxN 1.0

where Td is the average number of thunderstorm days per year (which can beobtained from isokeraunic maps from local meteorological services).

For some tropical regions correlations have been found [5], which are differentfrom the more general one given above:

12.1024.0 dg TxN

for mountainous regions in Mexico12.1030.0 dg TxN for mountainous regions in Brazil

56.10017.0 dg TxN for Colombia.

Another estimate ofNgmay be obtained from thunderstorm hour records by:

1.1054.0 hg TxN

where Th is the average number of thunderstorm hours per year[6].

ForNgequal or higher than 4, lightning protection is strongly recommended.

2.1.1 Frequency of lightning strikes

It is necessary to distinguish between the following frequencies of lightning strikeswhich can be relevant for a building or structure:

Nd Frequency of direct lightning strikes to the building or structure;NM Frequency of close lightning strikes with electromagnetic effects;NL Frequency of direct lightning strikes in utility lines entering the building or

structure;NI Frequency of lightning strikes adjacent to utility lines entering the building or

structure.


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2.1.2 Direct lightning strikes to a structure

The yearly lightning strike frequency (Nd) to a structure (as a CTBTO/IMS site) isdetermined by the following equation [1]:

610xCxAxNN

ddgd

where:Nd is the yearly lightning strike frequency to the siteNg is the yearly average flash density in the region where the structure is locatedAd is the collection area of the structure (m

2)Cd is the Location Factor (see Table 1).

Table 1: Location Factor Cd

Relative location Cd

Structure surrounded by higher objects or trees 0,25Structure surrounded by objects or trees of the same heights orsmaller

0,5

Isolated structure: no other structures in the vicinity 1

Isolated structure on a hilltop or a knoll 2

The Collection Area (Ad) refers to the ground area having the same yearly directlightning flash probability as the structure. It is an increased area for the structure thatincludes the effect of the height and location of the structure.

For isolated structures on flat ground, the collection areaAd is the area defined bythe intersection between the ground surface and a straight line with 1/3 slope whichpasses from the upper parts of the structure (touching it there) and rotating around it(see Figure 1). Determination of the value ofAdmay be performed graphically (Figure1) or mathematically:

Ad= L x W + 6 x H x (L+W) + 9 xx H2

with L, W and H expressed in meters. For more complex and non rectangular

structures see e.g. IEC 62305-2, A.2.1.1.


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Figure 1: Calculation of the collection areaAd for a rectangular structure

In case of a typical CTBTO/IMS site with underground vaults, a surrounding fenceof height HFand a radio tower of height HT the collecting areaAdcan be estimated asshown in Figure 2.

Figure 2: Collection areaAdfor a rectangular structure and a radio tower of height HT

2.1.3 Assessment of the average annual number of dangerousevents due to flashes near a structure NM

Average annual number of dangerous events due to flashes near a structure NMmay be evaluated as the product:

NM = Ng (AmAd x Cd) 106

where

Ng is the lightning ground flash density (flash/(km2 x year));

Ad is the collection area for the structure (see Figure 1)Am is the collection area of flashes striking near the structure (m

2).


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The collection area Am extends to a line located at a distance of 250 m from theperimeter of the structure (see Figure 3)

Figure 3: Collection areaAm, Al, andAifor a rectangular structure

For a rectangular structure of length L and width W the collection area Am is givenby:

Am = L x W + 2 x (L+W) x 250+2502x

2.1.4 Direct and indirect lightning strikes to an incoming overhead

line or cableThe frequency of lightning to or nearby a service line (power, data line,

telecommunication, etc) entering a building or a station can be estimated by:

NL = Ng Al Ce 106 (direct strikes)

Ni = Ng Ai Ce 106 (indirect strikes)

Ce is the environment factor (in rural areas Ce is 1).

Alis a function of the type of line (overhead line or buried cable) and the length LCof the line. In the case of buried cables, it is also a function of the earth resistivity ;and for overhead lines it depends on the height of the line (H) above ground level(see Table 2).


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Table 2: Equivalent interception areas Aland Aifor utility lines

Overhead Line Underground cable

Al CbaC HxxHHxL 63 )( xHHxL baC )(3

Ai CLx1000 xLx C25

Al is the collection area of flashes striking the service (m2);

Ai is the collection area of flashes to ground near the service (m2);

Hc is the height of the service conductors above ground (m);Lc is the length of the service section from the structure to the first node (m). A

maximum value Lc= 1 000 m should be assumed;Ha is the height of the structure connected at end a of service (m);Hb is the height of the structure connected at end b of service (m); is the earth resistivity (m) in or on which the line is laid, up to a maximum

value of

NOTE: The structure to be protected shall be assumed to be the one connected at b end ofservice


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2.2 Lightning Protection System (LPS)

a) Definitions:

The lightning protection system is the complete system used to reducephysical damage due to lightning flashes to the structure and lightning causedsurges on power and data lines.

The external LPS is intended to intercept direct lightning flashes to thestructure and conduct the lightning current from the point of strike to groundand to disperse this current into the earth without causing thermal ormechanical damage, nor dangerous sparking which may trigger fire or

explosions.In most cases, the external LPS is attached to the structure to be protected.An isolated external LPS should be considered when the thermal andexplosive effects at the point of strike, or on the conductors carrying thelightning current, may cause damage to the structure or to the contents (seeparagraph 5.3.2 in 62305-3).

b) Lightning Protection Level Classification:

According to [1] (IEC 62305-1) there exist four Lighting Protection Levels(LPL): LPL I, II, III and IV. For each LPL a set of maximum and minimumlightning current parameters is fixed. Only 1% of lightning events will exceedthe maximum values of lighting current parameters specified for LPL I. ForLPL II the parameters are reduced to 75% of the values of LPL I. These valuesare 50 % of LPL I for LPL III and IV.

Lightning protection classes I, II, III and IV refer to the LPL I, II, III and IVdescribed above.

Lightning protection measures specified in this document are based on LPL II,which provides sufficient protection for the IMS installations.

The components of a LPS are Air-Termination System, Down-conductors, Earth-termination System, Equipotential Bonding and Surge Protection based on LightningProtection Zone (LPZ) concept.

All connections within LPS must provide permanent galvanic and mechanicalconnection between the components. The method of connection depends on thematerials used for the LPS and can be made by brazing, welding, pressing, screwingor riveting, for example.


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2.2.1 Air-termination system

The probability of structure penetration by a lightning current is considerablydecreased by the presence of a properly designed air-termination system.

Air-termination systems can be composed of any combination of the followingelements (examples see Figure 4):

a) rods (including free-standing masts);

b) catenary wires;

c) meshed conductors.

a) b)

Figure 4: Two examples of air-termination system on buildings. a) Roof with air terminationconductor and b) air-termination rod for chimney (adapted from [8])

Air-termination components installed on a structure shall be located at corners,exposed points and edges in accordance with one or more of the following methods:

the protection angle method;

the rolling sphere method; the mesh method.

The values for the protection angle, rolling sphere radius and mesh size for eachclass of LPS are given in Table 3.


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Table 3: Maximum values of rolling sphere radius and mesh size corresponding to the class of LPS

Class of LPSRolling Sphere

Radius r(m)Mesh Size W

(m)

I 20 5 x 5

II 30 10 x 10

III 45 15 x 15

IV 60 20 x 20

Figure 5: Protection angle corresponding to the class of LPS as a function of the height Hof air-termination above the reference plane

The mesh method is a suitable form of protection where plane surfaces are to beprotected.

