scientific validation of a placement method and proposal for inclusion lightning protection standards.

Presented at ICLP, Cracow, Poland, Sept. 2002

SCIENTIFIC VALIDATION OF A PLACEMENT METHOD AND PROPOSAL FOR
INCLUSION IN LIGHTNING PROTECTION STANDARDS

F. D’Alessandro fdalessandro@erico.com ERICO Lightning Technologies Australia

Abstract: This paper reviews the application of the Collection Volume Method for the placement of air terminals on practical structures, summarises the field studies that have been carried out to validate the method, describes the software support for the method and outlines the advantages it can offer as an additional placement method in lightning protection Standards.

Keywords: CVM, field testing, validation, air terminal placement, Standards.

1. INTRODUCTION

Lightning rods or “air terminals” are placed on structures with the objective of capturing lightning strikes and protecting them against the deleterious effects of a direct strike. A “placement method” is used to identify the most suitable locations for the air terminals, based on the area of protection afforded by each one. These air terminal placement methods typically fall into one of four categories: (a) Pure geometrical constructions, such as the “Cone of Protection” or “Protection Angle” method [1], which is commonly found in national and international Standards; (b) Faraday Cage concepts, in which a “meshwork” of conductors or air terminations is placed at set intervals over a structure [2]; (c) Electrogeometric models (EGM’s), in which empirical relationships for striking distance and lightning peak current are invoked [1]. The most common example is the “Rolling Sphere Method” [3,4], which is also partly a geometric construction; (d) Physical models, where air breakdown mechanisms are applied to the lightning scale; these models have been derived from laboratory investigations of long sparks and, to a lesser extent, field studies of natural lightning [5-10].
The methods in categories (a), (b) and (c) are conceptually simple and relatively easy to apply. However, many scientists and engineers agree they have inherent limitations that make them difficult to apply reliably across a wide range of structures, particularly over a large range in height. The Rolling Sphere method (RSM) is the most common air terminal placement method in use today. However, the simplicity of the method means that it does not take into account the difference in field intensification created by corners, edges and flat surfaces of structures. Hence, it is unduly conservative for large flat surfaces, such as on the roof of a structure and the sides of tall structures, both of which are unlikely to be struck by lightning. A partial solution to the problem is a simple modification to the RSM proposed by Darveniza [11]. However, it is arbitrary and there is a clear need for a more rational approach. These aspects introduce the concept of “efficiency”. A lightning protection system (LPS) can be defined as “efficient” if it achieves the desired level of protection and safety at the lowest possible cost for the end user. The optimised placement of air terminals ensures that the most efficient LPS is installed on the structure. The theoretical and experimental investigations behind category (d) show that cost efficient systems can be obtained through the study of air breakdown characteristics and the criteria for inception of the upward intercepting leader from the preferred attachment point. These features can then be applied as the key parameters within computerised modelling, the output of which allows the lightning attachment probability of the air terminals to be directly compared with competing points such as the corners, edges and other prominent locations on structures. In this manner, air terminals can be strategically located for lightning attachment. In the existing air terminal placement methods, there is no quantitative procedure for taking into account the: (i) main lightning-related parameters, such as leader charge and its distribution, leader propagation etc., (ii) electric field intensification created by the structure and its features as a basis for upward leader inception, (iii) effects
Presented at ICLP, Cracow, Poland, Sept. 2002
of altitude on air breakdown parameters and the subsequent results, (iv) wide range of structure shapes and dimensions found in practice, (v) height and position of the air terminals used for protection, and (vi) variation in the lightning attachment probability of different competing features on structures. The Collection Volume Method (CVM), which is based on the model of Eriksson [5], takes all of the above parameters into account. In this sense, it is a muchimproved EGM for the placement of air terminals on structures. The main feature of the method is the emphasis it places on the electric field intensification of the critical points on a structure. A paper describing the CVM was presented at the Rhodes ICLP [12] and, since that time, a full journal paper has also been published [13]. Hence, only a brief summary is presented here. The CVM takes a more physical approach than the simple EGM by using the fact that the striking distance, ds, is dependent on both the peak stroke current (or downleader charge) and the degree of electric field enhancement, hereafter termed the “field intensification factor”, Ki, of the prospective strike point. For structures, the Ki is determined to a large extent by their height and width, but the shape and radius of curvature of the structure or structural features are also important. In the case of air terminals, the Ki depends on the height and tip radius of curvature as has been demonstrated in numerous papers by Moore et al., e.g., [14-16]. When air terminals are placed on buildings, the Ki’s are multiplied up by a factor which depends on the structure dimensions and their location on the structure. Hence, an improved approach is to assume all points on a structure are able to launch an intercepting upward leader, but to differentiate those points based on the local field intensification factor. The CVM considers the approach of the lightning downward leader to a structure and, using the Ki(x,y,z) of the air terminals and structural features, determines the point at which an upward leader will be launched. Eriksson’s original model used the critical radius concept [17], but other leader inception criteria may be used. The CVM goes beyond the above fundamental improvement of the simple EGM by stipulating that interception will occur only if an adjacent competing feature does not “win the race” to interception with the downward leader. This criterion introduces a leader propagation variable. The above analysis results in the definition of a parabolic-like volume above a point on a structure, e.g., edge, corner, parapet, mast, air terminal, etc.. This defined region represents the three dimensional “capture” or “collection volume” of that point. It is used to obtain the “attractive radius”, Ra, of the point, which in turn, is used to compute the “attractive area” or “protection zone” of a given structure, structural feature or air terminal. The remainder of the paper highlights the additional justifications for the inclusion of another placement method in Standards. This discussion focuses on the field studies that have been carried out to validate the method
and the software support for the practical implementation of the method.

