© Strojni{ki vestnik 49(2003)2,119-126 © Journal of Mechanical Engineering 49(2003)2,119-126 ISSN 0039-2480 ISSN 0039-2480 UDK 533.5:621.792.4 UDC 533.5:621.792.4 Strokovni ~lanek (1.04) Speciality paper (1.04) Tehnolo{ke zahtevnosti pri izdelavi plinskega odvodnika A Precision Techniques for Gas-Arrester Manufacturing Andrej Pregelj - France Brecelj - Andrej Pirih - Vladimir Murko Plinski odvodniki so hermetično zaprte celice, v katerih je med okrovom in elektrodo ujet argon ali njegova mešanica s primernim drugim žlahtnim plinom. Ob določenih pogojih (napetostni udar) plin lahko ionizira in s tem postane celica električno prevodna. Plinski odvodnik večjih moči, kakršnega razvijamo, je s se drugimi električnimi elementi povezan v sklop, ki deluje zaščitno, tako da omogoči takojšen odvod toka strele v zemljo. Čeprav je konstrukcija na videz preprosta, saj celoto sestavljajo le štirje glavni sestavni deli, je pa izdelava dokaj zahtevna. Za zagotavljanje čistosti in ohranjanje nespremenljivega tlaka plina v celici je treba uporabiti metode vakuumsko tesnega spajanja, ki so podobne onim pri proizvodnji elektronskih cevi. V prispevku predstavljeni postopki so: izdelava spojev steklo - kovina, testiranje tesnosti in trdo spajkanje v zaščitni atmosferi. Na kratko je opisan razvoj tehnoloških postopkov do sedanjega stanja in nakazane so možnosti za zmogljivejšo opremo za industrijsko proizvodnjo. © 2003 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: odvodniki plinski, spoji steklo-kovina, preskušanje tesnosti, spajkanje trdo) Gas arresters are hermetically sealed cells in which argon, or a mixture of argon and another noble gas, is captured between two isolated electrodes. Under defined conditions (e.g. lightning strike, etc.) it can become ionised, i.e. capable of conducting an electric current. The gas arrester designed for higher powers that has been developed in our laboratory is connected with other electro-elements in a special device that acts protectively in such a way that it diverts the lightning strike to the earth. The construction of the cell is relatively simple: no more than four main pieces are joined in a compact compound, but the procedures are not simple. To ensure high levels of cleanliness and required constant gas pressure in the cell for a period of years, it is necessary to use methods of vacuum-tight joining that are similar to those used for electron-tube production. In this paper the presented techniques are the manufacturing of glass-to-metal joints, leak detection and hard brazing in an inert atmosphere. A short description of the technology developed so far and the capabilities for more productive equipment are also given. © 2003 Journal of Mechanical Engineering. All rights reserved. (Keywords: gas arresters, glass-to-metal joints, tighteness tests, brazings) 0 UVOD Strele, preklopna rokovanja v stikalnih postajah v spletu porabnikov električne energije in drugi nenačrtovani pojavi povzročajo v električnem omrežju udarne sunke in valovanja. Posledica so prenapetosti, ki v priključenih električnih napravah (računalniki, telefoni, hi-fi instrumenti, itn.) lahko povzročijo poškodbe. Obramba pred njimi so zaščitne naprave, ki se jih vgrajuje v razdelilne omarice na vstopu električne mreže v stavbe. Njihov pomemben sestavni del so prenapetostni odvodniki, med katerimi zavzemajo posebno mesto plinski odvodniki. 0 INTRODUCTION Switching manipulations in an electrical network caused by electric consumers, lightning and other unexpected phenomena cause waves and strikes in the network. The results are overvoltages, which can damage sensitive electrical equipment: like computers, telephone apparatus, hi-fi devices etc. The defense against such phenomena is protection elements that are built into electric installations at the points (distribution boxes) where buildings are connected to the network. The important components of these elements are overvoltage gfin^OtJJIMISCSD 03-2 stran 119 | ^BSSITIMIGC Pregelj A., Brecelj F., Pirih A., Murko V.: Tehnolo{ke zahtevnosti - A Precision Technique Prednost plinskih pred drugimi vrstami odvodnikov arresters, and among them a special place is reserved je v tem, da so majhni po prostornini ter da v njih for so-called gas arresters because