1

2

3

This manual is intended to provide guidance on the principles and practices of

lightning protection for a wide range of structures and systems.

In general, it is not economically possible to provide total protection against all

the possible damaging effects of lightning, but the recommendations in this

manual will reduce the probability of damage to a calculated acceptable level.

Will minimize any lightning damage that does occur.

The decision to provide lightning protection may be taken without carrying out

a risk assessment . Where doubt exists as to the need for lightning protection,

further advice should be sought from a lightning protection consultant.

First decision has to be whether lightning protection is needed or not.

Realization that it is possible to provide effective protection against lightning

began with Franklin and for over a hundred years international standards

have been developed to provide guidance on the principles and practice of

lightning protection. Until about ten years ago, risk assessment was used to

determine if there was a need to provide lightning protection

This selection takes into account both efficiency and cost of their provision.

In the risk management approach, the lightning threats that create risk are

identified, the frequencies of all risk events are estimated, the consequences of

the risk events are determined, and if these are above a tolerable level of risk,

protection measures are applied to reduce the risk (R) to tolerable level (Ra).

This involves a choice from a range of protection level efficiencies for

protection against direct (d) strikes to the structure and decisions about the

extent of other measures for protecting low-voltage and electronic equipment

against indirect (i) lightning stresses incident from nearby strikes. In short:-

Risk = Rx = Rd + Ri ( Total risk is the sum of the direct & indirect risks) Rx = Nx Px x Px = kx px

R Ra

4

where Nx is the frequency of dangerous events, Px is the probability of damage

or injury, x is the relative amount of damage or injury with any consequential effects, and kx is a reduction factor associated with the protection measure

adopted and equals 1 in the absence of protection measures when Px = px .

The lightning protection measures include an LPS for the structure and its

occupants, protection against the lightning electromagnetic pulse (LEMP)

caused by direct and nearby strikes, and transient protection (TP) of incoming

services. The LPS for the structure comprises an air terminal network to

intercept the lightning strike, a downconductor system to conduct the discharge

current safely to earth and an earth termination network to dissipate the current

into the earth. The LEMP protection includes a number of measures to

protect sensitive electronic equipment such as the use of a mesh of

downconductors to minimize the internal magnetic field, the selection of

lightning protection zones, equipotential bonding and earthing, and the

installation of SPDs. The TP for incoming services includes the use of isolation

devices, the shielding of cables and the installation and coordination of SPDs.

Air terminal

A vertical or horizontal conductor of an LPS, positioned so as to intercept a lightning discharge, which establishes a zone of protection.

Damage () Mean relative amount of loss to a specified type of damage due to a lightning event, when damage factors are not taken into account.

Earth impedance (Z) The electrical impedance of an earthing electrode or structure to earth, derived from the earth potential rise divided by the impulse current to earth causing that rise. It is a relatively complex function and depends on :

The resistance component (R) as measured by an earth tester.

The reactance component (X), depending on the circuit path to the general body of earth.

A modifying (reducing) time-related component depending on soil ionization caused by high current and fast rise times.

5

Earth potential rise (EPR) The increase in electrical potential of an earthing electrode, body of soil or earthed structure, with respect to distant earth, caused by the discharge of current to the general body of earth through the impedance of that earthing electrode or structure.

Finial It is a term referring to short vertical air terminals.

Lightning flash density (Ng) The number of lightning flashes occurring on or over unit area in unit time. This is commonly expressed as per square kilometre per year

(km2 year1). The ground flash density is the number of ground flashes per unit area and per unit time, preferably expressed as a long-term (>10 years) average value.

Protection level (I to IV) Four levels of lightning protection. For each protection level, a set of maximum and minimum lightning current parameters is fixed, together with the corresponding rolling sphere radius.

CONCEPT OF RISK In this Standard, risk R is the probable annual loss due to lightning. Expressed as a number, it represents the probability of loss occurring over the period of a year. Thus a risk of 10-3 represents a chance of 1 in 1000 of a loss occurring during a year.

6

DAMAGE DUE TO LIGHTNING

Sources of damage The current in the lightning discharge is the potential source of damage. In this Section, the following sources of damage, relating to the proximity of the lightning strike, are taken into account : S1direct strike to the structure. S2strike to the ground near the structure. S3direct strike to a conductive electrical service line. S4strike to ground near a conductive electrical service line.

Conductive electrical service lines include electricity supply service lines and telecommunications service lines. The greater the height and collection area, the more lightning strikes will influence the structure. Tall trees and surrounding buildings may shield a structure from lightning strikes.Incoming conductive electrical service lines add to the lightning collection area as they can conduct lightning current into the building. Direct strikes to structure or incoming conductive electrical lines may cause mechanical damage, injury to people / animals and may cause fire and/or explosion.Indirect strikes as well as direct strikes may cause failure of electrical and electronic equipment due to overvoltages resulting from coupling of the lightning current.

Risk components S1 Lightning strikes directly to the structure

These may generate:

o Component Rh due to step and touch voltages outside the structure around downconductors causing shock to human beings

o Component Rs due to mechanical and thermal effects of the lightning current or by dangerous sparking causing fire, explosion, mechanical and chemical effects inside the structure .

o Component Rw due to overvoltages on internal installations and incoming services causing failure of electrical / electronic systems.

7

S2 Lightning strikes to ground near the structure These may generate component Rm due to overvoltages on internal installations and equipment (mainly induced by the magnetic field associated with the lightning current) causing failure of electrical and electronic systems .

S3 Lightning strikes directly to conductive electrical service lines. These may generate:

o Component Rg due to touch overvoltages transmitted through incoming lines causing shock of living beings inside the structure .

o Component Rc due to mechanical / thermal effects including dangerous sparking between external installation and metallic parts (generally at the point of entry of the incoming line into the structure) causing fire, mechanical / chemical effects on the structure and/or its content .

o Component Re due to overvoltages, transmitted through incoming lines to structure, causing failure of electrical / electronic systems .

S4 Lightning strikes to ground near conductive electrical service line conductors. These may generate component Rl due to induced overvoltages, transmitted through incoming lines to the structure, causing failure of electrical and electronic systems.

PROCEDURE FOR RISK ASSESSMENT The procedure for the risk assessment requires:

(a) Identification or defining of the structure / facility to be protected.. In most

cases it is a stand-alone building. The structure may encompass a building and

its associated outbuildings .Under certain conditions, a facility that is a part of a

building may be considered as the structure for risk assessment purposes. An example might be a communications installation at the top of an office building.

This segregation of a part of a building is only valid under the following

conditions:

8

(i) There is no risk of explosion in the remainder of the building.

(ii) Suitable fire barriers exist around the structure being considered (fire rating

of not less than 120 min).

(iii) Overvoltage (SPD) protection is provided on all conductive electrical

service lines at point-of-entry to the structure being considered.

(b) Determination of all the relevant physical, environmental and service

installation factors applicable to the structure.

(c) Identification of all the types of loss relevant for the structure or facility.

Structures involved in the provision of public service utilities such as water, gas,

electricity and telecommunications.

(d) For each type of loss relevant to the structure, determine the relevant

damage factors x and special hazard factors.

(e) For each type of loss relevant to the structure, determine the maximum

tolerable risk,Ra.

(f) For each type of loss relevant to the structure, calculate the risk due to

lightning by

(i) identifying the components Rx that make up the risk (see Figure 2.1);

(ii) calculating the identified risk components Rx; and

(iii) calculating the total risk due to lightning, R. (g) Compare the total risk R with the tolerable value Ra for each type of loss relevant to the structure.

9

The selection of the most suitable protection measures shall be made by the

consultant according to the contribution of each risk component to the total risk,

and according to the technical and economic aspects of the different protection

measures available. Technical considerations include addressing the highest risk

components while economic considerations involve minimizing the total cost to

achieve a suitable level of protection.