1. FIELD VALIDATION OF THE CVM

It is generally accepted that it is difficult to “test” an air terminal placement method in a high voltage laboratory. Hence, the issue of testing in the field, under natural lightning conditions, and providing a scientific validation of the method, is extremely important. The issues of validation can be considered from two viewpoints. Firstly, it is necessary to make an assessment of whether the observed number of flashes intercepted by the LPS is in agreement with the expectations of the model. The second requirement is a quantitative measure of the interception efficiency achieved with the LPS. The field validation of the CVM commenced many years ago with data collection in Hong Kong and Malaysia. The former study constituted a verification of the attractive radius model underlying the CVM, and the latter study involved the quantitative measure of the interception efficiency mentioned above. The remainder of this section summarises the key results from these studies and their implications.

2.1 Hong Kong: Verification of the attractive radius model The first part of a scientific validation involves a comparison of the flashes intercepted by a LPS with the number that is expected from the model used to position the system on the structure. Recently, a novel statistical study was undertaken on strike data obtained from LPS installations in Hong Kong. The aim of this study was to assess the suitability of Eriksson’s attractive radius model. The detailed results of this study have been published by Petrov & D’Alessandro [18], but the main results are summarised below. The study analysed the observed incidence of lightning strikes to a wide range of structures in Hong Kong. Data were obtained for 161 structures and spanned the period 1988 – 1996. These structures were protected with lightning air terminals, placed in the optimum locations on each structure using the CVM. A total of 103 flashes were observed over a total observation period (number of structures multiplied by the length of observation of each) of 640 years. A wide range of statistical tests were applied to the data. The results of the analysis showed a significant positive correlation between expected and actual strike frequency. Estimates of the strike frequency demonstrate that the interception probability of the LPS is compatible with the initial design specifications, i.e., the interception frequency predicted by the CVM attractive radius model is in excellent agreement with the observed interception frequency. This means that the interception rate is at least
Presented at ICLP, Cracow, Poland, Sept. 2002
as high as the claimed protection levels, which lie in the range ~ 85 to ~ 98 %.

2.2 Malaysia: Quantification of the interception efficiency Arguably the most important criterion to be satisfied in a field test and scientific validation of a placement method is a quantitative measure of the interception efficiency achieved with the LPS. This requires measurements of the number of lightning flashes captured by the LPS and the number of interception failures or “by-passes”. In this way, the actual interception efficiency or “protection level” can be compared with the assumed or desired value. The detailed results of this study have been presented by D’Alessandro & Darveniza [19], but the main results are summarised below. In this study, data were obtained from buildings in Kuala Lumpur and other regional centres in the Klang Valley region of Malaysia. Klang Valley is ideal for such a study, since the region receives approximately 20 ground flashes per square km per year. All of the buildings had a proprietary LPS with counters installed for measuring the number of lightning events. The data sample comprised 47 sites (total of 63 buildings) with LPS’s, 162 intercepted flashes and evidence of 27 by-passes, accumulated over the period January 1990 to May 2000. The placement of air terminals at each site was checked with the CVM design software (see Section 3) and an estimate obtained for the interception efficiency predicted by the model. This analysis took into account all of the main parameters discussed previously [13], e.g., critical radius concept used for as the leader inception criterion, with a radius of 0.28 m for competing features and 0.38 metres for air terminals, air terminal height of 4 m, and assumed the buildings are rectangular, not significantly shielded etc. The analysis showed that the mean protection level across the whole sample was ~ 78%. This implies that the percentage of “allowable” by-passes may be up to 22% for the data sample in the study. The final task in the analysis was the identification of the number of damage points on each building. Assuming that each damage point corresponds to a unique by-pass event, a total of 27 by-passes were evident. Damage points were seen on 11 of the 64 buildings in the study. One building accounted for 11 of the 27 damage points, raising the issue of whether 11 by-passes caused the 11 damage points or a smaller number of by-passes created multiple damage points. Taking a worst-case approach, the by-pass rate was found to be ~ 14%. This observation fits within the range of values of up to 22% estimated from the CVM design criteria. Hence, subject to the assumptions, uncertainties and the correctness with which the data have were interpreted, it was concluded that the predictions of the CVM are consistent with the observed lightning interception efficiency and the occurrence of by-passes.

