S.3.7 Lightning Protection Systems Pure Performance Standard: Instruments and Methods for Field Monitoring

13/03/2014
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S.3.7 Lightning Protection Systems Pure Performance Standard: Instruments and Methods for Field Monitoring

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Lightning Protection Systems Pure Performance Standard: Instruments and Methods for Field Monitoring Veronica Pomar, Susana Polo, David Ruiz Aplicaciones Tecnológicas S.A. Valencia, Spain Abstract— A new project (prEN50622) has been launched as an European Standard by CENELEC. This new standard is intended to evaluate Lightning Protection Systems (LPS) from a purely performance point of view. The idea of the standard as settled in the introduction is that "the performance of a LPS can only be established in real installations with a significant number of sites, representing the different climatic and geographical conditions around the world". The first draft considers that the number of lightning strikes should be at least 500, which means that a great number of real lightning protection installations should be monitored during years for checking if the system meets the standard. Is it achievable? This paper also introduces instruments and methods that can be useful for a long-term, reliable monitoring of lightning protection systems under affordable costs. The observation of real lightning striking LPS installed on different types of buildings will not only be useful for application of the standard but also for improving the scientific and technological knowledge within the field of lightning protection. Keywords— field monitoring, lightning protection systems (LPS), pure performance standard (PPS). I.INTRODUCTION The standardization group of CENELEC for lightning protection (CLC/TC 81X) was asked in 2010 by the Technical Bureau (BT) "to examine the possibility to establish a pure performance standard, independent from any technology and enabling the development of existing and future technologies on lightning protection systems”. Actually, the directives of ISO/IEC for structure and drafting standards state that "whenever possible, requirements shall be expressed in terms of performance rather than design or descriptive characteristics". That is, the performance based standards are now preferred over prescriptive standards, because performance standards leave more possibilities of innovation and application of new technologies. A performance standard has to establish objectives to be achieved, while prescriptive standards prescribe the materials and the methods for the design and construction of the equipment. Nowadays all standards for lightning protection are prescriptive standards: they give the materials and dimensions of the lightning protection components, the instructions for installation and the tests that such components should pass. The target of writing a pure performance standard for lightning protection was therefore completely new. The document should describe methods that could be used to demonstrate whether or not the lightning protection systems meet their goals. II. THE PROJECT PREN50622 Lightning is a very complex phenomena and cannot be fully reproduced in a laboratory. Some investigations have been made using artificially triggered lightning - launching rockets - but the attaching process is not really the same as for natural lightning. LPS can only be proved with real lightning. These and other problems raised during the first meetings of the working group that was settled for establishing if the requirement of CENELEC was or not achievable. On April 5th, 2011, the group returned to CENELEC a document whose conclusion was: "..., to establish a pure performance standard is possible, even if its implementation is quite complex due to the time and the costs needed to perform the required field tests." The complexity in time and costs was for CENELEC not a reason for stopping the task, so the group continued, adding the challenge of writing a draft in a very short term. Such draft has now been launched as prEN50622. The method described in such draft is clearly statistic. Only real lightning on real installation can be part of the survey. Furthermore, the results should be applicable to any type of structure and at any latitude, altitude, climate or environment. Thus the adopted solution was to study a significant number of lightning protection systems installed on structures. One of this task's controversial points has been, from the beginning, how to count the number of strikes. The first draft of the standard uses the real number of strikes to the protected area, however it also considers that, if the real number is not available, then NDT will be used. This parameter, NDT, is the expected number of strikes, calculated as other recognized lightning protection standards do: i.e. multiplying the flash density by the collection area -calculated as 3 times the height of the structure at each point- and taking also into account the location factor, that is, if the structure is isolated or surrounded by other buildings. This is the formula used by most lightning protection standards to make the risk assessment, that is, for estimating how many expected lightning strikes to a structure and comparing it with the accepted number or the potential losses. The fact of multiplying the height by 3 for calculating the protection area is an approximation related to the electrogeometrical model (EGM). The calculation of the expected lightning strikes has therefore the