Placement Method Doc (2)

7/30/2019 Placement Method Doc (2)

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Lightning ProtectionInternational Pty Ltd

ABN: 11 099 190 897

Complex #1, 16Mertonvale Circuit, KingstonTasmania 7050, Australia

P.O. Box 678,Sandy Bay, Tasmania 7006,

Australia

Phone:

+61 3 6227 1955+61 3 6227 1944

Fax:

+61 (0) 3 6229 1900

Email:

[email protected]

Web:

www.lpi.com.au

LPI Guardian CAT TerminalsPlacement Method

By

Rohan Thurstans B.E.Electrical Engineer

Copyright 2003, LPI

Lightning Protection International Pty Ltd forms Partnerships with ourcustomers to provide complete lightning, surge and grounding solutions.Our core competence is the ability to consistently deliver superiorperformance, quality and value to customers in a broad range of marketsegments.

An LPI ENDORSEMENT labeled product or solution confirms ourassurance of a long-term partnership with our customers and endorsesthe product quality and performance. All products offered by LPI comewith quality endorsement and compliance with International standardsand bodies wherever possible.
http://www.lpi.com.au/


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Lightning Protect ion Internat ional Pty Ltd. ABN 11 099 190 897 Tasm ania 7050 , Au st ra lia

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LPI Placement Method

The aim of direct strike protection installation is to reduce the risks of damage and disruption caused by the

electrical and thermal affects of a lighting discharge. Lightning Protection International Pty Ltd (LPI) place air

terminals on or near structures to be protected based on the physics and statistical characteristics of naturallightning. The placement method used is based on the physics of lightning attachment and the statistical

distribution of various lightning parameters. By using well-established links between the amount of electrical

charge in a descending lightning discharge, peak current and the lateral distance an air terminal can extend an up-

leader, a design can be carried out to a required risk level. The design method used produces a desired statistical

advantage of a given set of air terminals in capturing lightning discharges in preference to the exposed features of

a structure.

LPI use a risk assessment method based on the method used in Australian Standard AS1768-1991 to determine

pre-existing risk level. The method is modified in that allowance is made for strike densities higher than those

seen in Australia. This method takes into account the type of structure (including usage and contents), structure

construction, structure height and elevation.

Given the object of installing lightning protection is risk reduction, LPI base their designs on the lightning strike

density in the area of the facility to be protected. This way the design is based on resultant risk rather than degree

of mitigation. Design methods based on percentage of protection merely give an indication of the degree of

mitigation.

The maximum lateral distance an exposed object can extend an up-leader and intercept a lightning down-leader is

proportional to the amount of charge in the down-leader. The amount of charge in the down-leader is

proportional to the resulting peak current. In summary, larger strikes are easier to capture safely with a lightning

protection system. By combining strike density information with required risk information and the statistics

shown below for the distribution of peak currents across all lightning discharges, we can determine the minimum

strike size that needs to be captured to give the required risk level.

Percentage of strikes exceeding value shown (%) 99 90 75 50 25 10 1

Peak Current (kA) 5 12 20 30 50 80 130

The statistics above are taken from Table A1 of AS1768-1991.

Once a minimum peak current level is determined, this figure can be used to determine the relative abilities of air

terminals and various features of the structure itself to capture a descending lightning discharge or down- leader.

A computer program is used by LPI to place lightning protection and evaluate effectiveness in an int eractive

process. In order to understand the methods used by LPI to determine effectiveness an understanding of the

lightning attachment process is required.

Sequence of events leading to a lightning strike

A lightning strike is not an instantaneous process. First a down leader forms from within the cloud due to an

imbalance of charge between the cloud and ground. A leader is a high temperature mass of ionised air, which

obtains energy from the very high electric fields formed between one mass of charge in the cloud and another

mass of charge in the earth. The down leader extends toward the earth at very high speed, with this extension

generally occurring in bursts or steps of high speed followed by pauses. The result being the overall speed in

general is 1/10th

that of the speed during the bursts. The burst speed depends on a number of factors but the

amount of charge in the leader has a strong influence. Burst speeds are generally in the millions of kilometres an

hour. As the leader approaches within a few hundred metres of ground the electric field around objects on the

ground begin to rise rapidly.


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Objects protruding from the ground cause the electric field nearby them to be somewhat higher than that above

level ground. This is called field intensification. The level of intensification is related to a number of factors, but

generally the taller, sharper and more exposed an object the more the field is intensified. Once the down leader

moves close enough the field directly above the taller sharper objects will reach or exceed ~3Mv/m. Once this

occurs corona will be developed through the ionisation of air. As the leader approaches the amount of coronaincreases and forms streamers of heated air. Depending on the geometry of the objects emitting the streamers and

the direction of approach of the down leader one of the streamers above one or more objects may form an up-

leader (this process is described in further detail below). The up-leaders will travel toward the down-leader

under the influence of the strong electric field between them. Eventually one of the up-leaders will meet or

intercept the down-leader. As a result a continuous conductive channel is formed between the cloud and ground.