P E R S O N A L S A F E T Y The 30/30 safety guideline. An approaching thunderstorm is treated as dangerous when the time interval between seeing a lightning flash and hearing the thunder is less than 30 s . A receding local thunderstorm is no longer a threat when more than 30 min have elapsed after the last thunder is heard. Dont shelter under trees, particularly an isolated tree. If surrounded by trees, seek position outside the foliage , crouch, keeping feet together.

If on a boat deck, keep a low profile and avoid contacts with or being close to masts, rails, or other metallic objects. Avoid unnecessary contacts with communication or navigation equipment. Do not enter the water, and in general avoid contact with it. Additional protection may be gained by anchoring under high objects :jetties and bridges, provided that direct contact is not made with them. Isolated buoys and pylons should be avoided. Do not take a bath / shower and do not wash hands or dishes. Do not use personal computers and other electronic / electrical equipment, and avoid contacts with sinks, refrigerators, metallic pipes and other large metallic objects in the house. Disconnect television sets, personal computers, and other electronic / electrical appliances from antennas, conductive telecommunication connections and electricity supply outlets to avoid damage to them. Switching off an appliance does not disconnect neutral and earth wiring. Isolation / unplugging is the safest.

10

P R O T E C T I O N O F S T R U C T U R E S Following are recommendations for installation practices and selection of equipment to prevent damage caused by lightning discharge. The recommendations apply generally to the protection of structures using LPSs comprising air terminals, downconductors, equipotential bonding and earth terminations. If, after completing the LPS risk assessment, it is evident that surge protection is required to protect internal systems within the building and services at entry , an investment shall not be ignored.

LPS DESIGN RULES It is obvious that air terminals must be installed providing conductive paths for the lightning current from the air terminals to the earth . The downconductors should assist in preventing side-flashes to nearby metal elements. This is best done by locating downconductors immediately below the air terminals used to protect the most vulnerable parts. Rules for air terminals

(a) First, provide air terminals to protect the most vulnerable parts (points and corners);second, use the RSM method to check if the less vulnerable parts(edges) are protected and, if not, add more terminals to protect them; third, also check if the least vulnerable parts (such as flat surfaces) are protected and, if not, add more terminals.

(b) Air terminals shall be placed close to the most vulnerable parts; if a strip conductor is used, it shall be directly on the part it is to protect; if a vertical rod is used, its length shall be not less than 50cm, and it shall preferably be mounted within 1 m or 1/2 its length . Maximum allowable length of a rod terminal is 6 m. (c) If the structure has horizontal or gently sloping upper parts that are essentially cylindrical or oval in shape, then the edges are the vulnerable parts and shall be protected by air terminals; if a strip conductor is used, it shall be run along the edge(s); if vertical rods are used, there shall be a minimum of two evenly spaced terminals.

11

Rules for downconductors (a) Main conductors shall interconnect all air terminals and shall form one or more paths to earth via downconductors, such that the spacing between the downconductors does not exceed 20 m. (b) A downconductor shall be connected directly below an air terminal used to protect most vulnerable parts.If air terminal is on an exposed roof corner, its downconductor will also act as a continuation of air terminal to protect vertical edge below it, as required for tall structures.

Rules for earth terminations (a) Low earth resistance is desirable and all practical measures should be taken to achieve 10 or less for the whole interconnected LPS earth termination network.There shall be equipotential bonding at ground level for all metallic surfaces. (b) There shall be one earth termination per downconductor.

ZONES OF PROTECTION FOR LIGHTING INTERCEPTION

The protection efficiency against direct lightning strikes is achieved by installing an LPS in such a way that its air terminals establish zones of protection enclosing the whole structure. For the calculation of these zones of protection, the RSM,with a modification for large flat surfaces, is used. The RSM generally ensures that for lightning striking distances determined by the radius of the rolling sphere, the shortest distance between a lightning leader tip and any part of the structure is an air terminal.This method of analysis is suitable for conventional lightning terminals, which may be vertical rods, horizontal wires or strip conductors, railings, metal sheets, fascias and so on.

12

In the rolling sphere technique of determining zones of protection, a sphere of specified radius (a) is theoretically brought up to and rolled over the total structure. All sections of the structure that the sphere touches are considered to be exposed to direct lightning strokes and would need to be protected by air terminals. In general, air terminals need to be installed so that the sphere only touches their interception surfaces. This is illustrated below in next page, showing that the top corner/edge of the structure requires protection by an air terminal but the sides and lower section do not. The values of the rolling sphere radius (a) for the four protection levels (PL) I, II, III, IV are given in Table 4.2 together with the corresp.min. lightning current that will be intercepted.

13

ZONE OF PROTECTION ON A STRUCTURE BY A ROLLING

SPHERE

It is common to consider that PL III using a sphere of radius a, 45 m provides standardprotection. PL I and II with a, 20 and 30 m provide higher degrees of protection and should be used if required by the risk management calculations. Conversely, PL IV with a, 60 m provides a lower degree of protection. For PL III, the protection ensures that, for striking distances of 45 m or more, the shortest distance to the structure is to an air terminal. From table in previous page, such striking distances correspond to peak currents of 10 kA , and interception effic of 91%, there being only of the order of 9% of strikes having a lower current. In the RSM, lightning is considered most likely to follow the path of shortest distance. This path will have the highest average electric field produced by the potential difference between the tip of the lightning leader and the structure .

14

Buildings without structural steel frames The required conditions of protection for non-metallic buildings are generally

met by placing metal air terminals on the uppermost parts of the building or its

projections, with conductors connecting the air terminals to each other and to

earth. By this means a relatively small amount of metal properly positioned and

distributed can afford a satisfactory degree of protection and, if desired, the

material may be placed so as to give minimum interference to the appearance of

the building. A typical LPS is shown in

15

Reinforced concrete buildings The following recommendations apply to the use of steel reinforcement in

reinforced concrete buildings as part of the LPS. As far as possible, the steel reinforcement should be made electrically continuous in all concrete elements

having a structural purpose, e.g. columns, beams and also in non-structural

concrete elements, e.g. concrete wall panels, where the element, or a part of it, if

dislodged, could endanger persons below.

Where the steel reinforcement is used as the downconductor system, an

effective electrical connection should be made from the air terminal network to

the steel reinforcement at the top of the building. Such connections should be

made, by means such as welding or clamping to a minimum of four vertical

and/or horizontal bars, to ensure a multiplicity of conductive paths for the

discharge of lightning current.

NOTE: Steel reinforcement that is overlapped and tied by means of wire is not considered to provide an effective electrical connection for the purpose of air termination connection.

MATTER CONSIDERED WHEN PLANNING PROTECTION Design considerations

(a) Metal used in the roof, walls, framework or reinforcement above. .

NOTE: For a non-metallic roof, the position of any conduit, piping, water mains or other earthed metal immediately beneath the roof should be noted, as this may inadvertently attract a discharge if not shielded by an adjacent roof or structure, or downconductor . (b) Available positions for downconductors providing the required number of

low impedance paths from the air terminal network to the earth termination.

(c) The resistivity of the soil to design a suitable earth termination.

(d) Services entering the structure above ground.

(e) Radio and television antennas and microwave communications antennae.

(f) Flag masts, roof plant rooms:- lift rooms, boiler rooms, and water tanks .

16

(g) The provision of bonding connections to steel frame, reinforcement rods or

internal metalwork to allow for the free passage of the lightning conductor.

(h) The choice of metal most suitable for the conductor, e.g. aluminium

conductors for structures where aluminium is employed externally.

(i) Accessibility of test joints; protection by non-metallic casing from

mechanical damage and hazard ; lowering of flagmasts / tall chimneys.

(j) Preparation of a drawing detailing and showing positions of the main

components to form the lightning protection system .

Route for conductors Conductors should be installed with a view offering the least impedance to the

passage of discharge current between the air terminals and earth. The

impedance to earth is approximately inversely proportional to the

number of widely separated paths, so that from each air terminal there should be

as many paths to earth as practicable. The number of paths is increased and the

impedance decreased by connecting the conductors to form a cage enclosing the

building.