same deficiencies than in other lightning protection standards: the poor accuracy of flash density values and the simplifications assumed by the EGM. The calculation is related to the structure, regardless of whether it is protected or not or by what type of system. It does not depend on the performance of the lightning protection system, so it is not understandable that many commentaries that the draft has received are against the use of NDT. Furthermore, it is perplexing that such method (EGM) is considered by the authors of such commentaries as scientifically proved and valid for designing a lightning protection system; whereas the same method could not be used for the estimation of the expected lightning with the purpose of its performance evaluation. Other commentaries question the installation of systems with new technologies on structures. Unfortunately, due to the difficulties of simulating lightning, that has been a constant in the development of lightning protection. Since the protective angle of Franklin rods to ESE air terminals or isolated cables, most lightning protection components have been installed before being standardized. At that stage it is responsibility of the proponent of the non- standardized technology to assure the safety and the option of the user to acquire it. Finally, there are commentaries pointing out the difficulties of measuring and controlling lightning strikes on real installations. Indeed it is an ambitious goal and there are many points that should be clarified. The final document should clearly define: 1.- The number and characteristics of the lightning protection installations for being considered as "significant" 2.- How to observe the lightning protection installations and how to detect when lightning has struck the protected area 3.- How to define "failure" or "flashover" This paper will explore the possibilities for observing structures where a lightning protection system has been installed with the possibility of discriminating whether such LPS or the supposedly protected area have been struck by lightning. The devices for observation should be reliable, robust and not excessively expensive. The paper will also analyse the accuracy of the data we can obtain by using the existing technology. III. INSTALLATIONS FOR MONITORING In the described project draft (prEN50622), the expected number of direct strikes per year is calculated and accumulated for each structure in order to obtain the total number of expected dangerous events NDT. The total number of NDT to calculate the performance with reliable statistics must be NDT≥500 [6]. Several factors have to be considered in order to get non biased information. With this purpose it is advisable to monitor a heterogeneous set of structures with different particularities (such as high and low flash density, different altitudes and materials for the structure, etc.). [6]. On another side, the efficiency of a lightning protection system determines the expected number of failures. For example, the protection levels defined in [1] give an efficiency of 80% for LPL IV, 90% for LPL III, 95% for LPL II and 99% for LPL I,[1][2] Using the next formula to calculate the representative sampling of a data set, it is possible to determine a minimum number of samples (N.S.) to obtain reliable statistics [7]. Where k is determined by the confidence level; N is the total number of samples; e is the sampling error requested; p is the ratio of samples with the property under evaluation; q is the ratio of samples without the property under evaluation. When no information is available, the most secure option is p=q=0.5. Considering 95% as a confidence level, 5% as a sampling error and taking a total number of 500 events, the number of samples to estimate a certain LPL is 217 (rounding up 220). But, in any case, if information related with the ratio of the installations with a LPL is available, a more accurate calculus of N.S. could be obtained. IV. METHOD OF EVALUATION The first question that arises in the PPS proposal is evident, What do we need to measure? How should we make the measurement? How will we treat the obtained data? The first information that is necessary to begin with the evaluation consists of knowing whether lightning has struck the LPS of a protected structure. The most common way to evaluate this parameter is by a visual inspection that could show some traces of the current. For instance if the lightning has been intercepted by the LPS, there would be traces of some melting or charcoal at the air terminal. A flashover could be identified if damages are found and there is no doubt they have been caused by lightning (i.e. if typical ramified current marks are founded near the fault). Further information could be achieved with data about the number of lightning strikes intercepted by the LPS. In this way, a traditional lightning counter can help with this task. Getting the lightning peak current or other information related with the intercepted lightning, if possible, would be useful to correlate it with expected values. In this way, smart counters that register the peak amplitude of lightning are available in the market. Additional information regarding the flashover can be obtained by means of more advanced measuring systems capable of counting electric, magnetic or electromagnetic pulses (or any other physical magnitude) in a specific area but discerning if the current flows through the downward conductor or not. The point is comparing the number of identified failures with the number of expected failures. If