The protection angle method is suitable for simple-shaped buildings but it issubject to limits of air-termination height Has indicated in Figure 5.

The cone-shaped protected zone provided by a vertical rod of height h and thecorresponding angle ( being a function of h and class of LPS) is shown inFigure 6.


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Figure 6: Protected zone of a vertical rod for a protection angle

For class II LPS, the protection angle and radius R of protected area at thereference plane are given in Table 4.

Table 4: Protective angle and distance R inFigure 6as a function of height h for class II LPS

2.2.2 Down-conductors

Down-conductors shall be arranged in such a way that from the point of strike to

the earth several parallel current paths exist. The length of those current paths mustbe kept to a minimum.


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An equal spacing of the down-conductors is preferred around the perimeter of astructure. Typical values of the distance between the down-conductors are given inTable 5.

Table 5: Typical values of the distance between down-conductors and between ring conductorsaccording to the class of LPS

Class of LPS Typical Distance (m)

I 10

II 10

III 15

IV 20

A down-conductor should be installed at each exposed corner of the structure,where this is possible.

Figure 7: Example of down-conductor mounted on a brick wall

2.2.3 Earth-termination system

The main task of the earth termination system is to:

efficiently dissipate the lightning surge energy that may arrive via down-conductors of the lightning protection system;

efficiently dissipate electrical surges and faults in order to minimize thechances of human injury from either step potentials or touch potentials;

provide a stable reference for electrical and RF circuits at the facility in orderto minimize noise during normal operation;


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be properly bonded to provide an equipotential plane under fault or lightningstrike conditions;

be electrically and mechanically robust in order to assure performance overthe life of the facility (nominally several 10s of years from construction date).

The shape and dimensions of the earth-termination system are the importantcriteria when dealing with the dispersion of the high frequency lightning current intothe ground. In general, a low earthing resistance is recommended [3], with valueslower than 10 when measured at low frequency.

From the viewpoint of lightning protection, a single integrated earth-terminationsystem is preferable and is suitable for all purposes (i.e. lightning protection, powersystems and telecommunication systems). Underground metallic piping and anyother existing earthing system shall be bonded together to form a single integratedearth-termination system.

The earth-termination system shall have low electrical impedance, with conductorslarge enough to withstand high fault and lightning currents. In addition, in case ofhighly corrosive environment, the size of the electrodes should be large enough inorder to provide sufficient life time (minimum 20 years for IMS applications). Thelower the earth electrode impedance, the more effectively the earth electrode systemcan dissipate high energy impulses into the earth.

2.2.3.1 Earthing electrodes

The earthing electrodes are the conducting elements used to connect electricalsystems and/or equipment to the earth. The earthing electrodes are placed into thesoil to maintain electrical equipment at the potential of the earth and to dissipatecurrents into the soil. Earthing electrodes may be earthing rods, metal plates, inconcrete encased conductors, earthing ring conductors, electrolytic earthing rods orthe metal frame of buildings and includes also foundation earth electrodes. Typicalearthing electrodes are shown in Figure 8.


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Figure 8: Typical earth electrodes and their installation

The requirements for earthing electrodes are:

The material and dimensions used for earth electrodes should conform to thematerials listed inTable 6.

The behavior of the metal with respect to corrosion in the soil and inconnection with dissimilar metals should always be taken into account (seeAnnex D ).

The vertical earth electrodes shall have a minimum length of 3 m. The actualdiameter, length, and number of rods required may vary with site dimensionsand/or as determined by an engineering study based on the soil resistivityprofile of the site.

Where multiple connected earth electrodes are used, the separation betweenany two electrodes shall be at least the sum of their driven depths (wherepracticable).

The method of bonding earthing conductors to earth electrodes shall becompatible with the types of metals being bonded (seeTable 17in Annex D).

Earth electrodes shall be free of paint or other nonconductive coatings.

Where applicable, the earth electrodes shall be buried below the permanentmoisture level.

Earth electrodes shall be buried to a minimum depth of 0.8 m below finishedgrade, where possible, or buried below the freeze line, whichever depth islarger.

Earth electrodes that cannot be driven straight down, due to contact with rockformations, may be driven at an oblique angle of not greater than 45 degrees


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from the vertical, or may be buried horizontally in a trench at least 0.8 m deepperpendicular to the building.

Table 6: Material, configuration and minimum dimensions of earth electrodes [3]

Note: In some areas bare steel is also allowed to be used in the soil, notembedded in concrete. In [7] for bare steel electrodes in soil (electrodes were 2.4 mlong and had a diameter of 16 mm) a 7.6 % corrosion weight loss was determinedafter 7 years. As the actual corrosion of bare steel is determined by local soilconditions, regular measurements of the earthing resistance of such electrodes shallbe done to ensure proper earthing conditions and initiate improvements, whennecessary. Corrosion is phenomena that can occur at the boundaries between two

distinct soil layers. Therefore, earthing electrodes of sufficient corrosion resistivity(e.g. stainless steel, copper) shall be used.


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2.2.3.2 Earthing conductor

Earthing conductor is a conductor connecting the system component to be earthedto an earth electrode and which is installed above the ground or insulated in theground.

2.2.3.3 Minimum site earthing requirements

As the earthing system is used for several functions in a structure (e.g. signalreference ground and lightning protection), the request for a minimum earthingresistance may have different reasons.

For lightning and overvoltage protection the absolute value of the earthing systemresistance is not as important as to ensure that all equipment and conductingservices are connected to a more or less equal potential plane (equipotential bondingis a must). The need to ensure an equipotential plane becomes obvious by thefollowing simple calculation: When a typical lightning current of 10 kA is injected in an

earthing system of 10 , at the striking point a potential raise relative to the referenceearth of 100 kV (U = I*R= 10.000 A x 10 = 100.000 V) will occur and can causeflashover when bonding is not done properly.

A mesh of earth conductors with a mesh-size of about 5 m x 5 m, where towers,objects and equipment vaults are integrated, is suitable to limit potential differencesamong the installations and at the surface (risk of step-voltage) to acceptable values.

Since IEC 62305-3 [3] assumes a systematic lightning equipotential bonding, noparticular value is required for the earth electrode resistance. In this IEC standard theminimum length I1 of the earthing electrode is a function of the class of lightning

protection system (see Figure 9). For LPL II, being applicable to CTBTO/IMSstations, Table 7 shows l1as a functionof the soil resistivity in m.

