Industrial and Commercial Power SystemsTopic 8

LIGHTNING PROTECTION

The University of New South Wales

School of Electrical Engineeringand Telecommunications

People

Buildings and Contents

Aim is to protect:

against damaging effects of lightning strikes.

Lightning is very common event. Worldwide, some 30 lightning flashes occur in every second on average.

Frequency of occurrence of lightnings and thurderstorms varies significantly with location.

Severity of lightning storms also varies with location.

Local topographical features may cause variations in occurrence of ground flashes.

Tall objects (building rooftop, tree top, overhead lines) tend to attract lightning flashes to themselves and thus shielding surrounding area from direct strikes.

Distribution of worldwide lightning strikes (flashes/km2/yr)[Source: NSSTC]

2

PHYSICS OF LIGHTNING

Storms

Collision among ice crystalsand water droplets

Charge separation

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Lightning = sudden discharge of electricity between differently charged regions.

Cloud flash

Ground flash, less common.Downward leader (stepped leader).Upward leaderReturn stroke

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Lightning protection systems (LPS) are designed to ensure that lightning terminates on an air terminal (lightning rod) instead of on some other parts of building.

Ground flash consists of a sequence of high-amplitude short-duration current impulses (strokes).

Currents are uni-directional, and usually negative(negative charge injected into struck object).

Stroke considered as generated from a current source, i.e. current waveshape and magnitude not affected by characteristics of ground termination.

Characteristics of ground flashes[Table B1, AS/NZS1768:2007]

Potentials during a lightning flash to earthed conductor.

Modes of entry of lightning impulses [Fig 5.1, AS1768:2007]

Electrical.

Thermal

Mechanical

Principal effects of lightning discharge to object:

Direct strikes to person causing heart failure, brain damage, suspension of breathing, burns, etc.

Asphyxiation or injury due to fires or structural damage

Side flashes

Electric shock from step, touch, or transferred voltages

Cause death or serious injury in various ways :

3

ELEMENTS OFA LIGHTNING PROTECTION SYSTEM

Air terminals.

Down conductors

Earth termination network

Equi-potential bonding

Over-voltage protection

Protection system components:

4

STANDARDS ONLIGHTNING PROTECTION

AS/NZS 1768:2007 Lightning Protection.Provide guidelines for protection of

people, buildings and structures, and sensitive electronic equipment.

Applicable to conventional lightning protection systems (LPS) and surge protective devices (SPD).

4.1 Risk assessment & management

Risk management used to determine whether protection is needed and if so selection of adequate protection measures to reduce risk to below a tolerable level.

Risk R is defined as probability of loss occurring over a one-year period.

Table 2.1, AS/NZS1768:2007

Damage due to lightnings:

4.2 Protection of structures

Note: common to consider PL III as standard.

Typical LPS using metal in or on a building [Fig 4.4 AS1768:2007]

Using horizontal and vertical air terminals [Fig 4.5 AS1768:2007]

4.3 Voltage calculation

See Appendix D of AS1768:2003

Idealised lightning stroke currents.[Fig D1, AS/NZS1768:2007]

Approximate breakdown strength of air[Fig D2, AS/NZS1768:2007]

4.4 Earthing and bonding

Bonding of services [Fig E1, AS/NZS1768:2003]

Methods of equipotential bonding:

Combined utilities enclosure [Fig E2, AS/NZS1768:2003] (Preferred method for new buildings)

Common bonding network (CBN) [Fig E3, AS/NZS1768:2003](Building with properly bonded reinforced concrete floor)

Ring earth [Fig E4, AS/NZS1768:2003]

4.5 Transient waveshapes

See Appendix F of AS/NZS1768:2003Majority of transients encountered in

practice can be classified in terms of three standard waveshapes:1.2/50µs unidirectional pulse.8/20µs unidirectional pulse.0.8µs/100kHz ring wave.

Standard uni-directional waveshape[Figure F1, AS/NZS 1768:2003]

0.5µs/100kHz ring wave (open-circuit voltage)[Figure F2, AS/NZS 1768:2003]

Recommended application for waveshapes of Figs F1 and F2[Table F1, AS/NZS 1768:2003]

Typical voltage/time tolerance of computing equipment.[Figure F3, AS/NZS 1768:2003]

5

OVERVOLTAGE PROTECTIONIN LOW-VOLTAGE SYSTEMS

Crowbar devicesAir spark gaps or gas discharge tubesSCR and triacs

Clamping devicesMetal oxide varistors (MOV)Avalanche diodes (Zener diodes)Switching and rectifier silicon diodes

IsolatorsOpto-isolatorsIsolation transformersCommon-mode filters

Surge diverter protection for electricity supply circuits

Multi-stage protection for telephone and signalling circuits.

Low-pass filter to reduce rate of voltage rise.

Combination units.

Floating computer common (separate earth).

Appendix

OTHER DESIGN METHODSFOR

LIGHTNING PROTECTION

1. Cone of Protection Method

Volume protected by a catenary wire air termination.

Volume protected with vertical rod near building’s edge.

2 Faraday Cage Method

Also called mesh method, comprised of a series of horizontal air terminals such as copper tape which are bonded to vertically descending down-conductors.

Minimum mesh sizes (IEC61024-1 Standard):

3 Collection Volume Method

An improved Electrogeometric model developed by Eriksson.

Allows for computation of parabolic-like lightning collection volumes for all potential strike points on a building .

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