Economy of installation Economy of installation can be effected by taking advantage of constructional

features already installed as far as practicable.

Corrosion Corrosion resulting from contact of dissimilar metals can exist where a

conductor is held by fixing devices on or against external metal surfaces of a

building or structure.

Bonding connections between air terminal and down conductor should be sealed

against the ingress of moisture.

17

18

The design of the earth termination network should assume that each earthing

electrode will be bonded, directly to the following : the electrical installation

earthing system , the building structural steelwork , any water, sewer and fire

system supply pipes ( if metallic ) and pipelines for gaseous or liquid fuels( if

metallic ).

There is a hazard arising from the bonding of other service earthing electrodes.

The electricity supply service has many loads connected to it that

generate a direct current component; this direct current is an electrolytic hazard

to other earthing systems to which the electricity supply service earth is bonded.

The amount of direct current is limited but it is still sufficient to place at risk

some types of earthing electrodes. In particular, steel rods clad with copper or

stainless steel suffer premature failure when a small amount of direct

current such as this perforates the cladding, initiating a process of self-

destruction of the rod core.

It will be clear that the selection of any common metal or alloy for the earth

termination network places either itself or other systems or services at some risk

from galvanic corrosion.For lower cost installations ,the use of one of the

common metals or alloys may be satisfactory. The extent to which the material

combination can be damaging is related to soil moisture. Soil resistivity

typically below 30 . Expert advice on the selection of an appropriate earth termination network should normally be where such soil conditions exist.

19

FORM AND SIZE OF DOWN CONDUCTORS Factors influencing selection The form and size of the down conductors of the LPS should be selected having

regard to their minimum cross-sectional area required of a main current carrying component of a LPS is 35 mm2.

The dimensions of typical conductors are given below in table.

Electrical and thermal considerations

Air terminals and downconductors that may carry the full lightning current,

should have an adequate cross-sectional area and conductivity such that

they are able to carry without attaining temperatures that may give rise to risk of

fire. Copper conductors having a cross-sectional area of not less than 35 mm2 will normally be necessary for this purpose.

20

Conductors, which because of their arrangement in the LPS, will carry only a

proportion of the lightning current, may have a cross-sectional area that is

proportionately reduced but should be not less than 1/5 th of the cross-sectional

area to carry the full lightning current, or 6 mm2, whichever is the greater.

JOINTS Effectiveness of joints The LPS should have as few joints as possible. Joints and bonds should be

mechanically and electrically effective, e.g. clamped, screwed, bolted, crimped,

riveted or welded. All mechanical connections should be inspected on a regular

basis in accordance to ensure integrity of the connection over time. Particular

attention should be given to joints of dissimilar metals.

Fasteners. Conductors should be securely attached to the building or other object upon

which they are placed. Fasteners should be substantial in construction and not

subject to breakage, and should be, together with the nails, screws, or other

means by which they are fixed, of the same material as the conductors, or of

such nature that there will be no serious tendency towards galvanic corrosion in

the presence of moisture because of contact between the different parts.

Downconductors should be fastened at spacings not exceeding 1.0 m on

horizontal runs and not exceeding 1.5 m on vertical runs.

Plastics may be used for the fixing of conductors provided such materials are

suitable long-term exposure to the outdoor environment against the harmful

effects of ultraviolet radiation .

AIR TERMINALS May consist of a vertical rod , a single horizontal conductor as on the ridge of a

small dwelling, or a network of horizontal conductors with vertical rods for the

protection of roofs of large horizontal dimensions .

Protection may also be provided with a horizontal overhead wire supported, if

necessary, independently of the building to be protected or by a vertical air

21

terminal network .The upper portions of the downconductors on tall buildings

should be regarded as a continuation of the air terminal network and should be

positioned so as to intercept side strikes to the building. Preference should be

given to placing downconductors as near as possible to the exposed outer

vertical corners of a building.

All metallic projections, on or above the main surface of the roof, should be

bonded to, and form part of, the air terminal network. In the case of

telecommunications antennas, which have to be insulated from earth, a spark

gap connection to earth or an SPD should be provided.

Where roof construction consists of electrically continuous metallic materials,

such metallic roofs may form part of an LPS, obviating the need for air

terminals. If portions of a structure vary considerably in height, any necessary

vertical air terminal or air terminal network of the lower portions should, in

addition to their own downconductors,be bonded to the downconductors of the

taller portions

Air terminals may be of any form provided the section used and the means of

attaching it to the building structure have adequate mechanical strength to

withstand the expected winds.

Protection of roofs

The parts of roofs most likely to be struck by lightning are corners and edges of

flat roofs, chimneys, and the ridges and eaves of sloping roofs.

Preference should be given to positioning the air terminals so as to protect these

highly exposed parts.The height of a vertical air terminal should be such that the

tip will be not less than 50 cm above the object to be protected. On large flat

and gently sloping roof areas , a number of vertical rods of greater than 50 cm

in height may be needed to establish a zone of protection over the whole roof .

Horizontal conductors such as strap on metallic objects such as flagpoles, metal

railings, steel plant surrounds and roof access ladders may be used as air

terminals to protect a planar roof surface. When positioned at a height of more

than 50 cm above the area to be protected , the conductors or objects will

be at a suitable height to achieve the selected interception efficiency.

22

All metallic objects at roof level such as sheeting, plant, plant screens, tanks,

gutters,walkways, ladders, antennas, masts, poles, vents, chimneys, conduits,

piping, cable trays, ..etc should be bonded to the air termination network.

DOWNCONDUCTORS Structures Downconductors should be installed at each external corner of the building and

additional downconductors , as necessary, at spacings not exceeding 20 m.

Any extended metal running vertically through the structure should be bonded

to the lightning downconductor,.

A structure on bare rock, to be with at least 2 downconductors equally spaced.

EARTH TERMINATIONS The design of earth terminations should be such that lightning currents

are discharged into the earth in a manner that will minimize step and touch

potentials and also side-flashing . This can be achieved by ensuring that the

potential with respect to earth at each earth termination is limited by enough

low resistance to earth, so that the discharged current flows in as close to

uniform manner as possible in all directions away from the structure.

Ionization of the soil near an earthing electrode carrying lightning current tends

to reduce the potential of the earthing electrode relative to remote earth to a

lower value than the potential that would be calculated using the earth resistance

measured at low currents.

Recommended values for earth resistance. In general, the whole of an interconnected LPS should have an earthing

resistance not exceeding 10 before any bonding is effected to services that are not part of the LPS. Where the installation has two or more air terminal

networks not directly interconnected,such as a twin-tower building, then for the

purpose of determining the required earthing resistance, it should be considered

as consisting of separate LPSs.

23

Where buildings are used for telecommunications services or sensitive

electronic equipment, an earthing resistance not exceeding 5 should be required.A reduction of earthing resistance can be achieved by extending or

adding to the earth termination network or by interconnecting the individual

earth terminations of downconductors.

Notwithstanding the above recommendations, earthing electrodes complying

with either of the following, need not comply with the 10 criterion:

(a) For a substantial structure effectively encircled by a buried earthing

electrode, an earthing resistance not exceeding 30 should be satisfactory. A buried earthing electrode covering at least three sides of

the structure may be regarded as effectively encircling the structure.

(b) For any system incorporating two or more downconductors, it should not

be necessary to install a total length of more than 50 m of widely separated

horizontal or vertical earthing electrodes per downconductor, regardless of the

earthing resistance.

Common earthing electrode Where conditions permit potential equalization techniques to be used, a

common earthing electrode may be installed to serve the LPS and other

appropriate services. The earthing electrode should comply with the

recommendations in standards and with any regulations that may govern the

appropriate services .

Where isolation is required, a common earthing electrode should not be used,

but the separate earthing electrodes should be bonded via an SPD to minimize

potential differences between the LPS earth termination network and other

earthing systems in the event of a lightning strike.

Communications earths Where a communications earth, such as a Telecommunications Functional

Earthing Electrode (TFEE), is required to be isolated from other earths, because

of noise or direct current conduction considerations, this earth should be bonded

through a normally nonconducting protector or SPD.