Figure 9: IEC 62305-3 specified minimum length l1 of each horizontal earth electrodeaccording to the class of the LPS


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Table 7: Minimum length l1of each horizontal earth electrode as a function of ground resistivity in maccording to LPL II

.m l1 .m l1 .m l1 .m l1 .m l1 .m l1

0 5,0 500 5,0 1000 9,1 1500 19,3 2000 29,5 2500 39,8

10 5,0 510 5,0 1010 9,3 1510 19,5 2010 29,8 2510 40,0

20 5,0 520 5,0 1020 9,5 1520 19,7 2020 30,0 2520 40,230 5,0 530 5,0 1030 9,7 1530 19,9 2030 30,2 2530 40,4

40 5,0 540 5,0 1040 9,9 1540 20,1 2040 30,4 2540 40,6

50 5,0 550 5,0 1050 10,1 1550 20,3 2050 30,6 2550 40,8

60 5,0 560 5,0 1060 10,3 1560 20,5 2060 30,8 2560 41,0

70 5,0 570 5,0 1070 10,5 1570 20,8 2070 31,0 2570 41,2

80 5,0 580 5,0 1080 10,7 1580 21,0 2080 31,2 2580 41,4

90 5,0 590 5,0 1090 10,9 1590 21,2 2090 31,4 2590 41,6

100 5,0 600 5,0 1100 11,1 1600 21,4 2100 31,6 2600 41,8

110 5,0 610 5,0 1110 11,3 1610 21,6 2110 31,8 2610 42,0

120 5,0 620 5,0 1120 11,5 1620 21,8 2120 32,0 2620 42,2

130 5,0 630 5,0 1130 11,8 1630 22,0 2130 32,2 2630 42,4

140 5,0 640 5,0 1140 12,0 1640 22,2 2140 32,4 2640 42,6

150 5,0 650 5,0 1150 12,2 1650 22,4 2150 32,6 2650 42,8

160 5,0 660 5,0 1160 12,4 1660 22,6 2160 32,8 2660 43,0

170 5,0 670 5,0 1170 12,6 1670 22,8 2170 33,0 2670 43,3

180 5,0 680 5,0 1180 12,8 1680 23,0 2180 33,2 2680 43,5

190 5,0 690 5,0 1190 13,0 1690 23,2 2190 33,4 2690 43,7

200 5,0 700 5,0 1200 13,2 1700 23,4 2200 33,6 2700 43,9

210 5,0 710 5,0 1210 13,4 1710 23,6 2210 33,8 2710 44,1

220 5,0 720 5,0 1220 13,6 1720 23,8 2220 34,0 2720 44,3

230 5,0 730 5,0 1230 13,8 1730 24,0 2230 34,3 2730 44,5

240 5,0 740 5,0 1240 14,0 1740 24,2 2240 34,5 2740 44,7

250 5,0 750 5,0 1250 14,2 1750 24,4 2250 34,7 2750 44,9

260 5,0 760 5,0 1260 14,4 1760 24,6 2260 34,9 2760 45,1

270 5,0 770 5,0 1270 14,6 1770 24,8 2270 35,1 2770 45,3

280 5,0 780 5,0 1280 14,8 1780 25,0 2280 35,3 2780 45,5

290 5,0 790 5,0 1290 15,0 1790 25,3 2290 35,5 2790 45,7

300 5,0 800 5,0 1300 15,2 1800 25,5 2300 35,7 2800 45,9

310 5,0 810 5,2 1310 15,4 1810 25,7 2310 35,9 2810 46,1

320 5,0 820 5,4 1320 15,6 1820 25,9 2320 36,1 2820 46,3

330 5,0 830 5,6 1330 15,8 1830 26,1 2330 36,3 2830 46,5

340 5,0 840 5,8 1340 16,0 1840 26,3 2340 36,5 2840 46,7

350 5,0 850 6,0 1350 16,3 1850 26,5 2350 36,7 2850 46,9

360 5,0 860 6,2 1360 16,5 1860 26,7 2360 36,9 2860 47,1

370 5,0 870 6,4 1370 16,7 1870 26,9 2370 37,1 2870 47,3

380 5,0 880 6,6 1380 16,9 1880 27,1 2380 37,3 2880 47,5

390 5,0 890 6,8 1390 17,1 1890 27,3 2390 37,5 2890 47,8400 5,0 900 7,0 1400 17,3 1900 27,5 2400 37,7 2900 48,0

410 5,0 910 7,3 1410 17,5 1910 27,7 2410 37,9 2910 48,2

420 5,0 920 7,5 1420 17,7 1920 27,9 2420 38,1 2920 48,4

430 5,0 930 7,7 1430 17,9 1930 28,1 2430 38,3 2930 48,6

440 5,0 940 7,9 1440 18,1 1940 28,3 2440 38,5 2940 48,8

450 5,0 950 8,1 1450 18,3 1950 28,5 2450 38,8 2950 49,0

460 5,0 960 8,3 1460 18,5 1960 28,7 2460 39,0 2960 49,2

470 5,0 970 8,5 1470 18,7 1970 28,9 2470 39,2 2970 49,4

480 5,0 980 8,7 1480 18,9 1980 29,1 2480 39,4 2980 49,6

490 5,0 990 8,9 1490 19,1 1990 29,3 2490 39,6 2990 49,8

500 5,0 1000 9,1 1500 19,3 2000 29,5 2500 39,8 3000 50,0


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The minimum length of each earth electrode is:

I1 for horizontal earth electrodes

I1 x 0.5 for vertical or inclined earth electrodes (with a minimum of 3 m)

The determined values of l1 apply to each individual earth electrode. For meshedearth electrodes and foundation earth electrodes the average radius r of the areaenclosed by the earth electrode must be not less than the given minimum length l1inFigure 9 according to the selected class of LPS. To determine the average radius r,the area under consideration is transferred into an equivalent circular area and theradius is determined as shown in Figure 10.

Figure 10: Equivalent radius of a residential building in order to compare with minimumlength l1 of each horizontal earth electrode according to the class of LPS [8]

In the example shown in Figure 10 the calculated equivalent radius of 5.89 m forthe given ring electrode around the buildings area A1 is larger than the minimumlength of 5 m (see Figure 9 for LPS class III) and hence no further earth electrodesare required.

2.2.3.4 Calculation of earth electrode resistances

Table 8 gives some formulas for estimating the earth electrode resistance of themost common types of earth electrodes. In practice, these approximate formulas arequite sufficient, when keeping in mind all the variable parameters of the ground sub-surface (soil humidity, temperature, homogeneity of soil, etc.)


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Table 8: Formulae for calculating the earth electrode resistance RA for different earth electrodes

2.2.3.5 Special earthing situations

The earthing resistance of the total earthing systems should be as low as possible,nevertheless it is always most important to ensure minimum potential differenceswithin the protected area including towers, buildings and equipment vault.Independent of the actual earthing conditions (e.g. on a mountain top, in arctic regionor coral environment) a safe operation of equipment is achievable when allequipment is on more or less the same potential and all lines entering theequipotential area are well bonded and protected by SPDs. Some examples aregiven below.

Towers with limited space for an earthing ring

Towers installed close to buildings may not have adequate space for acomplete tower earthing ring or for earthing rods spaced properly to achieve theresistance requirements of the site. The tower earthing shall be integrated with theearthing system of the adjacent buildings and an earthing mesh (with optionalearthing rods) shall be installed across the available space (example see in Figure11).


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Figure 11: Linear earthing electrode system in case of limited space (typically one rod every5 m or twice the length of the rod)

Earth electrodes in rocky soil, sand, coral, limestone, arctic ice or mountaintops

Some sites are located on mountaintops or in areas with rocky soil. In theinstances where there is no soil or very little soil at the site (shallow topsoilenvironment, arctic regions, sand, coral, or limestone environments, etc.) specialdesigns will be needed. Surface earth electrodes such as ring earth electrodes orstar-type earth electrodes are often the only way to create an earth-terminationsystem. When installing the earth electrodes, the flat strip or round material is laidon the stony ground, the ice or on the rock. Even where a foundation earthelectrode has a reduced earthing effect in rocky soil, it still acts as an equipotential

bonding conductor. If the earth electrode cannot be installed in the soil and has tobe mounted on the surface, it should be protected against mechanical damage.Radial earth electrodes lying on or near the earth surface should be covered bystones or embedded in concrete for mechanical protection. The clamped pointsshould be installed with particular care and be protected against corrosion(anticorrosive band).

In conditions described above, the main target to be achieved is an earthtermination system providing sufficient equipotential plane. The achievement of alow grounding resistance is a secondary task.