they are small ostajata plazma in oblok zaprta v celici (ognjevarnost). and because the plasma is always captured in a cell, Razvoj, katerega glavni namen je v prvi fazi which means they cannot result in burning. The nadomestiti uvoz tujih elementov, je zelo main goal of our activities is to reduce our interdisciplinaren, saj povezuje znanosti o materialih, dependence on the importation of these elements. o plazmi ter elektrotehniko in že na svojem začetku Initial development efforts have focused on tackling kaže, da bo ključno za uspeh prav postavljanje the difficult precision techniques for gas-arrester tehnoloških postopkov izdelave. finishing. 1 KONSTRUKCIJA CELICE 1 CELL DESIGN Plinski odvodnik, kakršnega razvijamo (sl.1), The gas arrester we are developing now (Fig. je sestavljen iz lončastega telesa, ki mu odprtino na 1) is composed of a metal pot that is closed at the top vrhu zapira strnjen sklop iz kovinskega obročka, stekla with a special feedthrough. The feedthrough is made in osrednje elektrode, imenujemo ga prevodnica ali of a ring, containing a central electrode that is sealed vtalek V celici je še tableta barijevega klorida in to both pieces with glass. The pot and the izbrana plinska mešanica. S steklom ločena lonček in feedthrough are brazed together using a silver braz- osrednji del prevodnice sta elektrodi, med katerima ing alloy. In the cell interior there is a tablet of barium pride do preboja.Vsi spoji med različnimi materiali in chloride, and a selected gas mixture is also trapped sestavnimi deli morajo biti hermetično tesni. inside. All the joints between the different materials Lončasto telo in obod stekla sta iz jekla or pieces have to be sealed hermetically. oziroma iz posebne zlitine, elektroda iz molibdena, The central electrode and the pot represent steklo je posebno s termičnim raztezkom, ki je two poles isolated by glass, and between them an prilagojen raztezkom molibdena in obodnega electric arc appears under defined conditions. The kovinskega obroča. V izdelavnem postopku sta pot and the ring are made of steel or a special alloy, posebej pomembna dva koraka. Prvi korak je zahtevna and the electrode is made of molybdenum; the glass izdelava prevodnice z dvema spojema steklo-kovina; is special in terms of its thermal dilatation and has to postopek je naslednji: na grafitni podstavek zložene accommodate the dilatations of the inner molybde- sestavne dele (obroček, steklena predoblika in num electrode and the metal ring. There are two basic elektroda) segrejemo v peči na okoli 1000 °C, steklo design steps in the manufacturing procedure. The se stali in oprime pripravljenih kovinskih površin. first step is feedthrough finishing: the electrode, the Drugi (in zadnji) korak je zatalitev te prevodnice s glass perform and the outer ring are placed on the trdo spajko (zlitina srebra in bakra) v ležišče telesa. centering tool, and then by melting the glass at high Pri obeh omenjenih korakih je glavna zahteva tesnost temperature the central electrode and the outer ring izdelanega spoja; še posebej je skrb za tesnost are coupled together into a compact element. The pomembna ob temperaturnih spremembah v drugem second and final step is brazing the feedthrough into koraku (spajkanje), da se ne poškoduje že prej izdelani the pot. The main difficulty with both operations is Sl. 1. Shema plinskega odvodnika Fig. 1. Scheme of gas arrester ______03 2 !Si"in^j(g)Jlp^gDilg[]CD | ^BgiTnj^DOCC | stran 120 I Pregelj A., Brecelj F., Pirih A., Murko V.: Tehnolo{ke zahtevnosti - A Precision Technique spoj steklo-kovina v prevodnici. Nujno je torej treba preverjati tesnost, za kar obstajajo posebne testne metode. Celoten postopek je seveda daljši, saj ga sestavlja cela vrsta opravil. Poleg že omenjene izdelave spojev steklo-kovina in zaprtja plina so to še naslednje: delo z vakuumskim sistemom na peči, nastavljanje temperaturnega režima pri spajkanju, doziranje čistih plinov, razplinjanje kovinskih delov, nanos galvanskih plasti, preverjanje tesnosti, spojev, izdelava spajkalnih obročkov in tablet BaCl2, čiščenje in shranjevanje sestavnih delov, električni preizkusi itn. V naslednjem odstavku predstavljamo tri izmed omenjenih, ki so ključnega pomena za kakovost končnega izdelka. 