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LIGHTNING PROTECTION PRINCIPLES

Dr. Asanka RodrigoDepartment of Electrical Engineering, University of Moratuwa

Necessity of a Lightning Protection System The purpose of a lightning protection system is to

protect buildings and structures from lightning strikes and possible fire, or from the consequences of the load-independent active lightning current.

Buildings and Structures Substation

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Introduction to Protection Methods and Risks The science of lightning protection is

best attributed to Benjamin Franklin. The story of his kite flying experiment

The first mention of the traditional lightning rod was published by Franklin in 1750 in Gentleman’s Magasine [sic] and then later in his treatises on the subject published in 1751. In this he recommends the use of lightning

rods to “... Secure houses, etc, from lightning”

Lightning Protection System (LPS) The design of a lightning protection system

needs to: 1. Intercept lightning flash (i.e. create a

preferred point of strike) 2. Conduct the lightning current to earth3. Dissipate current into the earth 4. Create an equipotential bond to

prevent hazardous potential differences between LPS, structure and internal elements/ circuit

Lightning protection system must: Not cause thermal or mechanical damage to the structure Not cause sparking which may cause fire or explosion Limit step and touch voltages to control the risk of injury to occupants Limit damage to internal electrical and electronic systems

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Damage due to Lightning This is sub-divided into: Damage to a structure

including all incoming electrical overhead and buried lines connected to the structure

Damage to a service service in this instance being part of

telecommunication, data, power, water, gas and fuel distribution networks.

Damages:1. Sources of damage (S) and2. Types of damage (D) .

Sources of Damage S1 – Lightning flash to the structure S2 – Lightning flash near the structure S3 – Lightning flash to the services S4 – Lightning flash near to the services

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Types of DamageD1 – Injury of living beings (humans and animals) due

to touch and step potentialD2 – Physical damage (fire, explosion, mechanical

destruction, chemical release)D3 – Failure of internal electrical/electronic systems

due to lightning electromagnetic impulse

Types of LossL1 – Loss of human lifeL2 – Loss of essential service to the publicL3 – Loss of cultural heritageL4 – Economic loss (structure and its contents,

service and loss of activity)

Damage and Loss in Structures

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Lightning Protection Zone (LPZ)

Lightning Protection Zone (LPZ)

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External LPS Design Considerations An external LPS consists of:

1. Air termination system Air rods Catenary (or suspended) conductors Meshed conductor network

2. Down conductor system3. Earth termination system

Type A arrangement Type B arrangement Foundation earth electrodes

Position of Air Termination System The three basic methods recommended

for determining the position of the air termination systems:1. The rolling sphere method2. The protective angle method3. The mesh method

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(1) Rolling Sphere Method The Rolling Sphere method is a

simple means of identifying areas that need protection, taking into account the possibility of side strikes to the structures.

The distance of the last step is termed the striking distance and is determined by the amplitude of the lightning current.

This striking distance can be represented by a sphere with a radius equal to the striking distance.

Striking Distance (Last Step) It can be clearly seen that the corners are exposed to a

quarter of the circular path of the sphere. This means that if the last step falls within this part of the

circular path it will terminate on the corner of the building.

Corners of structures are vulnerable to lightning strikes

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Application of Rolling Sphere Method

Lightning Protection Levels In order to define lightning as a source of interference,

lightning protection levels I to IV are defined;

Max values: Design of products such as LP componentsMin values: Design LP systems (eg: radius of rolling sphere)

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Maximum Values of Rolling Sphere Radius There are different radii of the rolling sphere that

correspond to the relevant Class of LPS

(2) Protective Angle Method The protective angle method is a mathematical

simplification of the rolling sphere method

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Application of Protection using Angle Method Unlike the rolling sphere, the protective angle method is

not used to determine which parts of a structure require protection.

It is used to show the effectiveness of the designed protection system.

Determination of Protective Angle

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(3) Mesh Method A “meshed” air-termination system can be used universally

regardless of the height of the structure and shape of the roof.

This method is suitable where plain surfaces require protection if the following conditions are met: Air termination conductors must be

positioned at roof edges, on roof overhangs and on the ridges of roofs with a pitch in excess of 1 in 10 (5.7º)

No metal installation protrudes above the air termination system

LPS Design Values Maximum values of rolling sphere radius, mesh size and protective angle

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Air Termination System Air rods (or finials) Free standing masts or linked with conductors

to form a mesh on the roof.

Catenary (or suspended) conductors, Supported by free standing masts or linked

with conductors to form a mesh on the roof.

Meshed conductor network Lie in direct contact with the roof or be

suspended above it (in the event that it is of paramount importance that the roof is not exposed to a direct lightning discharge).

Types of External LPS Isolated Non-isolated

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Positioning of Air Termination Systems

Down Conductor System The lightning current is shared between the down

conductors The greater the number of down conductors, the lesser

the current that flows down each

Typical values of the distance between down conductors

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Natural Components It is encouraged to use of fortuitous metal

parts on or within the structure, to be incorporated into the LPS If the reinforcing bars are connected for

equipotential bonding or EMC purposes then wire lashing is deemed to be suitable.

Internal reinforcing bars are required to be connected to external down conductors or earthing network

If the reinforcing bars (or structural steel frames) are to be used as down conductors then electrical continuity should be ascertained from the air termination system to the earthing system.

Earth Termination System Effective Earthing System Providing a low impedance network to dissipate the fast-rising lightning

impulse Minimization of touch and step potential hazards

Three basic earth electrode arrangements are used. Type A arrangement Type B arrangement Foundation earth

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Touch and Step Potential

Intermeshed Earth-Termination System

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The New Standards IEC 62305:2006, BS EN 62305

Substation Lightning Protection Lightning protection should be carried for open terminal

substations to prevent the followings: Damage to Substation Equipment Loss of Power to Public

Equipment in a substation may be exposed to lightning in two ways. By voltage and current waves travelling along the exposed lines

leading to the station. By direct lightning strokes to the station.

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Lightning Protection Outdoor substations and switchyards are shielded against

direct lightning strokes by: i) Earth Wires (Shield Wires) ii) Masts iii) Earth wires and Masts

BOUNDARY

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Comparison of shielding according to various methods

Shielding wires

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Shielding Provided by Two Equal Height Masts