LIGHTNING PROTECTION POSITIONING AND WARNING SYSTEMS

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1 LIGHTNING PROTECTION POSITIONING AND WARNING SYSTEMS by Phillip R Tompso BE(Hos) CPEg MIE(Aust) MIEE MIEEE NOVARIS PTY LTD Abstract Kowledge of the mechaism of the lightig discharge eables a uderstadig of the way i which lightig detectio ad warig systems operate. This paper ivestigates the discharge mechaism ad discusses various methods of detectig lightig discharges. Cocepts of the Faraday cage, groudig ad the etry of lightig eergy ito buildigs is also discussed. INTRODUCTION Lightig, which was believed by may paga people to be the dowward swoop of a avegig thuderbird, is still the subject of much cofusio ad icorrect assumptio. For example, oe rather widespread belief is that lightig will pursue ugrouded metal objects such as had carried golf clubs or radios. It is ot difficult to see how such otios come about. Some people, icludig professioal golfers like Lee Trevio, Bobby Nichols ad Jerry Had have bee ijured by lightig iduced groud currets while holdig golf clubs i cotact with the groud at the istat of a earby lightig discharge. The reactio of may people whe exposed to the threat of lightig death is to seek cover uder a loe, cospicuous, uprotected tree to escape the pourig rai. Each year may such people pay a fatal price for such igorace of lightig. THE HAZARD People are killed or ijured by lightig i oe of three ways: shelterig uder trees of sometimes overhagig roofs. 3. Idirect effects such as step ad touch potetials. People walkig ca have their feet touchig differet groud poits i the presece of a high potetial gradiet due to a ear groud strike. Alteratively icorrect earthig ca cause differet metal objects to assume high potetial differeces. A perso i cotact with both objects will be subject to this potetial differece. So a uderstadig of the mechaism of the lightig discharge will help us uderstad how to take to steps to protect ourselves ad our equipmet from lightig damage. THE LIGHTNING FLASH The trasiet discharge of electric curret that occurs betwee a egative charge cetre ad a positively charged regio is a lightig flash. A typical flash is made up of oe or more discharge compoets or pulses, with each pulse cosistig of a egative leader stroke ad a positive retur stroke. t s s rai t Up draughts of warm air Dow draughts of cold air Negative charge cloud base Positive charge o top 1. A direct stroke to the body. A most likely evet i ope areas such as fields ad golf courses. 2. Lightig stroke due to sideflashig. This evet occurs with people close to the path of a lightig strike. It typically strikes people 72 Brows Road, Kigsto, TAS AUSTRALIA Tel: FAX: sales@ovaris.com.au URL: Fig 1. Charge build up Sectio #: SK09 Revisio: 1 Date: Page: 1 of 1

2 O a ormal fie, suy day the earth has a slight egative polarity, ad the electric field so created is about 300V per meter. As a thuder cloud forms, separatio of electric charge i the cloud creates a surplus of egative charge i the base of the cloud ad over a period of may miutes this will build up to create a electric field i the order of 10KV per meter. Imagie the electric field as essetially horizotal lies i figure 1. As the leader approaches groud, the electric field builds up further ad field itesificatio occurs aroud those objects above groud level. The degree of field itesificatio depeds upo the height of the structure or object. At some poit the breakdow electric stregth of air will be reached ad ioisatio will occur. This gives rise to a upward streamer which heads up to meet the dowward leader. For tall structures streamer creatio is most likely aroud poits of high field itesificatio, ie at the tops of atea towers ad masts ad the corers ad other extremities of buildigs. Charge build up i cloud Electric field itesificatio Positive charge o groud Air breakdow leads to stepped leader Fig 2. Developmet of the dowward leader Positive upward streamer meets the dowward leader Coductig path forms Potetial is equalised by the "retur stroke" Visible lightig flash Charge build up cotiues util the electric field is so great that ioisatio of the air occurs. As ioisatio occurs, the stepped leader exteds from the cloud towards groud brigig electric charge with it. Figure 2 shows a buildig or other structure. The effect of this structure is to distort the electric field as it top is at earth potetial. The field lies ow must curl aroud the buildig, creatig field itesificatio at the top ad corers. s Cotiuatio of dowward leader Further electric field itesificatio Upward positive streamer geeratio Streamer meets dowward leader Fig 3. Developmet of the upward streamer Fig 4. The retur stroke Oe of the upward streamers will meet the dowward leader ad complete the ioisatio path. The electric charge is the discharged via this path ad the so called retur stroke curret flows. DETECTION There are a umber of ways i which lightig ca be detected. The traditioal methods relied upo physical observatio. However examiatio of the mechaism of the discharge provides other iformatio. The rise i electric field from aroud 300V per metre to 10KV per metre ca be detected ad provide iformatio o the presece of a storm cell. Similarly the radio oise associated with the discharge ad retur curret flow ca also be detected ad used to provide a vast amout of iformatio o storm patters. Physical Methods 72 Brows Road, Kigsto, TAS AUSTRALIA Tel: FAX: sales@ovaris.com.au URL: Sectio #: SK09 Revisio: 1 Date: Page: 2 of 2