24

Joints Joints between earthing conductors and earthing electrodes should be of

adequate strength and current-carrying capacity, and be arranged so as to ensure

that there will be no failure of the connection under conditions of use or

exposure that can reasonably be expected.

GENERAL CONSIDERATIONS FOR PROTECTION

Each building will require specific consideration of the protective measures that

should be applied. Particular attention should be given to possible entry and exit

points for lightning current, which may include one or more of the following:

(a) Rooftop or external structures : TV antennas, dishes, metals, gutters , pipes ,

metal windows, frames, and ventilation outlets not protected by the LPS for the

building structure . These features will invariably be possible entry points for a

lightning discharge.

(b) The electricity supply service entry .This will normally be an entry point for

lightning if the service is aerial or overhead.

(c) The telecommunications services entry .This may be an entry point if the

service is overhead using a dropwire or aerial cable. The service is more

commonly underground and in such cases could be either an entry or exit point.

(d) Gas supply systems These are usually exit points for lightning .

(e) Metallic water supply . This is usually exit points for lightning.

(f) The LPS for the building (if provided) By design these systems provide both

an entry and exit point for a lightning discharge but, because of bonding, will

present an EPR condition to other services.

25

An illustration of possible entry and exit points for a lightning discharge.

26

Installation of equipotential bonding Consequently, bonding conductors should not be grouped with other

cables that are sensitive to induction unless the other cables are also bonded to

the LPS. If the bonding conductor is long (some tens of metres) it shall be

considered as an impulse transmission line, in which mode the protection

afforded by the bonding will be limited.

(a) Rooftop antennae and dishes. The bonding conductor should be attached to the most substantial part of the structural metal supporting the

equipment consistent with it fulfilling the requirements of an air terminal for the

LPS of a building. The bonding conductor to the antenna or communications

hardware should be insulated , if run within the building, but may be

uninsulated if run externally. The cross-sectional area of the bonding

conductor should be not less than 16 mm2 if made of copper.

(b) The electricity supply service entry .There are two distinct considerations

that apply.Firstly, the electrical installation earth should be bonded to the LPS

earth termination network with a copper conductor of not less than 6 mm2

cross-sectional area.

Secondly, SPDs should be installed for each active conductor of the electricity

supply service. Where an SPD is mounted on, or in, the building, its earthing

system should be bonded to the LPS by a conductor having a cross-sectional

area of not less than that utilized for its own earthing conductor. Where SPD

equipment is separated from the building (e.g. mounted on a customers electricity supply service pole), the SPD earth should not be used as the earthing

termination for the building LPS, however, the LPS earth termination network

and the SPD earth may be bonded together, if desired.

(c) The telecommunications service entry . The service should be regarded as a

potential entry point for lightning and an SPD should be fitted, subject to the

requirements of the telecommunications regulatory authority. The telecoms

service earthing system shall be bonded to the LPS earth termination network.

The bonding conductor should have a cross-sectional area of not less than 6

mm2 if made of copper.

(d) Metallic water supply should be bonded to the LPS and connected to the

electricity supply service earth. Metallic piping systems associated with fire

sprinklers, water, hot water or flammable liquid, that are unavoidably in contact

with the exposed conductive parts of wiring enclosures, cable components or

other electrical equipment shall be connected to such equipment by means of an

equipotential bonding conductor.

27

(e) Building earthing systems frequently have several earthing systems that

may be installed independently at different times : Electricity supply service

earthing system, telecoms earthing system some times more than one, the LPS

earth termination network and other special purpose earthing systems. It is

generally desirable to bond all such earthing systems but there may be specific

reasons for not doing so.

(f) The LPS earth termination network .Where an LPS is in place all of the

services earth should be bonded to the LPS earth termination network.

PROTECTION OF EQUIPMENT Lightning induces overvoltages in electrical lines, telecom lines, signalling,

data, and coaxial lines.

Equipment overvoltages may be experienced in the following ways:

(a) By direct conduction of lightning current on the conductors feeding into the

building. Ex. would be lightning striking overhead lines. This mechanism is of

lower probability, but involves higher surge currents.

(b) Indirectly, (through magnetic induction, or electrostatic coupling) where

lightning strikes nearby, and surges are induced on conductors feeding building.

This mechanism has higher probability, but results in lower surge currents.

(c) Lightning striking the LPS or other nearby objects, resulting in an EPR. This

can cause potential differences in earthing systems, causing flashover and

equipmentdamage.

(d) Temporary overvoltages at mains a.c. system frequency that can occur for a

number of reasons. Strategies to deal with involve equipotential bonding of the

earthing systems, and the provision of SPDs.

Equipotential bonding for equipment protection Voltage differences that are insufficient to cause injury to persons can be

extremely damaging to equipment. It is possible to have voltage drops in

bonding conductors that are carrying lightning surges in excess of 1kV per

metre. It is important that bonding conductors be kept short to reduce this

voltage difference, and to achieve this, all services should enter in close

28

proximity. For protection of equipment, this concept can be extended to

particular areas within the building. For example, consider

the case of a multistorey building with incoming underground services, and a

telephone system installed on an upper floor. On the lower level, the required

equipotential bonding will be performed, and primary surge protection to both

power and telecom lines can be fitted at that location, and will connect to the

same equipotential earth system.

On this lower level, it may well be that the bonding conductor lengths are not

ideal, depending on where the services enter, and thus the protection provided is

compromised.However, on upper floor where the telephone system is installed,

the same concept can be repeated, but at this point more control is possible over

wiring and equipment locations. That is, at this location all the services should

enter the room at the same point, and secondary surge protection to both power

and telecommunications lines can be provided at this location with short, direct

connections to the common earthing point.

Surge protective devices (SPDs) An SPD is a device intended to mitigate surge overvoltages . That is, it behaves

in a benign state until a voltage or current exceeds a designed value, then the

SPD acts to reduce the voltage or current, in order to prevent damage to

equipment in protection.SPDs often have features, such as mechanisms to

indicate their operational status,normally indicated in colour.

VARISTORS. These are made from metal oxide and are known as metal oxide varistors

(MOVs) or Voltage Dependent resistors (VDRs). The resistance of varistors

drops significantly when the voltage exceeds a limit thus clamping the voltage

near the limit. It normally respond in nanoseconds.

SOLID STATE DEVICES. .

Consists of special zener diodes that exhibit voltage limiting characteristics and

are optimized to handle surge currents. The breakdown voltages of such devices

are typically in the range 5 V to 200 V. They have current ratings up to several

hundred amperes and response times of the order of 10 picoseconds. Another

form consists of thyristors, that switch when their operation voltage is exceeded,

29

and act to clamp overvoltages. Their reduced voltage during conduction means

they can handle higher surge currents, compared with the zener diode type.

SPD configuration SPDs are configured as being either shunt or series protectors :

(a) Shunt protector .

Known as a one-port SPD, connected in shunt with the circuit to be protected,

as shown in Figure 5.3(a). A one-port device may have separate input and

output terminals without a specific series impedance between these terminals, as

shown in Figure 5.3(b). The latter arrangement is sometimes known as a series

connected, shunt protector.

A shunt protector with just two terminals (i.e. does not have separate input and

output terminals) has no limitation with regard to the load current of the circuit

to which it is applied. However, its ability to clamp overvoltages is reduced by

the additional voltage drop that occurs across its connecting leads. For this

reason, some shunt protectors are manufactured with separate input and output

terminals. This arrangement substantially reduces the connecting lead voltage

drop problem, but does mean that the full load current passes through the

device, which needs to be designed to handle this current.

FIGURE 5.3 EXAMPLES OF SHUNT (ONE-PORT) PROTECTORS

(b) Series protector known as a two-port SPD, is an SPD with two sets of

30

terminals, input and output. A specific series impedance is inserted between

these terminals. Typical examples are shown in Figure 5.4.

