20121206-lightning lecture note
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Hong Kong Polytechnic Universityg g y yDepartment of Building Services Engineering
Lightning Protection System Design and Installation Reviewand Installation Review
Organised by Prof YP Du
Dr Tony Sung BSc(Hon) MSc PhD FIET FCIBSE MIEE SenMIEEEAdj t P fAdjunct Professor
Chairman of CIBSE Electrical Services Grouphttp://www.cibse-electricalservicesgroup.co.uk
Dec 2012 Slide no: 1Adjunct Prof Tony Sung
Outline of LectureOutline of Lecture
Fundamental of Lightning PhenomenaPhenomenaStandards and Design GuidesImportant TermsQuantifying RisksQuantifying RisksControl MeasuresTh R l W ldThe Real WorldConclusion
Dec 2012 Slide no: 2Adjunct Prof Tony Sung
Theory of LightningTheory of Lightning (ref: Furse Guide to BS EN 62305)
Dec 2012 Slide no: 3Adjunct Prof Tony Sung
Separation of ChargesSeparation of Charges
How charges build up in the cloud is not well understood but the separation of charges in thethe separation of charges in the cloud is the source of lightning.
As the build up and separation continues until the voltage difference between the cloud and the ground exceeds the air ginsulation strength, it resulted in a lightning discharge as the air resistance is overcomeresistance is overcome.
(we can visualise it by analogy with HV overhead lines)
Dec 2012 Slide no: 4Adjunct Prof Tony Sung
)
St d d Li ht iStandards Lightning Protection SystemsProtection Systems
IEC EN 62305:2006 (due to be revised in 2013)BS EN 62305:2006 (due to be revised in 2013)( )Lightning Protection Standard (NFPA 780)IEEE Guide for Residential Lightning Protectiong gCIBSE Guide K (updated by CIBSE Electrical Services Group)p)
Dec 2012 Slide no: 8Adjunct Prof Tony Sung
Issues to be considered• Rise of Earth Potential. Soil has significant resistance, so
lightning strokes can cause large potential differences betweenlightning strokes can cause large potential differences between areas nominally at the same 'earth' potential. [8] shows that the 'traditional' practices (which are not recommended in this series) of star earthing and bonding cable screens at only one end makes this sort of damage more likely.
• Magnetic induction Very high surge voltages can be induced• Magnetic induction. Very high surge voltages can be induced into any conductors by magnetic coupling from lightning strikes up to 100 metres away.
• Surge Current injection. Direct strikes to external equipment or cables often results in damage to the internal equipment they are connected to and can damage unrelated equipment due to sideconnected to, and can damage unrelated equipment due to side-flashes in shared cable routes or terminal cabinets.
Dec 2012 Slide no: 11Adjunct Prof Tony Sung
Issues to be considered• Electric induction. Electric fields of up to 500kV/m can occur p
before a lightning strike, over an area of up to 100m from the eventual strike point. These can induce damaging currents into conductors and devicesconductors and devices.
• Lightning Electromagnetic Pulse (LEMP). This 'far-field' effect can be caused by cloud-to-cloud lightning as well as by distant y g g ycloud-to-ground strokes.
• Thermal and mechanical effects due to the intense energies i t d ith li ht i M tl ff t t t ' f b i dassociated with lightning. Mostly affects a structure's fabric and
its lightning conductors. Fire is one of the deadly results.• Multiplicity and duration of strokes in a single strike This isMultiplicity and duration of strokes in a single strike. This is
important for error-correction and system software recovery.
Dec 2012 Slide no: 12Adjunct Prof Tony Sung
Lightning StrokesLightning StrokesA t i l li ht i Lightning Stroke ShapeA typical lightning
'stroke' can last for over one second
g g p
over one second and consist of many 'strokes' (discharges)(discharges), sometimes over ten, each with an 'arc-each with an arcchannel' current of between 2kA and 200kA (1% of200kA (1% of strokes exceed 200kA).
Dec 2012 Slide no: 13Adjunct Prof Tony Sung
200kA). Another standard waveform is 20/80s normally consideredafter zone 0.