An example for the extension of the earthing system of a tower to nearby areaswith low soil resistivity is shown in Figure 12.


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Figure 12: Example of site on mountain top with earthing wires from the tower to nearbyareas with low soil resistivity

2.2.4 Equipotential bonding

Lightning strikes can give rise to harmful potential differences in and on abuilding/structure. The major concern in the protection of a building/structure is the

occurrence of potential differences between the conductors of the lightning protectionsystem and other grounded metal bodies and wires belonging to the building. Thesepotential differences are caused by resistive and inductive effects and can result indangerous sparking or damage of electronic equipment.

2.2.4.1 Bonding to the earth termination system

Equipotential bonding is required to remove or to reduce potential differencesbetween various installations. Bonding prevents e.g. hazardous touch voltagesbetween the protective conductor of the low voltage electrical power consumersinstallations and metal, water, gas and heating pipes.

The equipotential bonding consists of a main equipotential bonding bar (MBB)where the following extraneous conductive parts shall be directly connected (seeFigure 13):

main equipotential bonding conductor

foundation earth electrodes or lightning protection earth electrodes

conductive parts of the building structure (e.g. lift rails, steel skeleton,ventilation and air conditioning ducting)

metal drain pipes

internal gas pipesearthing conductor for antennas


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earthing conductor for telecommunication systems

protective conductors of the electrical installation (PEN conductor for TNsystems and PE conductors for TT systems or IT systems)

metal shields of electrical and electronic conductors

metal cable sheaths of high-voltage current cables up to 1000 V

Figure 13: Principle of lightning equipotential bonding consisting of lightning and mainequipotential [8]

Different types of equipotential bonding bars and their installation are shown inFigure 14.

In case, when a direct connection to the bonding bar is not possible, theinstallation components have to be integrated indirectly into the main equipotentialbonding via isolating spark gaps (e.g. measuring earth in laboratories, if they have tobe separate from the protective conductors). During normal operation the spark gapwill keep the circuits insulated and whenever lightning strikes the spark gap willprovide short time interconnection.

Lightning equipotential bonding connections shall be made as direct and straight

as possible. For external conductive parts, lightning equipotential bonding shall beestablished as near as possible to the point of entry into the structure to be protected.


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Figure 14: Examples of practical design an installation of equipotential bonding bars

The minimum cross-section values of the bonding conductors connecting differentbonding bars and of the conductors connecting the bars to the earth-terminationsystem are listed in Table 9.

Table 9: Minimum dimensions of conductors connecting different bonding bars

or connecting bonding bars to the earth-termination system [3]

The minimum cross-section values of the bonding conductors connecting internal

metal installations to the bonding bars are listed in Table 10.Table 10: Minimum dimensions of conductors connecting internal metal installations

to the bonding bar[3]

According to [9] the minimum cross-section for earth conductors of antennas is16 mm2 Cu, 25 mm2 Al or 50 mm2 steel.


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In order to minimize the induction loops within buildings the following isrecommended:

Cables and metal pipes shall enter the building at the same point;

Power lines and data lines shall be laid spatially close and have to beshielded;

Avoid unnecessarily long cables by laying lines directly;

Integration of the cable shields into the equipotential bonding by bonding theshield at both ends. In case when a permanent bonding of the cable shield atboth ends is not possible, the shield may be bonded with a spark gap.

2.2.4.2 Sub System Equipotential Bonding Bar (EBB)

Sometimes supplementary local equipotential bonding is useful. The reasonbehind is to interconnect all simultaneously accessible parts as well as the stationaryoperating equipment and also extraneous conductive parts. The aim is to keep anyvoltage differences between systems as low as possible.

The difference to the main equipotential bonding is the fact that the cross sectionsof the conductors can be chosen to be smaller, and also this supplementaryequipotential bonding can be limited to a particular location. For minimum cross-section of conductors connecting internal metal installations to the bonding bar seeTable 10. All connections to the bonding bar shall be as short as possible and without

loops.

2.2.4.3 Internal perimeter earthing bus conductors

Enclosures and racks of electronic devices and systems should be integrated intothe equipotential bonding network with short connections. This requires sufficientnumbers of equipotential bonding bars and/or ring equipotential bonding bars in thebuilding or structure. The busbars, in turn, must be connected to the equipotentialbonding network (Figure 15,Figure 16).

Figure 15: Example of earthing bus


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Figure 16: Connection of the ring equipotential bonding bar with the equipotential bondingnetwork via fixed earthing point

2.2.4.4 Equipment Earthing

Protective conductors (PE) and cable shields of the data links of electronic devicesand systems must be integrated into the equipotential bonding network inaccordance with the instructions of the system manufacturer. The connections canbe made either in the shape of a star or as a mesh (seeFigure 17andFigure 18).

Figure 17: Star shape integration of

electronic systems into the equipotentialbonding network (ERP is the earthing

reference point)

Figure 18: Mesh shape integration ofelectronic systems into the equipotential

bonding network

When using a star point arrangement (Figure 17), all metal components of theelectronic system must be suitably insulated against the equipotential bondingnetwork. A star-shaped arrangement is therefore usually limited to applications insmall, locally confined systems, such as IMS stations. In such cases, all lines mustenter the building or structure, or a room within the building or structure, at a singlepoint. The star point arrangement must be connected to the equipotential bondingnetwork at one single earthing reference point (ERP) only.


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2.2.4.5 Equipotential bonding of AC power service connection and data lines

Equipotential bonding of AC power service connection and data lines, as part ofthe internal lightning protection, represents an extension of the main equipotentialbonding. In addition to all conductive systems, this also integrates the supplyconductors (hot wires) of the low voltage consumers installation and data lines into

the equipotential bonding. The special feature of this equipotential bonding is the factthat a tie-up to the equipotential bonding is only possible via suitable surge protectivedevices (SPDs). Analogous to the equipotential bonding with metallic installations(see section 2.2.4.1), the equipotential bonding for AC power and data line shall alsobe carried out immediately at the point of entry into the structure.

If the step down transformer from medium to low voltage is located in the vicinity ofIMS installation the earthing of medium voltage and low voltage system shall beinterconnected, except it is in contradiction with local regulations.

2.2.5 Surge Protection

The protection of electrical and electronic systems in a structure against surgesresulting from the lightning electromagnetic pulse (LEMP) is based on the principle ofLightning Protection Zones (LPZ) [4]. According to this principle, the building orstructure to be protected must be divided into a number of internal lightningprotection zones according to the level of threat posed by LEMP (Figure 19). Thisallows categorizing areas of different LEMP risk levels and to adjust protectionmeasures to the immunity of the electronic system.

2.2.5.1 Lightning Protection Zones

Depending on the type of threat caused by the lightning, the following lightningprotection zonesare defined (see alsoFigure 19):

External

LPZ 0A: Zone is at risk from direct lightning strikes, from impulse currents up tothe whole lightning current and from the whole electromagnetic field ofthe flash of lightning.

LPZ 0B: Zone is protected against direct lightning strikes, but at risk from thewhole electromagnetic field of the flash of lightning. Internal systemscan be exposed to (partial) lightning currents.

Internal

LPZ 1: Impulse currents limited by the splitting of the current and by surgeprotective devices (SPDs) at the zones boundaries. Theelectromagnetic field of the lightning flash can be attenuated by spatialshielding.

LPZ 2, 3, . Impulse currents further limited by the splitting of the current andby surge protective devices (SPDs) at the zone boundaries. Theelectromagnetic field of the lightning flash is usually attenuated by

spatial shielding.