The series current limitation of the series protector will typically be determined

by the series impedance. Sometimes a protector is referred to as an n-stage

protector, and although this term is not applied, the n should refer to the number of shunt overvoltage stages that are separated by series.

Multi-service surge protection device (MSPD) An MSPD is a combination protector that combines both power protection and

signalling/telecommunications protection in one . Is an effective way of

protecting IT and sensitive equipment that has more than one connected service.

By including all the protection in the one device, the distance between the SPD

earth connections is very short, which greatly reduces the potential difference

between these services under incident surge conditions. A general diagram of an

MSPD is shown in Figure 5.5.

31

Parameters of an SPD. The primary parameters relate to how well an SPD limits overvoltages, how

much surge current it handles, and what voltage system is it designed for. The

following parameters are listed together with the IEC symbols for these

parameters.

Maximum continuous operating voltage (Uc)This is the max voltage that can be continuously applied to protector. For the power system, this should be at

least 275 V for SPDs connected between the phase and neutral conductors.

Rated load current (IL)Max continuous rated r.m.s. or d.c. current that can be supplied to a load connected to the protected output of an SPD.

Maximum surge current (Imax)This is the peak value of the 8/20 s waveshape current impulse that the protector can handle. The protector only has

to be able to withstand this surge current once. Known as the single shot rating.

Nominal surge current (In)This is the peak value of the 8/20 s waveshape current impulse that protector can handle many times. A protector must be able

to withstand at least 15 impulses at In.

Voltage protection level (Up)This is the peak voltage that the protector protects to (limits the voltage to). It is sometimes referred to as the let-through, ,

voltage.

Some SPDs include shunt capacitance that provides a filtering effect at

EMI/RFI frequencies, but provides little benefit at typical lightning surge

frequencies. Similarly, surge current levels may cause inductor saturation in

standard EMI/RFI filters, which will degrade the filter action.

Temporary overvoltage (TOV)

It is when the power voltage on an a.c power system rises above its normal

value. This can be caused by many factors including poor regulation,

faults on the LV or HV distribution system (including phase shorts to neutral or

earth, and loss of neutral conductor), capacitor switching, HV contact on

LV circuits.

These events typically last from 0.2 s up to 5 s. And considered a surge.

32

Application of SPDs The following aspects should be considered :

(a) Modes of protection . With any signal or electrical transmission system employing two lines

and a separate protection earth, two types of transients can occur. The first

type appears as a difference between the two lines, independent of their

potential differences to earth; this is known as a differential mode transient ,

where the transient voltage source is superimposed onto the normal signal

carried by the lines.

The second type appears as a transient between each line and the earth,

and is known as a common mode transient , where the transient voltage sources

are superimposed onto the normal potentials between the lines and earth.

The use of two non-earthed lines is common. Telephone lines use two wires

over which the signal is transmitted, with neither line tied to earth. RS-422

signalling for computer data uses two lines for each data channel, which is

known as balanced-pair signalling.When protective equipment is connected to

such lines, both differential and common mode transients must be suppressed.

Placing a protective device across the two signalling lines alone is not

sufficient. The high potentials to earth created by common mode transients can

cause insulation breakdown and arc-over, and damage electronic components.

Protection against transients can be achieved by the provision of voltage

clamping or diversion devices between the lines, and between the

lines and earth. These will shunt common mode transients to earth before they

are allowed to reach breakdown potentials.

Equipment to be protected is typically more robust to transients from line to

earth . Experience has shown that mains equipment is more easily damaged

from line to neutral transients, and although protection could be provided in all

modes , good protection is usually obtained by providing LN and NE protection modes only.

33

SPD location.

There are many possible locations where SPDs could be installed within a

facility. The aim is to provide effective protection to the equipment nominated

for protection, and to do so economically. The most effective is to provide SPDs

at the building service point-of-entry (known as primary protection), and then, if

necessary, to provide additional surge protection within the building closer to

the equipment to be protected (known as secondary protection).

Primary protection is important because the main function of such an SPD is to

keep most of the surge current from entering the building, by diverting it

directly to earth.

When considering mains power circuit protection, the concept of location

categories can be used . Building point-of-entry is where primary protection

would be installed. Further within the building are for secondary protection, to

be fitted as required, especially in the following situations:

(i) Where sensitive equipment is present; cctvs , Pcs, telecoms equip,etc.

(ii) Where the distance between the SPD located at the entrance and the

equipment to be protected is too long

(iii) Where there is internal equipment generating switching surges, or other

internal interference sources, inside the building.

Surge ratings

A lightning surge, travelling within a building is attenuated by the SPDs. Thus

higher levels of surge current are likely to be encountered at the building point-

of entry, compared to the distant end of a branch circuit;

The lightning surge current to be handled by a point-of-entry SPD has

traditionally been considered to come into the building via the service

conductors. If lightning strikes the building LPS, or even the ground or an

object nearby, a local EPR occurs. The incoming service conductors are

typically referenced to a distant earth (such as the neutral conductor grounded at

the secondary transformer some distance down the street, with the phase

conductor also being referenced to that distant earth by virtue of the transformer

winding).

34

The effect of the local EPR is that a proportion of the lightning current flows

out through the point-of-entry SPDs on its way to reaching the distant earth. The

surge current in the SPDs in this case is very large, being a significant

proportion of the lightning current itself.

While Table below gives a surge rating for SPDs in this case (Category C3)

using the 8/20 s waveshape, it should be acknowledged that the IEC standards make reference to a 10/350 s waveshape for use in this case, and the symbol given to the current rating using this waveshape is Iimp.

For example, an SPD withstanding a 100 kA 8/20 s impulse might be expected to withstand a 10 kA 10/350 s impulse. Given this discussion, for mains power system SPDs, the following surge current ratings are recommended, where the

surge rating is the Imax, or single shot, 8/20 s value, and apply for each SPD from the phase to neutral conductors.

Coordination

Often the approach taken is to have the primary SPD handle the bulk

surge current. A secondary protector that will not need to handle such a high

value of surge current,can be installed close to the equipment and can be chosen

to have an acceptable Up value.

However, to achieve this result, careful coordination between the two devices

needs to be undertaken. This is quite a complex matter, and a total examination

of the issues is beyond the scope of this Standard. However, simple rule is to

ensure there is at least 10 to 20 m of electrical cabling between two . If

this cannot be achieved, purpose built inductors are available that can be placed

in the circuit to achieve this effective separation.

35

Wiring considerations. It is essential to provide a fuse or circuit breaker ahead of the SPD to provide

for the safe disconnection of a failed SPD. SPDs should be installed after the

main switch but prior to any RCD.

If the terminals of the SPD are rated for the required load current level, a

configuration as shown in Figure below (B) is preferred.

This wiring method is known as Kelvin connection . Failing this, twisting the

connecting conductors together as shown in above ( C ) can have a substantial

impact on reducing the voltage drop.

Failure modes of SPDs Typically an SPD initially fails to a low impedance state, and the resulting

current that then flows into the SPD either causes the SPDs internal fusing to

operate, or causes external fusing/protection to operate. Consideration needs to

be given to the most desirable location for the external fuse, and whether it is

desirable for power to be disconnected from the load when the SPD fails. If the

external fuse/protection is in series with the load current, power will be

disconnected from the load when the SPD fails. In some applications this is

considered beneficial, since the SPD is no longer protecting the load. However,

locating the fuse in the non-load carrying SPD connection wiring, means that a

ruptured fuse will isolate the SPD, but allow power to continue to the load.

SPDs are fitted with a visual indication to show their operational status, and

may additionally be fitted with contacts to allow for remote monitoring.

36

Withstand voltage of equipment.

The equipment to be protected may have a level of resistability to surges

designed into it. In particular, a resistance level to electromagnetic interference

(EMI) disturbances is mandated. Ideally, the Up of the externally provided SPDs should be lower than the Up of any equipment internal surge protection components, otherwise the internal components may be damaged instead of

being protected by the external SPDs.