3 A lightig strike is highly visible, its locatio ca usually be reasoably accurately assessed. However this method will ot give ay prior warig of a storm buildig overhead, it may provide iformatio o the approach of a storm but is otoriously ureliable. Physical methods ca also be used to more accurately assess the distace to a discharge. Timig of the flash to thuder iterval ca be used to measure this distace. Kowig the speed of soud the distace ca be estimated by allowig 3 kilometres for every secod. Field Mills Field Mills are the traditioal ad first istrumets to be used by golf courses, miig operatios ad blastig crews. A field mill cosists of two rotatig segmeted plates. The earthed upper plate alteratively exposes ad covers the lower active plate. Hece the electric field is screeed ad uscreeed by the rotatig plate. The resultat from the active plate is a alteratig voltage whose magitude is i proportio to the stregth of the field. This voltage ca be detected ad displayed. A sigle field mill will provide iformatio about local coditios oly ad gives o iformatio o the progressio or directio of a storm. A umber of field mills located some distace apart ad with iformatio trasmitted to a cetral poit ca be used to locate a thuder cell ad track its movemet withi the area of iterest. Vibratig Capacitor The operatio of the vibratig capacitor is similar to the field mill yet cotais o movig parts like the field mill. The device is similar to a small loudspeaker diaphragm which moves back ad forth i the electric field. By movig the diaphragm to differet field potetials, displacemet currets are set up similar to the field mill. Vertical Rods The field itesificatio aroud a poited rod will produce coroa discharge, which although ot ecessarily visible ca be measured i the form of curret flow. This curret will deped upo the magitude of the electric field. The additio of radioactive sources has bee used i the past. The operatio of these devices also suffers from the disadvatages above. Radio Detectio Methods The basic priciple o which these radio systems operate is that each retur stroke produces a radio frequecy impulse which ca be detected for hudreds of miles. The sigal produced from a cloud to groud strike has a differet sigature from cloud to cloud lightig ad from ma made oise. So moder detectio systems ca differetiate betwee groud strikes ad other oise. A etwork of such devices ca accurately track storms over large distaces. A sigle strike will be received at a umber of locatios simultaeously ad this iformatio set to the cetral positio aalyser. Based upo magitude ad time of arrival the locatio of the storm ca be accurately located ad displayed o a scree. The etwork ca display the progressio of a storm, its directio ad severity. Commercial Systems There are a umber of lightig detectio systems o the market. They are either electric field detectors, radio types or a combiatio of both. Combiatio uits provide iformatio o ear ad distat storms. However their ability to distiguish betwee high itesity distat strikes ad low itesity close strikes is limited. Furthermore they caot detect the path of the storm. Nevertheless such devices ca provide effective warigs agaist storm activity. PROTECTION Methods of lightig protectio for buildigs have bee aroud sice the 1750 s whe Bejami Frakli proposed his ow famous Frakli rod. As figure 5 shows Frakli appreciated the eed for proper earthig. This method of field detectio works satisfactorily i the laboratory but is ureliable uder field coditios. I the author s experiece the presece of rai ad moisture as well as wid affects the operatio of the device so much that it is ot a recommeded techique. 72 Brows Road, Kigsto, TAS AUSTRALIA Tel: FAX: sales@ovaris.com.au URL: Sectio #: SK09 Revisio: 1 Date: Page: 3 of 3

4 I the first istace a lightig warig system ca war of the approach of a impedig storm. Secodly safe haves ca be provided. These ca be the clubhouse, itself suitably protected, or shelter huts which may be simply all metal sheds or purpose built shelters of all metal or metal mesh costructio. Beig a cotiuous metal structure, it will act as a Faraday cage ad effectively scree the occupats. Fig 5. I 1762, Frakli ispected oe of his lightig rods, which had bee istalled o a house belogig to a Mr. West, who had complaied of lightig damage to the stoes of his house foudatio. Frakli oted that the rod should have bee brought away from the house ad... suk deeper so as to come to earth moister. Step ad Touch Potetials Frakli was correct i thikig that a lower earth resistace would elimiate the problem with the stoes by lowerig the earth potetial rise. Ufortuately we almost ever obtai a perfect zero ohm earth. For this reaso earth potetial rise, which i tur gives rise to step ad touch potetials, must be carefully cosidered whe desigig ay protectio system aroud which people may gather. Figure 6 shows the meaig of these terms. Golf Courses Because golf courses gather umbers of people who may be exposed to lightig dagers, there has bee mometum by course maagemet towards assumig some resposibility ad providig protectio. X is i cotact with the groud at a ad b. Y is i cotact with the groud at c ad coductor at d. Z is i cotact with the coductor at e ad hadrail at f. X is subject to step potetial. Y is subject to touch potetial. Z is subject to trasferred potetial. Fig 6. Potetial Differeces durig a lightig flash. Earthig To avoid the problems of step ad touch potetial i the golf course shelter shed the floor should also be of metallic costructio, or at least metal mesh boded 72 Brows Road, Kigsto, TAS AUSTRALIA Tel: FAX: sales@ovaris.com.au URL: Sectio #: SK09 Revisio: 1 Date: Page: 4 of 4

