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Fig.1: The Lightning Protection Design Process |
Step#1: Characteristics of the Structure to Be Protected
Explained in above Article
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Step#2: Risk Assessment Study
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Also, In above Article, I indicated that the risk assessment study can be done by (4) different methods as follows:
Methods Of Calculations For Risk Assessment Study
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Articles
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First: Manual Method (Equations And Tables Method) as per IEC 62305-2
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Design Calculations of Lightning Protection Systems – Part Two
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First: Manual Method (Equations And Tables Method) as per NFPA 780
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Second: Software Method For Performing The Risk
Assessment Study
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Third: Excel Sheets Method For Performing The
Risk Assessment Study
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Fourth: Online Calculators Method Used for Need for Lightning Protection calculations
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Today, I will I will continue explaining other steps of Lightning Protection Design procedure.
For more information, please review the following Articles:
Step#3: Selection Of External LPS Type and Material
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1- The Correct Choice Of Lightning Protection Components (LPC)
- The correct choice of
material, configuration and dimensions of the lightning protection components
is essential when linking the various elements of an LPS together.
- The
designer/user needs to know that the components, conductors, earth electrodes
etc will meet the highest levels when it comes to durability, long term
exposure to the environmental elements and perhaps most importantly of all,
the ability to dissipate the lightning current safely and harmlessly to
earth.
- Various standards series have been compiled with this very much in
mind. At present these standards are as follows:
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Standards for Lightning Protection Systems |
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1- Within Europe:
Various standards series
have been issued by (2) National Committees which are:
- The European
Committee for Electrotechnical Standardisation (CENELEC).
- The International
Electrotechnical Commission (IEC)
The CENELEC has
released the EN 50164 series of standards. The EN 50164 series are component
standards to which the manufacturers and suppliers of lightning protection
components should test their products to verify design and quality.
The EN 50164 series
currently comprises of:
- EN 50164-1 Lightning
protection components (LPC) – Part 1: Requirements for connection components
- EN 50164-2 Lightning
protection components (LPC) – Part 2: Requirements for conductors and earth
electrodes
- EN 50164-3 Lightning
protection components (LPC) – Part 3: Requirements for isolating spark gaps
- EN 50164-4: Lightning
Protection Components (LPC) – Part 4: Requirements for conductor fasteners
- EN 50164-5: Lightning
Protection Components (LPC) – Part 5: Requirements for earth electrode
inspection housings and earth electrode seals
- EN 50164-6: Lightning
Protection Components (LPC) – Part 6: Requirements for lightning strike
counters
- EN 50164-7: Lightning
Protection Components (LPC) – Part 7: Requirements for earthing enhancing
compounds
Notes:
- The standards
generally have an IEC prefix to their number (CEI for French versions). IEC
standards are produced in English and French languages.
- IEC and CENELEC
generally work in parallel, and CENELEC members vote to adopt new IEC
standards as CENELEC standards. The committees of CENELEC may choose to make
some alterations to the IEC version.
- Additionally, CENELEC
produce their own standards to which IEC have no counterpart. CENELEC
documents are produced in English, French and German and an approved CENELEC
standard will have an EN prefix (or NE in the French language versions).
For example:
IEC 62305-1 (IEC
version) is parallel to EN 62305-1 (CENELEC adopted copy of the above)
And both are parallel
to BS EN 62305-1 (British National Standard adoption ofthe above)
2- Within USA:
Various standards series have been issued such as:
- Underwriters Laboratory
(UL96 & 96A),
- The National Fire
Protection Association (NFPA 780),
- The Lightning Protection
Institute (LPI-175).
Note:
For heavy fault
conditions, Conductor Size should be calculated in accordance with IEEE Std
80.
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2- Material Requirements for Conductors and Air Terminations
- All lightning
protection materials should conform to EN 50164-1 and EN 50164-2 Lightning
Protection Components requirements. The exceptions to these requirements are non-current
carrying devices such as down-conductor fixings (clips), anti-vandal guards
and mechanical supports.
- The IEC
and EN standards prescribe the minimum material requirements as summarized in
the following Table:
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Table#1: Material Requirements for Conductors and Air Terminations |
Note:
- The
standards do not prescribe any relative performance advantages between these
choices; all are adequate to conduct the lightning current. But, when
considering service life, aesthetics, galvanic compatibility with building
materials and ease of installation, there is generally a preferred clear
choice of material for a given structure.
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3- Comparison between Copper and Aluminum
as a Lightning Conductor Material
A
conductor material should be chosen that is compatible with the surface it is
to be located upon and that which it is to connect to. As a typical lightning
protection system requires frequent bonds to nearby metallic items,
compatibility with this should also be assessed.
Comparison
between Copper and Aluminum Conductor Materials is indicated in the following
table:
Aluminum
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Copper
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lower cost
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More expensive than Aluminum
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lighter weight
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More heavy than Aluminum
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Less compatible with many building materials
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More compatible with many building materials
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Cannot be buried in the ground
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Can be buried in the ground
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the aesthetics of eventual white corrosion of aluminum
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the aesthetics of green verdigris of copper
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Aluminum is prone to corrosion when in contact with limestone, plaster, mortar and cement. For this reason, aluminum conductors should not be placed in direct contact with such surfaces.Stand-off fixings can be used (see below figure), or PVC covered aluminum conductors used
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care should be taken in areas with
sulphurous atmospheres (e.g. smoke stacks), where stainless steel or lead covered copper may be more appropriate
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Aluminum is prone to corrosion in marine or sea-side environments
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Aluminum should not be installed where it will be exposed to water run-off from copper (or copper alloy) surfaces
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Copper should not be installed above galvanized, zinc or aluminum parts where water run-off occurs (unless parts are protected such as by PVC covering). Water run-off from the copper surface carries fine copper corrosion particles, which when deposited on lower galvanized, zinc or aluminum parts may results in severe corrosion.