Lightning surges need not physically damage equipment for it to experience

problems. Erratic operation, that may or may not require manual resetting, can

occur and lead to data loss at surge levels which are lower than those required to

cause hardware failure.

Rapid changes in the voltage supply, even those for which the

amplitude does not exceed the normal mains power peak, can cause problems.

This dv/dt problem can be reduced by utilizing SPDs with filters.

Magnetic shielding and line routing. Magnetic shielding reduces electromagnetic fields, and can also provide a

reduction in the emissions from electrical noise. The complexity of shielding

can range from the use of metallic conduits, to simple metal enclosures or

cabinets, and up to whole rooms being comprised of shielding

materials. Such shields need to be earthed to be effective, and any SPDs

provided to conductors entering the shielded area need to be effectively

connected to the shield.

The amount of surge energy directly induced into building conductors from

nearby lightning strikes depends on the closeness of the current source, and the

loop area formed by the conductors. Conductors of the same service should be

run together, along with an earthing conductor, or otherwise run in close

proximity to other earthed components, such as earthed cable trays. To reduce

the inductive loop area, such cables should be neatly tied together and not

allowed to splay out over the whole tray width.

37

Practical installation examples.

Determine protection needs. Simple methodology for determining where SPDs are required. Method asks

what requires protection , and then drawing an imaginary box around it. Each

location where electrical lines cross the box is a potential location for SPDs, and

they should be provided where the particular electrical line is prone to having

surges on it. Then the earths of the SPDs are connected together to the earths of

the equipment, and taken to earth.

Typically it will be necessary to consider the following points for SPDs:

(a) At point-of-entry of external services e.g. electricity supply and telecoms.

(b) At the connection of the external services to the equipment.

(b) At the connection of long internal cabling to the equipment e.g. communications and LAN. The two latter mentioned can damage

equipment:

(i) An excessive voltage/current enters the building via a service due to either a

lack of protection or incorrectly installed protection. Both the mains and

telecomms point of entry SPDs earths are bonded to the main earth bar by conductors of 1.5 m or less where SPDs are installed.

(ii) An excessive voltage/current is induced into the internal wiring loop.

The procedure for protecting equipment is as follows:

(a) Install secondary protection at equipment when risk of damage due to

induction into the external service conductors and the building conductors

exceeds an acceptable level.

(b) Install point-of-entry protection when the risk of damage due to a direct

strike to the structure or service conductors exceeds an acceptable level. A

prime role of the point-of-entry protection, apart from preventing dangerous

discharges is to protect the secondary protection from damage.

38

Protection examples.

Example 1.

A central PLC and remote sensor . It is determined that a particular industrial process must be protected. It consists

of a central controller (PLC), and various sensors and controls. For simplicity,

the example will show two sensors, one at a considerable distance from the

PLC, and another close to the PLC. The arrangement is shown in Fig. below.

39

EXAMPLE OF CENTRAL PLC AND REMOTE SENSOR.

The PLC has a console and modem to allow communication, and consequently

has electrical and telephone services. The signal line is over 1 km in length.

Since the operator console is close to the PLC there is no reasonable subjecting

of surges impinging on that connection. Likewise, any sensors or controls close

to the PLC would not normally require protection.

SPDs should be installed close to equipment they are to protect, and must have

their earths connected together, and connected to the PLC earth, and taken to

earth. The PLC and modem are mounted next to each on a rack. Just below ,

them, the required SPDs are mounted in a row, with an earthing busbar

immediately below them, connected at each end to the rack frame. Each SPD

will have a short direct connection to the busbar not exceeding 10 cm, and the

PLC will also have a direct connection to the busbar. In this manner, effective

equipotential bonding will be achieved .

The SPD on the signal line at the PLC end does not provide protection for the

remote sensor. The only line here is the signal line, and so an SPD is provided

on it. This SPD is earthed with a direct short connection to the sensor earth (and

associated pipe ), and taken to a local earth. There is no point in trying to

connect this very distant earth to the PLC earth via a bonding conductor.

In addition to secondary SPDs installed closed to the equipment, considerations

as whether it is run in metallic conduit or not, it may be prudent to include

primary point-of-entry protection on the PLC signal lines where they enter the

building, in addition to the secondary protection shown at the PLC.

Example 2.

A video surveillance system .

This example is similar in eg 1. The DVR will have surge protection applied to

the long camera feeds and to the a.c. power line. The video feeds that are

selected for protection will need all their signal lines protected, and this may

include the video feed, the power supply leads, and any pan and tilt control

signals.

The earths of these SPDs will connect to each other and to the equipment earth.

Video cameras that are located nearby in the same building may not need surge

protection fitted. Those video cameras that are located a long distance from the

central monitoring and recording equipment will need the same protection fitted

at the remote end as was fitted at the central end.

40

The earths of these SPDs must be connected to the video camera earth, and then

be taken to a local earth. Although all earth connections should be as short as

possible, it is particularly important to keep the length of the SPD earths to

video camera earth short.

Example 3

A multistorey building with PABX on upper floor .

his is a multistorey building with services entering on grnd floor, and PABX on

an upper flr. Regardless of whether the point-of-entry location of the telecoms

and power services are co-located , there can be induction into the internal

wiring between the grnd floor and PABX. Therefore the electricity supply, the

exchange line and the outdoor extensions will require protection at the PABX.

The local handsets do not need protection . To ensure adequate protection of the

PABX it is necessary to have a DB and a telecoms distributor (IDF) co-located

with the PABX. SPDs are installed in the DB and the IDF. Where direct strike

protection is required SPDs need to be installed at the point-of-entry .

41

Example 4

A domestic computer and ADSL modem . The simplest way to provide SPDs with short bonding conductors to a common

earth point is to use an MSPD so this has been used, along with a power board

to provide additional protected outlets. A Fax machine at the same physical

location has also been protected using a second MSPD.

42

Example 5

A rooftop cellular base station. In this example a cellular base station is located on the roof of a multistorey

building.Fig belowshows an effective means of providing lightning, earthing

and surge protection.

43

The a.c. power supply is fed to the base station dist.board from a main switch

board located in the basement of the building. An SPD (diverter )would be

installed at the MSB. A second SPD (surge filter) is installed on the a.c. power

feed to the rooftop distribution board.

All other metallic services, for example antenna feeders should be bonded and

fitted with SPDs. All equipment should be referenced to a common earth

bonding bar in the cabin and this in turn bonded to the tower and building LPS.

There will be a connection to the main a.c. earth via the earth conductor in the

power cable to the roof.

Good practice suggests that all metallic services should enter the cabin on the

same side and the common earth to the tower and LPS should exit the same

side.

It should be noted that in the event of a lightning strike to the tower, conducted

currents will flow through the power earth conductor, and care should be taken

to segregate cables if possible.Antennas installed on the tower that contain

electronics may require additional protection measures.

P R O T E C T I O N O F M I S C E L L A N E O U S

S T R U C T U R E S A N D P R O P E R T Y.

STRUCTURES WITH ANTENNAS Indoor antenna system.

This may be indoor radio and television receiving antennas .

Outdoor antennas on protected structures

This may be outdoor radio and television receiving antennas without

further precautions, provided that every part of the antenna system, including

any supporting metalwork, is within the zone of protection of the LPS. Where

these conditions cannot be fulfilled, precautions should be taken to ensure that

the lightning current can be discharged to earth without damage to the structure

or injury to its occupants with an antenna system fitted : -

44

(a) Directly onto a protected structure. This can be done by connecting the

antenna bracket structure to the LPS at the nearest point accessible below the

antenna installation; or

(b) On a metallic support structure that projects above the LPS. This can be

accomplished by connecting the antenna support structure to the LPS at the

nearest point accessible below the antenna installation. Consideration should be

given to the fitting of SPDs in the conductors connected to the antenna system.

Antennas on unprotected structures.

Before installing an antenna on an unprotected structure, the need to provide an

LPS should be assessed . The earthing electrode of the LPS may also be used

for the purpose of earthing a radio system.