Once this occurs the other leaders will fail to progress so only one up-leader will be successful in intercepting the

lightning.

Formation of the successful up-leader

The success of an up-leader, in being the first to intercept the lightning down leader, is determined by its distance

from the down leader and the relative timing of its development. It was once thought that the earliest an up-leader

could be launched from an object was determined by the time at which the field immediately above the object

reached ~3Mv/m. This requirement corresponds to the time that ionisation of air begins to occur. Some

alternative lightning protection placement methods are based on this requirement alone.

More recent research shows that in addition to this requirement, high field strengths approaching 300kv/m must

be present ahead of the object for some distance. Generally it is considered that the critical distance is up to a

metre. The 300kv/m requirement corresponds to the time at which enough energy is obtained from the electric

field by the up-leader to keep it stable and progressive through this critical distance. The 300kv/m figure is based

on the more common positively charged up-leader, a figure approaching 1Mv/m needs to be used for the less

common case of a negatively charged up-leader.

When the two requirements are not met simultaneously, as often occurs with tall/sharp objects, corona or space

charge forms around the object. Where this occurs for a relatively long period, the affect of this space charge is to

create a relatively conductive section of air above the object. Often the position of this cloud corresponds with the

critical distance at which we need to see 300kv/m. This relative conductivity causes the field strength in this area

to be lower than it otherwise would be.

Many old placement methods assumed that up-leader progression occurred at a constant velocity. Recent research

shows that up-leaders do start from relatively low speed and accelerate as they develop.

As we can see an accurate placement system needs to take into account a number of mechanisms.

LPI Placement Process

The method used by LPI to determine the effectiveness of a particular air terminal is based on principles similar to

those developed by Eriksson and the methods given in Australian Standard AS1768-1991 that are based on his

work. However the LPI placement method extends this work further based on improved scientific understanding

of the lightning attachment process gained since the publishing of that document.

Using a computer program the LPI placement process starts by drawing a three dimensional representation of the

structure to be protected. Terminals are placed on the structure in strategic places according to the experience of

the operator. The program then determines the potential of the air terminals and various sections of the structureto capture lightning. In particular structure features at the extremities of the influence of the air terminals are

considered.


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The program calculates the maximum distance away a down-leader will cause an up-leader to be launched from

each terminal and structure feature. This distance is calculated for all directions from these points, resulting in a

three dimensional surface. We call this surface the striking distance surface. The program calculates the striking

distance surface of each terminal and structure feature as if it were a stand-alone object placed on the ground.Initially each object is approximated as a semi-ellipsoid as shown below.

A field intensification factor is then applied based on the relative field intensification of the object compared to a

semi-ellipsoid. The field intensification factor for each object is calculated based on a number of accepted

relationships for field intensification of common objects. An altitude factor is also applied to account for the

affects of air density on striking distance. Based on the above information a striking distance surface is

represented in three dimensions for each terminal and structure feature. A two dimensional section is shown

below.


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In the first instance it is checked in plan view that the striking distance surfaces of the air terminals completely

cover the striking distance surfaces of all relevant structure features. This ensures that an up-leader will always be

launched from an air terminal prior to an up-leader being launched from any structure feature. Successful

coverage of the structure by the air terminals is then determined based on vertical distances between the striking

distance surface of the air terminals and the striking distance surfaces of the structure features beneath them. Thediagram below depicts a basic example.

Successful coverage is determined based on vertical separation, x, between the striking surfaces and relative

distances, y and z to down-leader position as the down leader reaches the respective striking surfaces. This

calculation is made based on typical up-leader velocity and acceleration figures.

This placement method is conservative in that it does not take into account the affects the up-leader launched from

the air terminal will have in reducing the electric field above nearby structure features. This method is also

conservative in that it does not take into account evidence that suggest the down-leaders begin to divert towards

an air terminal or structure feature at about the time an up-leader is launched from that object, thus giving

advantage to the object that launches the first up-leader.

Where coverage is not successful the designer may move the air terminals or place additional air terminals until

successful coverage is achieved.

Summary

LPI placement method is related to installed risk, rather than percentage risk mitigation, through links to

lightning strike density at the location of the installation.

The placement method used by LPI makes use of the latest scientific principles from the quickly developingscience of lightning attachment.

Through the use of up to date lightning attachment models far more efficient placement of air terminals can

be carried out. By placing terminals only where they are required less terminals can be used without

compromising the risk of bypass.

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