identified failures are less or equal than expected failures, then the LPS will be in accordance with this pure performance standard. A more accurate indication can be obtained using measured parameters instead of expected values. The efficiency can be obtained comparing the number of intercepted strikes with the number of identified failures. This result can be compared with the theoretical efficiency determined by the LPL [1][2]. Furthermore, more advanced systems can be used to obtain extended information involving intercepted strikes or flashovers. For this, the most important magnitude to acquire is the current waveform, which offers additional information useful to validate the models, dimensioning of the external and internal protection systems, etc. In Table 1 different parameters to measure and its possible utility are shown. Table 1: Utility for different parameters of interest. V. TOOLS FOR THE DATA COLLECTION Tools to make the data collection task are not defined by the standard, but in a general way they could be classified according its capabilities and taking into account the data that could be considered, i.e as noted previously, a mechanical lightning counter gives information related with the number of intercepted lightning, but not about any other parameter. As far as determining the performance is concerned the key parameters are the number of intercepted strikes and flashovers and, if possible, their the peak current [4] [5]. Consequently, Table 2 shows different sensing principles that may be useful to identify those main parameters. The chief problem arises when measuring the peak current of a flash-over. In this case an accuracy of at least 1kA would be necessary, for example, to discern whether a lightning strike was lower than the "accepted" by LPL I. If this parameter is available a complete experimental evaluation of the LPS could be done. Otherwise, expected values (of failures) should be used in order to check if the performance of the LPS corresponds with what was expected. Anyway, equipments and tools can be based in one or more types of sensors depending of the parameter to measure and the particularities of the installation under evaluation. VI. DATA ANALISYS As previously noted, the data number must be large enough to obtain reliable statistics. An amount of at least 500 lightning strikes provides statistically significant results if all LPL are considered. In order to assess an specific LPL, 220 lightning strikes can be considered as statistically reliable. The draft is focused on evaluating the performance interception by means of an analysis of the failures, noting that failures caused by an improper installation will not be considered as failures. A book of installation is also advisable for performance verification purposes. This book is an useful tool to identify the particularities of the installation and results in maintenance procedures. When the amount of available data is large enough, the number of failures are then evaluated for all the installations as follows: The parameter Nf is the total number of measured flashovers and Nw is the weighted number of expected flashovers. For the evaluation of a determined LPL, Nw is the number of expected flashovers considering the efficiency determined by the LPL.[3] For being according to this standard, the parameter α should be less or equal than 1. VII. CONCLUSSIONS Nowadays different standards, air terminals and methods for positioning are employed for designing a LPS. But when it comes to the application of prescriptive standards, there is not a structured way of continuous evaluation of the performance of such LPS. Moreover, the efficiencies of the LPS are based on extrapolated data, that is, there is also a lack of experimental information about all the parameters affecting LPS installed on real structures with different environmental circumstances. With all this background, the group of CENELEC for lightning protection (CLC/TC 81X) was asked in 2010 by the Technical Bureau (BT) "to examine the possibility to establish a pure performance standard, independent from any technology and enabling the development of existing and future technologies on lightning protection systems” and a new project of standard (prEN50622) is nowadays in draft version. The aim of this project is the continuous monitoring of many LPS with different particularities, such as latitude, altitude, climate or environment and evaluate statistically its performance. In this paper, the project of standard is presented together with different ways, tools and methods to achieve the goal of the prEN50622. Nevertheless, different ways of monitoring and evaluation are possible in order to collect data. REFERENCES [1] IEC 62305-1:2011 “Protection against lightning – PART 1: General principles”. [2] IEC 62305-3:2011 “Protection against lightning- PART 3: Physical damage to structures and life hazard” [3] UNE 21186:2011 “Protection against lightning by means of Early Streamer Emmiters” [4] K. Berger, R.B. Anderson & H. Kröninger,Parameters of lightning flashes,” CIGRE ELECTRA No 41, p. 23-37, 1975 [5] R.B. Anderson, A.J. Eriksson, Lightning parameters for engineering application, CIGRE ELECTRA No 69 p. 65-102, 1980 [6] DRAFT prEN50622 “Lightning protection systems pure performance standard”. [7] http://es.wikipedia.org/wiki/Tama%C3%B1o_de_la_muestra [8] Internal document of CENELEC April 2011. [9] V. A. RAKOV, M. A. UMAN, and K. J. RAMBO - A Review of Ten Years of Triggered-Lightning Experiments at Camp Blanding, Florida. Atmos. Res., 76, 503-517, (2005).