STRUCTURES NEAR TREES

When a tree is struck by lightning, a voltage drop develops along its branches,

trunk and roots. The side-flash clearances between the tree and adjacent

structures are set by taking 100 kV/m as the flashover strength of unseasoned

wet timber and 500 kV/m as the breakdown strength of air.

If tree is less than the height of the structure , its presence can be disregarded. If

taller , the following clearances between the structure and the tree may be

considered as safe:

(a) For normal structures; one-third of the height of the structure.

(b) For structures with explosive or highly-flammable contents; the height of

the

structure.If the clearances cannot be met then the structure should be fitted with

lightning protection in such a manner that the side-flash always terminates on

the protection system.

If the tree is fitted with LPS, no further protection is necessary for the structure ,

provided that conditions for zone of protection and separation are fulfilled.

PROTECTION OF TREES.

Protection of trees needs to be considered only where the tree is desired for

historical or other reasons.

45

A main downconductor should be run from the topmost part of tree to the

earth termination . Large upper branches should be provided with branch

conductors bonded to the main conductor. Allowance should be made for

swaying in the wind and the natural growth of the tree.

The earth termination should consist of two rods driven into the ground on

opposite sides of, and close to, the trunk of the tree. A strip conductor should be

buried to a depth of 3mtrs to encircle the roots of the tree at a minimum distance

of 8 m radius from the centre of the tree or at a distance equal to 1 m beyond the

spread of the foliage, whichever is the greater. This conductor should also be

bonded to the rods by two radial conductors.

Where two or more trees are so close together that their encircling earth

conductors would intersect, one conductor adequately connected to the earth

rods should be buried so as to surround the roots of all the trees.

The recommended is to protect the roots of the tree and to reduce the potential

gradient, in the event of a lightning discharge to the tree.

CHIMNEYS, METAL GUY-WIRES OR WIRE ROPES

Metal guy-wires or wire ropes attached to steel anchor rods set in earth may be

considered as sufficiently earthed. Other guy-wires or wire

ropes should be earthed . Metal chimneys or flues need no protection against

lightning other than be properly earthed.

Metal ladders .

Where chimneys have a metal ladder , they should be connected to the LPS at

their upper and lower ends.

Chimneys Chimneys consisting partly reinforced concrete should be electrically

connected together and electrically connected to the downconductors

at the top and bottom of the concrete.

46

PROTECTION OF BOATS

A boat is at risk both because of its method of construction (except

for metal-hulled boats) and because it forms a marked protrusion above the

surrounding water surfaces. Overseas statistics show that in excess of 10 percent

of fatalities occurring on cruising sailing boats are due to lightning.

While the principles to be applied will not differ from those for land-based

structures, the methods employed will depend on the form of construction and

the type of boat to be protected.

Elements of the protection system

Air terminal A metal mast or the metal fitting on a timber mast will act as an adequate air

terminal. The mast, if metallic will both act as downconductors and each should

be connected to an earth termination.

Stays as small as 3 mm diameter steel wire will serve as effective

downconductors, but may be damaged under severe lightning discharges.

Earthing

Any metal surface that is normally submerged in the water will provide

adequate earthing. Propellers, metal rudder surfaces and metal keels may be

used. The earth plate for the radio transmitter may also be used, providing that it

is constructed of solid material and not of the porous type. A metal or a ferro-

cement hull also constitutes an adequate earth.

Metallic objects Metallic objects that are permanent parts of the boat and whose function would

not seriously be affected by earthing should be made part of the LPS by

interconnection with it. The purpose of interconnecting the metal parts of a boat

with a downconductor is to prevent side-flashes to metal objects .

A general rule is, that if the non-conducting part of the alternative path through

such objectis less than one-eighth of the length of downconductor bridged out,

then that object shouldbe electrically interconnected with the downconductor.

47

Radio transceivers

A whip antenna consisting of a fine wire embedded in a glass fibre tube cannot

be considered a satisfactory lightning conductor and should be folded down

during a lightning

storm.

All radio equipment or other navigational equipment with exposed transducers

such as radar, wind speed/direction indicators, and the like, should be fitted with

effectively-earthed spark gaps or SPDs. Alternatively, input cabling should be

disconnected from the equipment if there appears to be imminent danger of the

boat being struck by lightning.

Corrosion

Care should be taken that the design of the LPS does not promote the

occurrence of electrolytic or galvanic corrosion. Bonding of dissimilar metals

and interconnection of the earth terminals of different pieces of electrical

equipment should not be undertaken without expert knowledge .

Protection of boats with masts

Sailing or power boats that have a mast or masts of sufficient height must

consider if they give an adequate zone of protection . They may be protected by

earthing the lower ends of the standing rigging and the base of a metallic mast,

or the lower end of a continuous metal sail track on a timber mast.

Where the mast of a boat is stepped on deck, particular care should be taken to

ensure that the conductor from the base of the mast follows a direct route if it

passes through the accommodation section of the boat.

A typical small sailing boat with aluminium mast stepped on deck, glass fibre

hull with the metal ballast encapsulated in the glass fibre and with chainplates

moulded into the hull provides something of a problem.In such cases, it is

suggested that some protection be sought when necessary by temporarily

connecting the mast and stays together at deck level by a length of chain or

other flexible conductor and allowing a short length of chain or the conductor to

hang in the water at each chainplate.

48

Protection of boats without masts

Boats without masts do not constitute as high a risk as boats with masts.

However, where the size of the boat is such as to cause a marked protrusion

above the surrounding water surfaces, should be fitted with air terminals that

will give at least the protection for land-based structures.

Precautions for persons and maintenance suggestions

During a lightning storm, persons should remain inside a closed boat and avoid

contact with metallic items such as gear levers . Persons should stay as far as

practicable from any parts of the standing rigging or other items forming part of

a downconductor. No person should dangle arms or legs in the water.

If a boat has been struck by lightning, compasses and navigation instruments

should be checked for calibration. Protective coatings on steel hulls and glass

fibre sheathing over ballast keels should also be checked for damage. All

standing and running rigging and associated fittings should be checked in detail.

Bonding the LPS to the vessels electrical wiring system earth.

The bonding of LPS on a boat should recognize that the electrical

wiring system on a boat is commonly only a final subcircuit. As such, the

wiring will be very light, and neither the live conductors (whether or not

energized) nor the earthing arrangements, are capable of carrying lightning

discharge current. Even with a larger vessel, where the wiring is for a submain

or a complete installation with a generator, this will often still be the case,

though larger wire sizes would be in use.

The ship installation may have common fed accessories off the isolation

transformer secondary, and all systems may incorporate RCD protection. One

side of the isolation transformer secondary may be a pseudo neutral with ship earthing.

When the LPS is completed, the earthing conductor at its final point connection

to its chosen earth termination network should be bonded to the ship earth or

ship bonding point at its termination.

49

FENCES If an extended length of metal fence is struck it is raised to a high potential

relative to earth. Persons or livestock in close proximity to, or in contact with,

may therefore be exposed to danger.Fences that give rise to the most risk are

those constructed with posts of poor conducting material, such as wood or

concrete. Fences built with metal posts set in earth are less hazardous, especially

if the electrical continuity is broken. Breaking the electrical continuity prevents

a lightning stroke from affecting the entire length of a fence, as it can if the

stroke is direct and the fence continuous, even though earthed.

In addition, persons or livestock can be endangered by potential differences in

the ground in the proximity of fences . The risk is greatest on rocky ground.

No value can be given for the earth termination resistance, since this must be

largely governed by the physical conditions encountered, but the lower the

resistance to earth the less risk will result to persons and livestock. It should be

borne in mind that because of large body spans and bare contact areas many

types of livestock are more susceptible to electric shock than humans.

50

MISCELLANEOUS STRUCTURES

Metal scaffolding , including overbridges. Where metal scaffolding is readily accessible to the general public, particularly

when it is erected over and on part of the common highway or may be used in

the construction of public seating accommodation, it should be efficiently

bonded to earth.