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Figure 19: Lightning protection zones concept according to IEC 62305-4 [4] (figureadapted from [8])

Generally, the lightning current arrester from LPZ 0 to LPZ 1 acts like a kind ofwave breaker and it conducts a large part of the interference energy away and thusprotects the installation in the building from damage. Additional surge protective

devices are installed at the LPZ boundary from LPZ 1 to LPZ 2 to ensure asufficiently low level of residual interference adjusted to the immunity of the terminaldevice.

2.2.5.2 Types of overvoltages

Overvoltages (surges) in low-voltage systems are caused by several types ofevents or mechanisms [11]:

(1) Ligh tning overvo ltages

Lightning overvoltages are the result of a direct flash to or near the power

system, structures (with or without lightning protection system) or to the soil.Distant lightning flashes can also induce overvoltages in the circuits of aninstallation.

Lightning is a natural and unavoidable event which affects low-voltage systems(power systems as well as signal/communication systems) through severalmechanisms. The obvious interaction is a flash to the power system, but othercoupling mechanisms can also produce overvoltages (seeFigure 20)


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S1: flashes to the structure

S2: flashes near the structure

S3: flashes to the services

connected to the structure

S4: flashes near the servicesconnected to the structure

Figure 20: Different lightning flash coupling mechanisms to a structure

For a given flash, the severity of the overvoltage appearing at the end-usersfacility reflects the characteristics of the coupling path such as distance andnature of the system between the point of flash and the end-users facility,earthing practices and earth connection impedance, presence of surge-protective devices (SPDs) along the path, and branching out of the distributionsystem. All these factors vary over a wide range according to the generalpractice of the utility as well as local configurations.

(2) Switchin g overvo ltages

Switching overvoltages are the result of intentional actions on the powersystem, such as load, inductor or capacitor switching in the transmission ordistribution systems by the utility, or in the low-voltage system by end-useroperations. They can also be the result of unintentional events such as powersystem faults and their elimination.

(3) Tempor ary overvoltages (TOV)

Temporary overvoltages occur in power systems, as the result of a wide rangeof system conditions, both normal operation and abnormal conditions. Theiroccurrence is relevant to the selection of suitable surge-protective devices.

TOV are power frequency overvoltages of relatively long duration (severalseconds) and may be caused by faults within the medium and low-voltagenetworks. The Temporary overvoltage specification of the SPD (UT) shall begreater than temporary overvoltage of the network. Otherwise the SPD will bedestroyed due to an overload when TOVs occur.

2.2.5.3 Expected surge currents due to lightning flashes

For direct lightning flashes to connected services, partitioning of the lightningcurrent in both directions of the service and the breakdown of insulation must betaken into account. As an example a 2 wire 0.5 mm2 cross section data cable will beunable to carry a 100 kA lightning current into a building. Insulation breakdown (cabledamage) will occur along the line.

Considering these limitations expected surge currents for power andtelecommunication/data lines are shown in Table 11.


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Table 11: Expected surge currents due to lightning flashes

2.2.5.4 Impulse withstand categories of installed equipment

So called overvoltage categories or impulse withstand categories I, II, III, and IVare specified in the standards for testing of equipment taking into account thecharacteristics of the system to which it is intended to be connected [16].

Equipment of overvoltage category IVis for use at the origin of the installation(e.g. electricity meters and primary over-current protection equipment).

Equipment of overvoltage catego ry IIIis equipment in fixed installations and

for cases where the reliability and the availability of the equipment is subject tospecial requirements (e.g. switches, distribution boards, electric wall mountedsockets in the fixed installation and equipment for industrial use withpermanent connection to the fixed installation).

Equipment of overvoltage category IIis energy-consuming equipment to besupplied from the fixed installation (e.g. appliances, portable tools and otherhousehold and similar loads. If such equipment is subjected to specialrequirements with regard to reliability and availability, overvoltage category IIIapplies).

Equipment of overvoltage category Iis equipment for connection to circuits inwhich measures are taken to limit transient overvoltages to an appropriately low level(e.g. PCs).Table 12 shows the rated impulse voltages for equipment energizeddirectly from the low-voltage mains. Thus installation of appropriate (and coordinated)SPDs shall limit the overvoltages to the values shown in Table 12. As an example, ina typical 230/400 V 3-phase network limitation of overvoltages to 1500 V is requiredfor Category I equipment such as PCs or other sensitive electronic devices.


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Table 12: Rated impulse voltage for equipment energized directly from the low-voltage mains [16]

Nominal voltage ofthe supply system

Voltage line to neutralderived from nominal

voltages a.c. or d.c. upto and including

(V)

Rated impulse voltage in (V)

Overvoltage category

Three phase

(V)

Single

phase(V) I II III IV

50 330 500 800 1500

100 500 800 1500 2500

120-240 150 800 1 500 2 500 4 000

230/400277/480

300 1 500 2 500 4 000 6 000

400/690 600 2 500 4 000 6 000 8 000

1 000 1 000 4 000 6 000 8 000 12 000

2.2.5.5 Surge Protection Devices

A Surge Protect ive Device(SPD) is a device that is intended to limit transientovervoltages and divert surge currents and it contains at least one non-linearcomponent.

2.2.5.6 SPD selection

The primary function of SPDs is to protect downstream terminal devices. Theyalso reduce the risk of cables from being damaged. The choice of SPDs depends,among other things, on the following considerations:

Lightning protection zones of the installation site (see 2.2.5.1)

Energies to be discharged

Arrangement of the protective devices

Immunity of the terminal devices

Protection against differential-mode and/or common-mode interferences

System requirements, e.g. transmission parameters

Compliance with product or user-specific standards, where required

Adaption to the environmental conditions / installation conditions.

2.2.5.7 SPD Technologies

SPDs are installed external to the equipment to be protected and to limitovervoltages to values below the test voltages of equipment in Table 12. Undernormal conditions, the SPD has no significant influence on the operationalcharacteristics of the systems to which it is applied. Under abnormal conditions(occurrence of a surge), the SPD responds to surges by lowering its impedance andthus diverting surge current through it to limit the voltage to its protective level. Uponreturn to normal conditions, the SPD recovers to a high impedance value after thesurge and a possible power follow current.


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A SPD can fail or be destroyed when surges are larger than its designedmaximum energy and discharge current capability. Failure modes of SPD are dividedroughly in open-c ircu i t mod eand short -c i rcu i t mod e.

For an open-c ircu i t mod ethe system to be protected is no longer protected.

In this case, a failure of the SPD is usually difficult to detect because it hasalmost no effect on the system. To ensure replacement of the failed SPD beforethe next surge, an indicating device of the SPD failure may be required.

For a short -c ircu i t mod ethe system is severely affected by the failed SPD.Short-circuit current flows through the failed SPD from the power source.Thermal energy can be produced there and can result in a fire hazard beforeburning out and open circuit. In case the system to be protected has no suitabledevice to disconnect the failed SPD from its circuit, a suitable, additionaldisconnecting device may be required for an SPD with short-circuit failuremode.

The main components of SPDs belong to two categories [13]:

(1) Voltage-limiting type, as varistors, avalanche or suppressor diodes, etc.These SPDs are sometimes called "clamping type". Figure 21 shows theresponse of a typical voltage-limiting SPD to an impulse applied via acombination wave generator.