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Aluminum should not be installed on surfaces coated with alkaline based paint.
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Aluminum should not be installed in locations subject to excessive moisture (i.e. in gutters, or on surfaces where water may be retained).
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Therefore,
most lightning protection systems are entirely copper or utilize an upper
aluminum portion connecting to a copper earth termination system. As aluminum
and copper are not compatible, a bimetallic joint should be used to
interconnect these two materials.
Note:
As aluminum and copper are not compatible, a bimetallic joint should be used to interconnect these two materials.
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Bimetallic Connector |
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4- Use of Dissimilar Metals
- Galvanic
corrosion occurs when two dissimilar metals are in contact with each other in
the presence of an electrolyte. In this situation, one metal becomes the
anode and the other the cathode. The anode will tend to go into solution and
therefore corrode. The electrolyte can be water with impurities from the air,
other surfaces or from the metal itself
- The
following Table shows the potential difference between dissimilar metals.
Combinations of metals with potential differences above 0.5 V should be rejected
to avoid excessive corrosion.
For example:
A bare
copper conductor should not be directly connected to steel, as the
electrochemical potential difference is 0.53 V (≥ 0.5 V). However, if the
copper is tin plated then the difference becomes that of tin (0.24 V), which
is acceptable.
- One method
of reducing the effects of corrosion is to use plating of one or both of the
metals to reduce the electrochemical potential difference. Commonly, tin
plated copper conductors are used for this purpose. Tin plating also has the
advantage of stopping the appearance of green verdigris coating and reducing
the chance of theft (as the conductor no longer looks like copper). Tin
plated copper should be used for connections to:
- Lead,
- Grey cast
iron,
- Steel
(stainless steel connections do not need to be tinned),
- Aluminum,
- Cadmium.
The
following Table shows the material of structure and its LPS compatible
material:
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5- Temperature Rise Criteria
In non-isolated
lightning protection system, the down-conductors are primarily mounted
directly onto the structure (with no distance).
But,
sometimes a separation distance is required between the down conductors and
the structure. This is governed by the criteria of temperature rise in the
event of lightning striking the lightning protection system.
Temperature
Rise Criteria:
The
temperature rise criterion is based on the level of flammability for the
structure walls; we have two cases as follows:
Case#1: If
the wall is made of flame-resistant material or material with a normal level
of flammability
- In this
case, the down-conductor systems may be installed directly on or in the wall.
- For example,
Wood with a bulk density greater than 400 kg/m2 and a thickness greater than
2 mm is considered to have a normal level of flammability. Hence the
down-conductor system can be mounted on wooden poles.
Case#2: If
the wall is made of highly flammable material
In this
case, we have two sub-cases according to considering the temperature rise
(when lightning currents flow) is a hazard or not, this is can be known from
table as follows:
- The below Table states
the maximum temperature rise ΔT in K of the various conductors for each class
of lightning protection system. These values mean that, generally, it is even
permissible to install down conductors underneath heat insulation because
these temperature rises present no fire risk to the insulation materials.
This ensures that the fire retardation measure is also provided.
- So, the two
sub-cases for case#2: If the wall is made of highly flammable material, are:
Case#2-A:
The temperature rise of the down-conductor systems is not hazardous
- In this
case, the down conductors can be installed directly on the surface of the
wall.
Case#2-B: The temperature rise of the down-conductor
systems presents a hazard
- In this
case, the down conductors must be mounted (by using Standoff brackets for
example) to ensure that the separation distance between the down-conductor
and the wall is greater than 0.1 m. The mounting elements may touch the wall.
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Standoff brackets |
Notes:
- When
installing the down-conductor system in or underneath heat insulation, the
temperature rise (on the surface) is reduced if an additional PVC sheath is
used. Aluminum wire sheathed in PVC can also be used.
- The erector
of the structure must state whether the wall, where a down-conductor is to be
installed, is made of flammable material.
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6- Natural Components
- Natural conductive
components can be used as an integral part of the lightning protection
system. Natural components are typically metallic structural items that will
not be modified during the life of the structure, such as reinforcing steel,
metal framework and roofing/cladding.
- Natural components
must meet minimum material requirements which listed in below Table and be
electrically continuous with secure interconnections between sections such as
brazing, welding, clamping, seaming, screwing or bolts.
- The above Table
provides the thickness requirements for natural air terminations. Where
combustible materials are not present, and water ingress can be tolerated
from a puncture due to lightning, then thinner material is permitted for
air-terminations. If the materials do not meet these requirements, then they
must be protected by the lightning protection system.
- Metal pipes and tanks
on roofs can be used, provided they meet the requirements of Tables#1 and 2. Refer to the
standard for requirements of tanks and piping that contain combustible or
explosive mixtures. It is not desirable to use vessels and pipe work which
contains gas or liquids under high pressure or flammable gas or liquids.
- The requirements for
natural air-terminations differ from natural down-conductors. Down-conductors
and air-terminations need to withstand the ohmic heating and
electromechanical/magnetic forces, but air-terminations also need to
withstand the heat of the lightning plasma arc.
- The following parts of a
structure can be used as “natural components” of the lightning protection
system:
- Metal Installations
- Facade Elements, Mounting
Channels and the Metal Substructures of Facades
- Metal downpipes
- Rebar in Reinforced
Concrete
- Rebar in Precast
Concrete
- Rebar in Prestressed
Concrete
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Facade Elements as Down Conductors
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In the next Article, I will explain Step#4: Sizing of Air Termination System Components. Please, keep following.
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