A simple method of bonding such structures consists of running a strip of metal

other than aluminium, 20 mm 3 mm size, underneath and in contact with the base plates carrying the vertical members of

the scaffolding and earthing it at intervals not exceeding 20 m. With public

seating accommodation only the peripheral members of the structure need

bonding to earth. Other such as those used for pedestrian bridges over main

trunk roads, are frequently sited in isolated situations where they may be prone

to lightning strikes and should be bonded to earth, at the approach points.

Tall metal masts, towers ,and cranes.

Masts and their guy-wires, floodlighting towers and other metallic construction,

particularly those to which the general public have access, should be earthed.

Cranes and other tall lifting appliances used for building construction purposes,

shipyards and port installations should also be bonded to earth. For cranes or

revolving structures mounted on rails, efficient earthing of the rails, preferably

at more than one point, will usually provide adequate lightning protection.

In special cases, where concern is felt regarding possible damage by lightning to

bearings, additional measures may be justified.Mobile towers, portable cranes

and similar structures mounted on vehicles with pneumatic

tyres can be given a limited degree of protection against lightning damage by

drag chains or tyres of conducting rubber such as are provided for dissipating

static electricity.

51

PROTECTION OF HOUSES AND SMALL BUILDINGS

Houses / small buildings vary greatly in the degree to which their construction

provides inherent lightning protection. Small buildings with mainly non-

metallic materials offer little or no protection against lightning, whereas a

building with a metallic roof, metallic gutters, and metallic downpipes leading

into the ground has a high degree , since the main elements of an LPS are

already present.

If lightning strikes a house with little lightning protection, the lightning is

likely to penetrate the roof and attach to electrical wiring in the roof area. This

will usually result in damage to electrical equipment in the house, and in some

cases, may result in a fire, or in hazard to persons within the house.

The objective in protecting small buildings should be to provide conductors to

intercept the lightning, to provide a low-resistance path to earth, and to provide

at least two earth earthing electrodes for taking lightning current to the earth.

Air terminal network for the building If building roof consists mainly of metallic materials, then it will serve as the air

terminal network. It is necessary to ensure that there is electrical continuity

between the various parts of the roof.

If building roof consists mainly of non-metallic materials, then an air terminal

network should be provided. Copper wire and copper strip are recommended for

their durability. At least one conductor should be run along the highest parts of

the roof, eg. the highest ridge of the building. If the roof has a complicated

shape, it may be necessary to run additional conductors along the highest parts

of each section of the roof. All conductors should be joined together.

The cross-sectional area of the conductors should be not less than 35 mm2,

achieved, for example, by copper strip 25 mm 1.5 mm. However, it should be realized that much thinner conductors are able to carry most lightning

currents without damage. Even if the conductor were to melt, it would have

carried out its function for that one strike, as the lightning current would flow

through the path of the molten metal, rather than penetrate below the roof of the

house.

For a large, flat roof of non-conducting material, the simplest form of air

52

terminal network may be a series of vertical metallic rods above the roof level,

all connected together. Metallic gutters may become a strike attachment point.

If there are metallic gutters around the roof, these should be connected to the air

terminal network. With metallic roofs, these connections may already exist in

the fastenings of the guttering to the roof. With non-metallic roofs, the guttering

should be connected to the air terminal network at no less than two points.

Provision of downconductors for the building There should be at least two low-resistance paths to convey the current from

any lightning strike to the roof down to earth. Metallic downpipes from metallic

gutters may be used for this purpose, provided they afford a direct electrically

continuous path for the lightning current. In the absence of any low-resistance

path from roof to earth, at least two conductors should be provided to serve as

downconductors. These may be continuations of the conductors forming part of

the air terminal network.

Provision of earthing electrodes

A path to earth for the lightning current should be provided at no less than two

well separated points, Eg. at opposite ends of the house. Preference should be

given to areas that are usually damp, such as gardens. A metallic water pipe

buried in the ground would be a satisfactory earthing electrode provided that the

water pipe is also connected to the electricity supply service earth.

Each downconductor should be connected to an earthing electrode by the

shortest possible route, with the proviso that downconductors and earthing

electrodes should not be placed close to entry doors, or places where persons are

likely to stay for long periods. Eg. close to swimming pools.

Earthing electrodes and their connected conductors should be examined

periodically to ensure that they are intact, and not suffering corrosion or

mechanical damage.

53

I N S T A L L A T I O N & M A I N T E N A N C E P R A C T I C E.

WORK ON SITE

Throughout the period of erection of a structure, all large and prominent masses

of metalwork,Eg. steel frames, scaffolding and cranes, should be effectively

connected to earth. Once work has commenced on the installation of an LPS, an

earth connection should be maintained at all times.

INSPECTION

All LPSs should be inspected after completion, or alteration . A routine

inspection should be made at least every two years. More frequent inspections

may be warranted in some circumstances. Such circumstances include, but are

not limited to

(a) Areas subject to severe weather and lightning activity.

(b) Structures located in areas where aggressive soil or other conditions may

accelerate corrosion or other aspects of system degradation.

(c) Changes in technology use within the structure that may necessitate a review

of the protection means and their continued effectiveness.

(d) Any other time where it is deemed necessary to update the original risk

assessment for lightning damage.

TESTING On the completion of installation or any modification to it, and at time of any

maintenance inspection, the resistance to earth of whole installation and of each

earth termination should be measured, and the electrical continuity of all

conductors, bonds and joints and their mechanical condition verified.

Where regular testing during maintenance reveals that the earthing resistance is

substantially unchanged, the frequency of maintenance testing may be reduced

to each alternate inspection.

54

RECORDS

The following records should be kept on site, and by the person responsible for

the upkeep of the installation:

(a) Scale drawings showing the nature, dimensions and position of all

component parts of the LPS.

(b) The nature of the soil and any special earthing arrangements.

(c) Date and particulars of salting, if used.

(d) Test conditions, date and results .

(e) Alterations, additions or repairs to the system.

(f) The name and contact details of the persons responsible for the installation

or for its upkeep.

Detection of occurrence of flashes to structure and magnitude of discharge

current may be estimated by magnetic links, or magnetic tape .

While the use of instruments to count the number of strikes intercepted by

the protection system is highly recommended, in practice, this may be

impractical to achieve on multiple downconductor LPSs.

MAINTENANCE

Some system components will lose their effectiveness over time because of

weathering, corrosion, and stroke damage. Both physical and electrical

characteristics of the LPS must be maintained .

The periodic inspection and tests described in report of maintenance,shall

indicate works done. Particular attention should be paid to any evidence of

corrosion of earthing and to any alterations or extensions to the structure that

may affect the LPS. Examples of such alterations or extensions are as follows:

(a) Changes in the use of a building.

(b) The erection of radio and television antennas.

(c) Installation / alteration to electrical, telecoms or It within the building.

A good maintenance program should also contain provision for the following:

(i) Inspection of all system components.

(ii) Tightening of all accessible clamps and splices.

(iii) Measurement of system resistance, including earth resistance of terminals.

(iv) Inspection or testing of SPDs.

55

EXAMPLE OF LIGHTNING RISK CALCULATIONS : Two storey house.

56

NATURE OF LIGHTNING & PRINCIPLES OF PROTECTION

THE NATURE OF LIGHTNING

Thunderstorms occur under particular meteorological conditions, and partial

separation of electrical charges within the thundercloud usually results in

regions with negative charge mainly in the lower parts of the thundercloud, and

regions with positive charge in the upper.

A complete ground flash consists of a sequence of one or more high amplitude

short duration current impulses. About 40% of ground flashes

have more than one ground termination, usually separated by distances up to a

few kilometres . The currents are unidirectional and usually negative, i.e. a

negative charge is injected into the object struck. For all practical purposes the

stroke can be considered to be generated by a current source whose waveshape

and magnitude are unaffected by the characteristics of the ground termination.

The lightning attachment process The first stroke of a ground flash is normally preceded by a downward-

progressing low-current discharge that commences in the negatively charged

region of cloud, progresses