Figure 21: Typical response of a voltage-limiting type SPD such as a varistor

(2) Voltage-switching type as air-gaps, gas discharge tubes, thyristors, etc.Figure 22 shows the response of a typical voltage-switching SPD to animpulse applied via a combination wave generator.

Figure 22: Typical response of a voltage-switching type SPD such as a spark-gap


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In practice often combination type SPDs, which incorporate both voltage-switchingtype components and voltage-limiting type components, are used.

2.2.5.8 AC power SPD classification (Type 1, 2 and 3)

SPDs employed as part of the fixed installation are classified according to the

requirements and loads on the installation sites as surge protective devices Type 1, 2and 3 and tested according to [12].

The highest requirements (class I test in [12]) with respect to the dischargecapacity are made on SPDs Type 1.The class I test is intended to simulatepartial conducted lightning current impulses. SPDs Type 1 are generallyrecommended for locations at points of high exposure, for example, lineentrances to buildings protected by lightning protection systems. Theseprotective devices must be capable of carrying partial lightning currents,waveform 10/350 s, many times without consequential damage to theequipment. SPDs Type 1 are called l ightn ing current arrestersand the

function of these protective devices is to prevent destructive partial lightningcurrents from penetrating the electrical installation of a structure.

SPDs tested to class II or III test methods are subjected to impulses of shorterduration. Type 2 SPDs are generally recommended for locations with lesserexposure to direct impulses. SPDs Type 2 are called surg e arrestersandemployed to protect against surges. Their discharge capacity is around some10 kA (8/20 s).

SPDs Type 3: The main function of a SPD Type 3 is to protect against surgesarising between L and N in the electrical system They may be installed in

supply networks where SDPs Type 1 and/or 2 already exist. They can beinstalled in fixed or mobile sockets.

Type 1 SPD Type 2 SPD Type 3 SPD

Figure 23: Examples of Type 1, Type 2, and Type 3 Surge Protection Devices

2.2.5.9 SPDs for information technology systems, RF receivers and GPS

In contrary to surge protective devices for AC power supply systems, whereuniform conditions can be expected with respect to voltage and frequency in230/400 V systems, the types of signals to be transmitted in information technology(IT), control and data networks differ with respect to their

Voltage (e.g. 0 10 V)

Current (e.g. 0 20 mA, 4 20 mA)


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Signal reference (balanced, unbalanced)

Frequency (DC, NF, HF)

Type of signal (analogue, digital).

Therefore, the signal must not be influenced intolerably by the use of lightningcurrent and surge arresters in measuring and control installations. Manufacturers ofSPDs offer a wide variety of surge protection elements with appropriatespecifications for many applications (Ethernet, Telephone, RS232, RS442, ISDN,etc.).

When selecting SPDs, the following aspects must be especially taken intoconsideration:

Protective effect (discharge capacity and protection level)

System parameters (system voltage, nominal current and transmission

parameters) Installation environment (design, conditions of connection and certifications)

Guidelines for the selection and testing of surge protective devices connected totelecommunications and signaling networks are given in [14], [15] and by themanufacturers specifications. IEC 61643-21 [14] specifies test procedures andparameters for IT system SPDs and they must be tested with at least one of thepulses listed in Table 13.

The maximum protection level of a given SPD arisen during these tests isindicated as voltage protection level Up measured at the output of the device.

Category C tests represent especially disturbing pulses with a steep rate of riseand less energy (surge arrestors), opposite to the disturbing pulses of category D,which is supposed to simulate high energy loads due to induced partial lightningcurrents (lightning current arrestors). The category is also indicated in the technicaldata sheet of the SPDs both in the description of the discharge capacity (In, Iimp)and protection level (Up).

Table 13: Voltage and current impulses for determining the voltage limiting characteristics of SPDs forIT systems adopted from [14]

Category

Type of

test

Impulse

voltage

Impulse

Current

Minimum

number ofimpulses

Test

for

C1

steeprate of

rise

0.5 kV or1 kV, 1.2/50 s

0.25 kA or0.5 kA, 8/20 s

300

surge

arrester

C22 kV, 4 kVor 10 kV,1.2/50 s

1 kA, 2 kAor 5 kA,8/20 s

10

C31 kV,

1 kV/ s10 A, 25 Aor 100 A,10/100 s

300

D1high

energy

1 kV0,5 kA, 1 kAor 2,5 kA,

10/350 s

2 *)

*) Lightning current arrester / Combined lightning current and surge arrester


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Immunity of IT terminal devices to be protected

Within the scope of the tests for electromagnetic compatibility (EMC), electricaland electronic equipment (devices) must have a predefined immunity against

conducted pulse interferences (surges). Requirements on the immunity and testconstructions are specified in EN 61000-4-5 [19].

Different electromagnetic environmental conditions make different demands on theimmunity of the devices. In EN 61000-4-5 test levels are subdivided into four differentstages. Test level 1 includes the minimum requirements on the immunity of theterminal device. Generally, the test level can be taken from the documentation aboutthe device or requested from its manufacturer.

Table 14: Test levels of devices according to EN 61000-4-5[19]

Test levels

according to EN 61000-4-5

Corresponds to charging voltage

of the test generator1 0.5 kV

2 1 kV

3 2 kV

4 4 kV

SPDs for use in IT systems need to limit conducted interferences to safe values toensure that the immunity of the terminal device is not exceeded.

Depending on the building structure and the protection requirements stipulated bythe Lightning Protection Zones Concept (see 2.2.5.1) it may be necessary to install

lightning current and surge arresters locally separated from each other or at onepoint of the installation.

Examples of different types of SPDs applicable for AC power, coaxial cables (e.g.GPS signals), Ethernet and low voltage signal lines (e.g. meteorological sensors) areshown in Figure 24.

Coaxial cable (e.g. GPS) Ethernet IT system

Figure 24: Typical SPDs for coaxial cables, Ethernet and IT data lines


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Protection of GPS receiver

The GPS receiver antenna shall be placed within the area protected by a lightningrod or a structure (LPZ 0B) to avoid any direct lightning strike to the antenna.

Appropriate SPD for coaxial lines shall be placed at the entry of the antenna cable

into the equipment vault or the building. Alternatively the SPD can be placed directlyat the GPS clock interface. Connection to the common ground of the GPS clock andthe SPD should be as short as possible.

Note: Check with the GPS clock manufacturer compatibility of the SPD with the GPSclock specifications to avoid unacceptable signal distortion.

Figure 25: Example of a SPD for coaxial cable with N-connectors

2.2.5.10 SPD for PV installations

Photovoltaic systems inevitably represent a connection to the electrical installationof a building. Today no final standard exists, specifying SPDs used to protect PVarrays, nevertheless SPD manufacturers offer appropriate SPDs. In a recent draftIEC document (IEC/82/514CD, 2008) the following specifications for surge arrestersto protect PV arrays from over-voltages caused by indirect lightning strikes arerequested:

a) maximum continuous operating voltage UC> 1,2 VOC STC,where VOC STC is the open circuit voltage of a PV module at Standard TestConditions

b) maximum discharge current Imax 5 kA

c) voltage protection level UC< Up < 1,1 kV

Some SPD manufacturers offer SPDs especially designed for the application in PVinstallations (e.g. DEHNguard, DG PV 500 SCP).

2.2.5.11 SPD Installation Requirements

It is highly recommended that the power and signaling networks enter thestructure to be protected close to each other and are bonded together at acommon bonding bar. This is especially important for structures made of non-shielding material (wood, bricks, concrete, etc.).