1. SOFTWARE IMPLEMENTATION

One of the criticisms that could be levelled at the more complex physical models, e.g., those highlighted in category (d) in the Introduction, is that they are difficult to put into practice. Clearly, it is important that the model is available in the form of an easy-to-use computer program which requires only a minimal level of technical knowledge. This makes it accessible to a wide range of users, e.g., lightning protection designers, consulting engineers, system installers, etc. This issue has been taken into account throughout the development of the CVM, and its current implementation is in the form of a software support tool that reduces the practical application of the method to a very simple exercise. A summary of the main features of the software follows. The software is a client-server application written in Java and created to allow the easy design of a lightning protection system for a structure. It can also be compiled as a stand-alone application. A user obtains the client side of the application and this enables them to “model” their structure and place air terminals. Upon completion of this task, the model is sent over the Internet to the server side of the software for calculations of the structure collection volumes and the volumes and protection areas of the air terminals. The server side performs all the calculations necessary and the results are then returned to the client side, where the user can view them graphically. The software has much of the standard Windows functionality in relation to the way objects are created, edited, saved etc. Three fundamental “building blocks” are available for creating a model of the structure, viz. rectangular, cylindrical and gable. This is done in a 2D format from a plan view, as shown in Fig. 1. Air terminal and design parameters such as interception efficiency and site altitude can also be varied to user specification prior to calculations. Horizontal and vertical conductors can also be added, automatically or manually with edit facility, during the design phase. The results can be displayed in 2D plan or elevation views (Figs. 2 & 3), as well as in 3D. Fig. 4 shows the final, 3D, perspective view of the structure and LPS with the collection volumes not displayed. The design can be saved for later use and output in hard copy format. A typical design takes less than 5 minutes to produce.

1. CODES OF PRACTICE AND STANDARDS ON LIGHTNING PROTECTION

Many countries have their own national standard on lightning protection, and Australia is no exception. The
Presented at ICLP, Cracow, Poland, Sept. 2002
first standard in Australia was published in 1969 and since then there have been three revisions. The current standard is AS 1768-1991 Lightning Protection, which covers the protection of buildings, along with the persons and equipment inside these buildings. This standard broke new ground when it was introduced in 1991 with the introduction of measures for the protection of electronic equipment within structures and the protection of personnel. Recognising the need to keep the Standard
relevant to the marketplace, the EL24 committee responsible for the standard has been busy for the last three years introducing some of the most recent advances in lightning protection. The specific details of the work being done by EL24 are confidential and cannot be presented here. However, the general aspects have already been made public and these are briefly summarised here.

Fig. 1: Design of the structure and LPS with the support software.

Fig. 2: Results shown in 2D plan view.
Presented at ICLP, Cracow, Poland, Sept. 2002

Fig. 3: Results shown in one of the 2D elevation views.

Fig. 4: Elevation view of final LPS design.

The two most significant proposals in the new draft of the standard involve the introduction of improved methods for positioning air terminals on structures and a more rigorous risk assessment procedure. The new approach to positioning of air terminals introduces a modification to the Rolling Sphere Method, namely that an enlarged sphere is used to more realistically cater for large
flat surfaces, and a completely new method of air terminal placement called the “Field Intensification Method”. This method is derived from the “Collection Volume Method” that has been reviewed earlier in the paper. The changes to risk assessment involve the replacement of the current “risk index” with a “risk management” approach based on the principles of an early draft revision of the first edition
Presented at ICLP, Cracow, Poland, Sept. 2002
of the IEC 61662 Management of risk due to lightning document. There is an increasing trend around the world for national committees to adopt the IEC standards. The Australian committee decided to issue its own interim standard because the IEC standard is currently undergoing major revisions. Over the next few years, it appears the IEC will be issuing a complete suite of standards documents on lightning protection. It has been reported that these will comprise five individual booklets, taking into account all aspects of modern lightning protection practice such as general principles, risk management, physical damage, LEMP and incoming services. Over the course of the next two years, EL24 will be reviewing these and assessing their suitability for use in Australia.

1. CONCLUSIONS

This paper has reviewed the practical and design aspects of the Collection Volume Method for air terminal placement, as well as providing a summary of the field testing that has been carried out to validate the method. The latter constituted two unprecedented studies to validate a placement method for application to practical structures by verifying the basic model and quantifying the interception efficiency. The advantages of the method include the following: (i) a sound scientific basis, (ii) undergone an unprecedented program of field testing and validation, (iii) poses no restrictions on building heights and the like, and (iv) enables the designer to produce the most costeffective system possible within risk management principles. In addition, the CVM has received an unprecedented level of evaluation and scrutiny on the Australian Standards committee. The method is supported by a software package, based on a user-friendly graphic user interface for ease of design and verification of the LPS. Taking account of the above information, together with the deficiencies of existing techniques for air terminal placement, it is proposed that the CVM be considered by the IEC and other Standards bodies for inclusion in the relevant lightning protection standards as an additional placement method. If necessary, the software support for the method can be made available for this purpose.

1. REFERENCES

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Presented at ICLP, Cracow, Poland, Sept. 2002
[19] D’Alessandro, F. & Darveniza, M., “Field validation of an air terminal placement method”, Proc. VI SIPDA,
Santos, Brazil, pp 234-239, 2001.