5 to the metal ski which is i tur coected to the earthig system. At a buildig likely to be struck by lightig, the earthig system must exted out beyod the limit of the dagerous potetial gradiet zoe. This meas that step potetials are reduced to a safe level ad the earthig system may also be used to bod other metallic objects. It is commo at commuicatios sites for example to exted the earthig system to the perimeter fece ad bod to it. LIGHTNING ENTRY Lightig ca strike buildigs directly ad as has bee show protectio of the buildig will coduct much of the lightig eergy to groud assumig that reasoable attetio has bee paid to earthig. However may cases of equipmet damage ca be traced to ot just direct strikes but to lightig etry via other meas. Some of these possible etry paths are show i figure 7. The solutio is to bod these structures to the lightig protectio system ad esure that cable sheaths are earthed at their poit of etry ad that cable coductors have adequate overvoltage protectio. b) The electrical power service etry. Regardless of whether the power feed is udergroud or overhead, this ca be a lightig etry poit. A direct strike to a power lie or a iduced evet ca result i lightig eergy etry. A direct strike to the buildig will raise the local earth potetial. Uder these circumstaces, the electrical power etry ca be a poit of lightig exit. The solutio is to esure that the mais earth is boded to the lightig protectio earth. Oe commo earth system is essetial. Overvoltage surge protectio must also be fitted. c) The commuicatios service etry. Agai regardless of whether the service is overhead or udergroud, the commuicatios service may be a poit of lightig etry exit. A direct strike to a commuicatios cable ca ofte be sufficiet to destroy the cable but iduced damage ca be miimised by proper protectio methods. The solutio is to bod the telecommuicatios earth to the other earthig systems ad provide overvoltage protectio o icomig metallic pairs. The ultimate solutio for buildigs with a large umber of telephoe services is to utilise optical fibre cable. Fig 7. Etry Paths for Lightig Impulses Particular attetio is required to the followig etry paths: a) Roof top structures such as TV ateas, commuicatio ateas, vetilatio ducts etc, all ca provide for lightig etry if ot protected by the lightig protectio system. d) Gas, water ad other coductive services. These services are almost always exit poits for lightig sice a direct strike to the buildig ad the subsequet earth potetial will cause lightig curret to flow alog these services to poits of lower potetial. The solutio is to earth these services to the buildig earthig system. By so doig these metallic coductors ca cotribute to the overall earthig system. CONCLUSION 72 Brows Road, Kigsto, TAS AUSTRALIA Tel: FAX: sales@ovaris.com.au URL: Sectio #: SK09 Revisio: 1 Date: Page: 5 of 5

6 The applicatio of overvoltage protectio devices is beyod the scope of this paper. However it should ow be apparet that proper buildig protectio, earthig systems which create a equipotetial plae, ad the appropriate utilisatio of overvoltage protectio devices ca provide a effective barrier to the etry of lightig eergy to most structures. REFERENCES M. M. Frydelud Lightig Protectio for People ad Property. Va Nostrad Reihold. J.L Norma Violette, Doald R. J. White, Michael F. Violette Electromagetic Compatibility Hadbook. Va Nostrad Reihold. NZS/AS , Australia, New Zealad Stadard Lightig Protectio Phillip R Tompso graduated from the Uiversity of Queeslad with a hoours degree i Electrical Egieerig i His early experiece was gaied as a commuicatios egieer with Telecom Australia, the with power utilities specialisig i commuicatios ad cotrol systems desig ad maagemet. His work ow ivolves cosultacy i the field of lightig protectio, power quality as well as product desig work for surge ad overvoltage protectio products. He is a chartered member of IE(Aust), IEE, ad IEEE as well as a member of the Australia Stadards Committee o lightig protectio.. 72 Brows Road, Kigsto, TAS AUSTRALIA Tel: FAX: sales@ovaris.com.au URL: Sectio #: SK09 Revisio: 1 Date: Page: 6 of 6

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