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Connecting conductors for SPDs and any external connectors connected inseries with the SPD shall be as short as possible. The residual voltagetransferred to equipment will be the sum of the residual of the SPD and theinductive voltage drop along the connecting leads as shown in Figure 26.

When the surge protective device in the conductor branch responds, thedischarge current flows through further elements (lead conductors, fuses)causing additional dynamic voltage drops Udyn1 and Udyn2, respectively,acrossthese impedances (Figure 26) and the protected equipment is exposed to thevoltage Utotal, given by

Utotal= Udyn1 + USPD + Udyn2

Figure 26: Connection of surge protective devices in cable branches [8]

For a high frequency event as a lightning flash, the resistive component isnegligible compared to the inductive component andUdynis determined by:

dt

dixLU

imp

dyn

where L is the inductance of the lead conductor. As a general rule, the leadinductance is assumed to be 1 H/m. This inductive voltage drop, when caused byan impulse with a rate of rise of 1 kA/s will be approximately 1 kV/m of lead length.Furthermore, if the steepness of di/dt is greater, this value will be increased. In order

to keep this dynamic voltage drop low, the inductance of the connecting cable andhence its length must be kept as low as possible (less then 0.5 m).

SPDs in the AC power installation

Due to short distance between entry point of the cables and the equipment in thevault combined lightning current and surge arresters (Type 1 +Type 2 SPD in asingle unit) are recommended when low impedance equipotential bonding from theprotective device to the terminal device can be assured (e.g. use shielded cable from

combined arrestor to the terminal device).


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When using separate Type 1 and Type 2 SPD in series, coordination of the SPDsis essential to ensure proper share of the surge energy among the involved SPDs.Manufacturers of SPD offer coordinated product lines and provide information aboutinstallation requirements (e.g. minimum cable length between Type 1 and Type 2unit). Coordination with the local power distribution network operator is required to

ensure installation of surge protection devices in line with local regulations andconditions (e.g. before or after the meter or different solutions are applicable in TN,TT or IT distribution network systems).

With respect to system earthing (TT, TN, IT) and position of Residual Current Device(RCD) the SPDs shall be installed as shown in Figure 27, Figure 28, and Figure 29,respectively [13], unless it contradicts local regulations. The main target is to ensureproper operation of the RCD even in case of a damaged SPD.

Figure 27: Installation of SPD in TN-system

Figure 28: Installation of SPD in TT-system

1 Origin of the installation2 Distribution board

3 Main earthing terminal or bar4 Surge protective devices5 Earthing connection of surge protective

devices, either location 5a or 5b6 Equipment to be protectedF Protective device indicated by the

manufacturer of the SPD (for example,fuse, circuit-breaker, RCD)

RA Earthing electrode (earthing resistance) ofthe installation

Rg Earthing electrode (earthing resistance) ofthe supply system

1 Origin of the installation2 Distribution board3 Main earthing terminal or bar4 Surge protective devices4a Surge protective device in accordance with

IEC 60364-5-53 (2.3.2) or spark gap5 Earthing connection of surge protective

devices, either location 5a or 5b6 Equipment to be protected7 Residual current protective device (RCD)F Protective device indicated by the

manufacturer of the SPD (for example,

fuse, circuit-breaker, RCD)RA Earthing electrode (earthing resistance) of

the installationRg Earthing electrode (earthing resistance) of

the supply system


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Figure 29: Installation of SPD in IT-system

SPDs in Telephone/Control/Data Network Circuits and RF installations

Lightning equipotential bonding requires that all metal conductive componentssuch as cable lines and shields at the entrance to the building shall be incorporatedinto the equipotential bonding to achieve impedances as low as possible. Examplesof such components include antenna lines, telecommunication and control lines withmetal conductors. The lines are connected with the help of elements capable ofcarrying lightning current (arresters and shielding terminals). A convenient installationsite is the point where cabling enters the building. Both the arresters and theshielding terminals must be chosen to be appropriate to the lightning currentparameters to be expected.

The individual cables must be integrated into the equipotential bonding as follows:

Unshielded cables must be connected by SPDs which are capable ofcarrying partial lightning currents (the expected partial lightning current perwire is the partial lightning current of the line divided by the number ofindividual wires).

If the cable shield is capable of carrying lightning currents, the lightningcurrent flows via the shield. However, capacitive/inductive interferences canreach the wires and make it necessary to use surge arresters.

o The shield at both ends must be connected to the mainequipotential bonding to be capable of carrying lightning currents

1 Origin of the installation2 Distribution board3 Main earthing terminal or bar4 Surge protective devices5 Earthing connection of surge protective

devices, either location 5a or 5b6 Equipment to be protected7 Residual current protective device (RCD)F Protective device indicated by the

manufacturer of the SPD (for example,fuse, circuit-breaker, RCD)

RA Earthing electrode (earthing resistance) ofthe installation

Rg Earthing electrode (earthing resistance) ofthe supply system


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Figure 30: Shield connection system capable of carrying lightning currents

Whenever possible all outside equipment (antennas, sensors, etc.) shall beinstalled in LPZ 0B.

When implementing measures to protect against disturbance variables fromnearby, distant and direct lightning strikes, it is recommended to apply a concept ofprotective devices with several protective stages. This reduces the high energyinterference (partial lightning current) in stages because an initial energy absorbingstage (LPZ 0/LPZ 1) prevents the main part of the interference from reaching thedownstream system. The subsequent stages (LPZ 1/LPZ 2) serve to reduce theinterference to values which the system can cope with.


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3 CTBTO/IMS Specific Part

This part is a complement to section 2 and describes specific requirements for theprotection of IMS field installations. The protection of the Central Recording Facility(CRF) and the remote elements is discussed, as well as some technology specificissues.

3.1 Classification of CTBTO/IMS stations

3.1.1 Classification in terms of lightning exposure

The CTBTO/IMS stations are installed at geographical locations with differentexposure to lightning. With respect to their location, the stations can be separated in

two classes, Class A and Class B.

Class A : Lightning Exposed

Lightning is not an unusual event in the region and therefore all protectionmeasures (SPDs, equipotential bonding, etc.) should consider direct lightningon site or to an entering service (e.g. use SPD Type I at the entry points ofservice lines and install a lightning protection system).

In terms of the Lightning Protection Level (LPL) defined and used inIEC 62305 a LPL II should be considered as a standard for Class A

CTBTO/IMS stations, since there is no risk of explosions and loss of humanlife involved in IMS stations.

Class B: None or very little lightning activity

Lightning is not expected in the region or is an extraordinary event. Extralightning protection is not necessary but typical measures for surge protectionand equipotential bonding as for Class A sites are recommended with the onlydifference that installation of SPDs Type 2 (see section 2.2.5.8) at the entrypoint of service lines is sufficient.

As for many sites reliable data of ground flash density or annual thunderstormdays are not available to the public, the classification should be based also oninformation from local authorities.

Class B should be used only, when the local authorities confirm the absence ofany lightning activity. The budget differences in the protection measures for Class Aand Class B stations are relatively small compared to the total value of installedequipment. The consequences of increased failure rate and costs for repair do hardlyjustify the acceptance of any lightning risk.

3.1.2 Classification in terms of Lightning Protection Zones

For a typical CTBTO/IMS site application of LPZ 0A, LPZ 0B, LPZ 1 seemssufficient with the following specifications:


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LPZ 0A: Exterior of the site

LPZ 0B: Outside areas protected against direct lightning strikes eitherby extra lightning rods or by IMS station construction elements (e.g.radio tower)

LPZ 1: Inside the equipment vault or the CRFAt the boundary LPZ 0A to LPZ 1 lightning current arrestors (Type 1 SPD) shall be

used whereas at the boundary LPZ 0B to LPZ 1 surge arrestors (Type 2 SPD) shallbe used.