Lightning Protection System (LPS) design
Comprising:• A risk assessment based on actual lightning exposure • Design of the air termination network and down-
conductors D i f th th t i ti t k d th• Design of the earth termination network and earth electrodes
• Bonding of the metalwork within a structure and of the• Bonding of the metalwork within a structure, and of the metallic services entering a structure, to the LPS
• It helps reduce the risk of FIRE and EXPLOSION• It helps reduce the risk of FIRE and EXPLOSION
Dec 2012 Slide no: 14Adjunct Prof Tony Sung
Lightning RisksLightning RisksRi k t i b d li ht i t ik d it• Risk assessment is based on lightning strike density maps called isokeraunic (or isoceraunic) maps, plus:
• The structure's 'effective collection area' forThe structure s effective collection area for lightning strikes.
• Its use. • Its type of construction. • Its contents. • The consequential effects of any damage. • The degree of its isolation from other structures. • The type of terrain.
Dec 2012 Slide no: 15Adjunct Prof Tony Sung
Components of an LPSComponents of an LPS (ref: Dehn (UK))
Dec 2012 Slide no: 17Adjunct Prof Tony Sung
External and Internal Parts of a LPS '( f D h (UK))
External part of a LPS
(ref: Dehn (UK))
A typical level 3 or 4 LPSInternal part of a LPSDec 2012 Slide no: 18Adjunct Prof Tony Sung
A typical level 3 or 4 LPSInternal part of a LPS
Down Conductors
Minimum number of down conductors to beMinimum number of down conductors to be based on typical separation distance.
Dec 2012 Slide no: 21Adjunct Prof Tony Sung
External and Internal Parts of a LPS '( f D h (UK))
External part of a LPS
(ref: Dehn (UK))
A typical level 3 or 4 LPSInternal part of a LPSDec 2012 Slide no: 27Adjunct Prof Tony Sung
A typical level 3 or 4 LPSInternal part of a LPS
A iAssessing RisksRisks
(LPS design is by riskby risk
assessments)
Dec 2012 Slide no: 32Adjunct Prof Tony Sung
BS7671 Lightning protection Requirements
• BS7671 does not prescribe rules for protection against Lightningprotection against Lightning -Exclusion
• BS7671 refers Designers and• BS7671 refers Designers and Installers to use BS EN 62305 to design and install LPS to protectdesign and install LPS to protect buildings
Dec 2012 Slide no: 37Adjunct Prof Tony Sung
BS7671 Surge protection Requirements
• Section 443 – Protection against Overvoltages of atmospheric Origin or due to switching within the installationinstallation
• Table 44.3 gives the required minimum impulse withstand voltage of apparatus within an installationwithstand voltage of apparatus within an installation
Dec 2012 Slide no: 38Adjunct Prof Tony Sung
SPD Selection Design ProceduresSPD Selection Design Procedures
Remember Kirchhoff’s Current Law?
Dec 2012 Slide no: 41Adjunct Prof Tony Sung
Limit SPD conductor lengthsLimit SPD conductor lengths
InducedInduced Voltage:
Dec 2012 Slide no: 43Adjunct Prof Tony Sung
Remember surge inputs can be direct or indirectcan be direct or indirect
S1 S3S1 S3
BuildingstructureS2 S4
Meterbox
S4Groundlevel
Dec 2012 Slide no: 49Adjunct Prof Tony Sung
A Typical Risk Assessment ReportA Typical Risk Assessment Report
Dec 2012 Slide no: 50Adjunct Prof Tony Sung
ConclusionsConclusions• Risk assessments should be carried out – either
to BE EN/IEC 62305 or to comply with HK COP_E (or other country’s WRs)
• Designers and Installers must incorporate SPDs to control/minimise risks to an electrical i t ll ti finstallation for:– Power circuits (motor controls/dampers etc)
Data Circuits (network equipment/routers etc)– Data Circuits (network equipment/routers etc)– Life safety circuits (fire alarm/security/medical
equipment etc)equipment etc)• Maintenance of SPD (i.e., replacement access)
must be allowed for the life of the building.Dec 2012 Slide no: 56Adjunct Prof Tony Sung
must be allowed for the life of the building.
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