2 ZAHTEVNI POSTOPKI IZDELAVE 2.1 Spoj steklo-kovina Spoj steklo-kovina izdelamo tako, da hkrati segrevamo dotikajoča se kosa kovine in stekla na temperaturo, pri kateri postane steklo toliko zmehčano, da zalije in omoči površino kovine (ali zlitine). Osnova tovrstnim spojem je kovinski oksid, ki se v času dotikanja žareče kovine in staljene steklene mase kemijsko spoji z oksidnimi sestavinami stekla. Med ohlajanjem je najbolj ugodno, da se oba partnerja enako krčita, sicer pride do pokanja stekla. Spoje steklo-kovina delimo glede na toplotne raztezke obeh partnerskih materialov, na usklajene in neusklajene. Pri usklajenih (sl.2a) spojih združimo täko steklo in täko kovino oziroma zlitino, ki imata v vsem temperaturnem območju ista ali vsaj čim bolj enaka toplotna raztezka. Največ uporabljani usklajeni spoj se izdeluje iz zlitine, poznane pod imenom kovar (Fe-Ni-Co) in iz “kovarskega” stekla (npr. Schott 8250) z raztezkom a = 50 x 107/K. Med neusklajenimi spoji je najširše uporabljan stisni spoj (sl. 2b), pri katerem je steklo z manjšim raztezkom ujeto v obroč kovine z večjim raztezkom. V tem primeru adhezija med kovinskimi oksidi in steklom ni toliko pomembna, kajti odgovornost za trdnost in tesnost spoja prevzame achieving a 100% hermeticity of the joint. During the second step it is important not to damage the very delicate glass-to-metal joint. The whole procedure is longer than the mentioned two steps: it also involves many smaller steps. These steps include: braze-ring making, operating the vacuum system and the furnace, adjusting the profile of the brazing temperature, pure-gas metering, manufacturing glass-to-metal joints, encapsulation brazing, leak-detection of the seals and joints, keeping the components clean, electrical testing, etc. In the next section three especially important and interesting operations are presented. 2 DEMANDING TECHNIQUES IN THE MANUFACTURING 2.1 Glass-to-metal joint Glass-to-metal joints are made by simultaneously heating a piece of metal (or alloy) and a piece of glass that are in contact at the melting temperature of the glass. The principle of such a seal is the union of two sorts of oxides: the oxide of the metal piece firmly bonds the oxide components of the glass. This is realized during the time of full contact between the glowing metal and melted glass mass. During cooling it is very important that both parts contract in harmony, otherwise the in-built glass might break. Regarding the thermal extensions of the partner materials, there are two sorts of glass-to-metal joints, i.e. matched and unmatched. Matched ones (Fig. 2a) involve the joining of glass and metal (or alloy) with equal, or nearly the same, thermal expansions across the whole temperature range. The most commonly matched joints are manufactured from an alloy known as Kovar (Fe-Ni-Co) and of so-called “Kovar glass” (e.g. Schott 8250) with an expansion of a =50x 10-/K. The most frequent unmatched joint is the compressed joint (Figure 2b), where glass with a smaller expansion is caught in a metal ring that has a larger expansion. In this case the adhesion between the metal oxides and a b Sl. 2. Dve vrsti spoja steklo-kovina (a - usklajeni in b - neusklajeni) Fig. 2. Two sorts of glass-to-metal joints (a-matched, b-unmatched) stran 121 | ^IMlFinRDO Pregelj A., Brecelj F., Pirih A., Murko V.: Tehnolo{ke zahtevnosti - A Precision Technique vstop zaščitnega plina "shielding gas inlet kvarčni zvon ceramic holder gap shield Sl. 3. Izdelava prevodnice z visokofrekvenčnim segrevanjem v inertni atmosferi Fig. 3. Feedthrough manufacturing using high-frequency heating in an inert atmosphere krčna sila obodne kovine, ki se pojavi po ohladitvi pri izdelavi spoja. Stisni spoj se največ uporablja za električne prevodnice, pri katerih skozi stisnjeno steklo poteka osrednje prevodno kovinsko steblo. Obodna prirobnica je navadno iz nerjavnega jekla ali železa z razteznostnim koeficientom 100 do 160 x 107/K, steklo pa izberemo tako, da ima koeficient okoli 90 x 107K. Na sliki 1 prikazani odvodnik je konstruiran z usklajenim spojem - zunanji obroč prevodnice je iz kovarske zlitine in uporabljeno steklo je kovarsko. V toku naših razvojnih dejavnosti izvajamo preizkuse tudi z izvedbo, ki ima stisni spoj. Ene in druge spoje smo prvotno izdelovali z visokofrekvenčnim segrevanjem (sl. 3), sedaj pa prehajamo na staljevanje stekla (sl. 4) v peči. V obeh primerih zagotovimo potrebno odsotnost zraka (da ne pride do prevelike oksidacije) z uporabo inertne atmosfere. Zaščitni plin je navadno argon ali dušik, ki mu lahko dodamo 3 do 10 % vodika. 