When the expected lightning strike rate is very high, implementation of LPZ 2inside the CRF building is highly recommended.

3.2 Protection of the Central Recording Facility

CRF is usually located in a separate building or in a dedicated room of an existingbuilding. The design of the LPS for the CRF installation has to comply with standardIEC 62305, LPL II.

3.2.1 Air Termination system and down-conductors

Typical design of the air termination and down conductor system of a smallbuilding with two down conductors placed at diagonal corners is shown in Figure 31.

Figure 31: Typical external LPS of a small building

Alternatively to the system of horizontal air termination conductors shown in Figure31 protection can also be achieved by vertical lightning rods of a sufficient height toplace the building within the protected volume.

3.2.2 Earth termination system

Foundation earth electrodes shall be the preferred earth electrode system for

buildings. When professionally installed, electrodes are enclosed in concrete on allsides, and hence, corrosion resistant. Because of the large area of this type of


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electrode, low earthing resistance can be achieved. The hygroscopic characteristicsof concrete generally produce a sufficiently low earth electrode resistance.

The foundation earth electrode must be installed as a closed ring in the stripfoundation or the bedplate and thus also acts primarily as the equipotential bonding

(Figure 32, Figure 33). For larger buildings (exceeding dimensions of 20 m) a divisioninto meshes 20 m x 20 m is required. The terminal lugs to the outside required toconnect the down conductors of the external lightning protection system, and to theinside for equipotential bonding, must be considered.

Figure 32: Preparation of foundation earthelectrode (spaces ensure that electrodes are

enclosed on all sides by concrete)

Figure 33: Foundation earth electrode in use

Terminal lugs to the outside into the ground must have supplementary corrosionprotection at the outlet point. Suitable materials are, for example, plastic sheathedsteel wire high-alloy stainless steel, Material No. 1.4571, or fixed earthing terminals.

When a foundation earth electrode is not possible, a ring electrode shall beinstalled. When choosing the material of the earth electrode with regard to corrosion,the local conditions must be taken into consideration. It is advantageous to usestainless steel.

In case of CRF being located in a dedicated room of an existing building, whoseearth termination system does not comply with LPL II requirements, an upgrade of

the earth termination system according to LPL II is required.

3.2.3 Equipotential bonding system

Equipotential bonding shall be done according to section 2.2.4.

3.2.4 Surge protection

All incoming and outgoing cables must be protected at the entrance of the building(LPZ 0A,B / LPZ 1 boundary) by appropriate SPDs.

When LPZ 2 needs to be provided, the CRF equipment must be installed in a

metal rack which defines the volume of the LPZ 2. Appropriate SPDs shall beinstalled at all lines crossing this boundary


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Special attention should be paid also to equipment not belonging to IMS stationbut is interconnected through power or signal connections with the IMS stationequipment.

In case of a dedicated room, the room boundaries define the LPZ 1 volume and

appropriate SPDs shall be installed at all lines crossing this boundary.

3.3 Protection of the remote elements

The remote elements shall be protected from direct lightning and lightning inducedsurges based on the lightning protection zones concept. The main equipment shallbe located in the LPZ 1. Some units, located outside, will be situated in the LPZ 0B(antennas and radio modems) and no equipment shall be installed in the LPZ 0A.

The protection zones at remote elements are shown in Figure 34 andFigure 35.The boundary between LPZ 0A and LPZ 0B is defined by the protection zones,

calculated for the LPL II, provided by the communication towers, power transmissionline poles or installed lightning attractors. The LPZ 0B and LPZ 1 boundary is definedby the SPDs at the entrance to the equipment vault.

Figure 34: Example for LPZ for an infrasound remote element (Note: it is assumed in thisexample that trees provide certain protection against direct strikes to installations at ground

level)


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Figure 35: Example of LPZ for a seismic remote element

A schematic LPZ concept and according SPD installations for a remote element isshown in Figure 36. All lines entering the equipment vault are protected by SPDs andthe GPS antenna and meteorological sensors are placed in LPZ 0B.

Figure 36: Lightning protection concept of an IMS station (schematic)

3.3.1 Air termination system

All electronic equipment, including solar panels, GPS and RF antennas, andmeteorological stations, installed at remote elements shall be located within LPZ 0B.


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This zone LPZ 0B is provided either by existing installations (e.g. radio tower) or extrainstalled lightning protection rods. Protection angles are computed according toLPL II (see section 2.2.1).

Figure 37: Example of air-termination for RF antenna at an IMS station

3.3.2 Down-conductors

Down-conductors are not required, except in case of lightning air-terminationinstalled on non conductive structures (e.g. wooden or fiber glass masts). When suchnon conductive structures are used, the size and material of the down conductor

shall comply with requirements in section 2.2.2.

3.3.3 Earth-termination system

The earthing of the remote elements should be designed in order to provide (a)equipotential plane and (b) low resistance to ground.Figure 38and Figure 39 presenttwo examples for a recommended design of the earthing system for CTBTO/IMSarray elements. As the size, the distances and the relative position between thetower, the equipment vault and the fences may vary from site to site, the followingmain aspects should be considered:


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Figure 38: Example of schematic design of earth termination system for an IMS infrasoundarray element. A minimum of 4 earthing rods per structure (communication tower, equipment

vault, and solar panel array) shall be installed


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Figure 39: Example of schematic design of earth termination system for an IMS seismic array

element. A minimum of 4 earthing rods per structure (communication tower, equipment vault,and solar panel array) shall be installed


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Create one integrated earthing systems for the tower and equipment vault (about5 m x 5 m mesh)

Ring conductor (distance about 1 m) around the tower and the equipmentvault shall be used in order to provide control of the step voltage in the vicinityof the constructions

Vertical ground rods shall be used when they result in a significant and costeffective reduction of earthing resistance (e.g. when they reach the groundwater level or other good conduction soil layers).

Ensure low impedance connection between tower and vault and/or buildingparallel to the cables.

To minimize potential differences all metal parts (e.g. tower base, guy wires, GPS

and meteorological mast, solar panel frames, conductive fences, borehole casing,etc.) shall be connected to the earth termination system.

Material and dimension shall be selected according to section 2.2.3.1

3.3.4 Equipotential Bonding system

The equipotential bonding at the remote elements has to be done according tosection 2.2.4 by the installation of SPDs at all incoming and outgoing cables andproper connection of cable shields at the cable entry point, as well as equipmentchassis interconnection.

To reduce the potential difference between the tower and the vault the followingmay be applied:

Several parallel bonding conductors running in the same paths as theelectrical cables, or the cables enclosed in grid-like reinforced concreteducts (or continuously bonded metal conduit), which have been integratedinto both of the earth-termination systems.

Shielded cables with shields of adequate cross-section, and bonded to theseparate earthing systems at either end.

Cables shall be placed inside the tower structure whenever possible(reduces induction effects).


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Figure 40: Preferential cable routing along a tower of triangular cross profile

3.3.5 Surge Protection

The Surge Protective Devices (SPD) shall be installed according to section 2.2.5at the entrance of all incoming cables to the equipment vault (boundary of the LPZ 0Bto LPZ 1). Examples are shown in Figure 36 and Figure 41.

Figure 41: Example of SPDs installed at the entrance of

Documents

Earthing and Lightning Protection Guide