2.2 Preverjanje tesnosti Eden glavnih pogojev za stabilno delovanje plinskega odvodnika je tesnost. Vsako puščanje ovojnice ima za posledico izgubo delovnega plina ali vdor zraka od zunaj, kar pomeni spremembo značilnosti elementa preko dopustnih meja. Prizadevanja za doseganje čim boljše tesnosti celice so torej razumljiva. Šteje se, da sestavni materiali ne prepuščajo plinov, nujno pa je treba preveriti tesnost oz. netesnost spojev. the glass is not so important because the firmness and the tightness are achieved by the contracting force of the metal ring, which appears during cooling (in the last step of joint manufacturing). A compressed joint is normally used for electrical single- or multi-tip feedthroughs, where a glass insulator with a central conductive wire is placed in a metal wall. The outer metal is usually stainless steel with an expansion factor from 100 x 10-7/K up to 160 x 107/K; a suitable glass would have a factor of approximately 80 x 107K. The arrester shown in Fig.1 is made with Kovar and Kovar’s glass, but we have also tried to develop a version with a compressed joint. At the beginning of our development work both kinds of joints were produced using high-frequency heating (Fig. 3), in later experiments, however, we began to melt the glass (Fig. 4) in a furnace. In both cases the absence of air (to prevent oxidation) is achieved with an inert atmosphere. 2.2 Leak detection Hermeticity is one of the first conditions for the stable operation of a gas arrester. The presence of a leak in the envelope causes a change of the gas composition (because air enters the chamber) and the arrester does not function appropriately. It is supposed that the component materials do not have leaks, but we need to test the quality of the joints. | ^j^tMgfJT [RODCC | stran 122 Pregelj A., Brecelj F., Pirih A., Murko V.: Tehnolo{ke zahtevnosti - A Precision Technique Sl. 4. Deli pripravljeni za izdelavo prevodnice s stalitvijo stekla Fig. 4. Pieces prepared for feedthrough manufacturing V primeru odvodnika izdelujemo spoje dvakrat; najprej ko izdelujemo vtalek (spoj s steklom) in na koncu, ko zapremo plin (trdo spajkanje kovin). Spoj steklo-kovina je pri vtalku za odvodnik dokaj zahteven, saj se pojavlja v stisnem spoju na dveh mestih (znotraj ob molibdenu in zunaj proti obodnemu obroču), poleg tega pa je to spojno mesto kasneje (ob spajkanju) ponovno izpostavljeno temperaturi. Med razvojem preverjamo najprej tesnost vtalkov in ob končnih preverjanjih tudi tesnost izdelanega odvodnika. Netesnost neke stene je podana s količino plina, ki vdre skozi netesno mesto v določenem času in pri določeni tlačni razliki z ene strani na drugo; enota je torej mbarl/s. Za natančna preverjanja tesnosti se uporablja plin helij, ki ima poleg vodika najmanjše molekule in hkrati ni eksplozijsko nevaren. Pri proizvodnji vtalkov testiramo tesnost s helijevim masnim spektrometrom, in sicer tako, da preizkušanec vpnemo v primeren nastavek (orodje) in le-tega priključimo na testirno napravo, ki ima vgrajeno zaznavalo za helij. Nato izčrpamo prostor pod vtalkom in od zgoraj obpihavamo kritični spoj s tankim curkom helija (sl. 5). Če zaznavalo zazna dotok testnega plina, to pomeni, da je na mestu dotekajočega curka netesno mesto. Pri testiranju končno izdelane celice je dostopna le ena stran spojev, druga pa ne. Zato izberemo metodo, imenovano “bombanje”. V tem In the case of gas-arrester manufacturing, two joints are present: at the glass-to-metal surfaces, at the brazed seal between the feedthrough and the body of the pot. A glass-to-metal joint is at two locations: inside, with molybdenum; and outside, with a metal ring. Therefore, there are three critical places where leaks can appear: twice at the glass-to-metal joint and once at the hard, brazed seal. The leak-detection on the three places is obligatory. It is also very necessary because of the delicate glass-metal area in the feedthrough, which is twice exposed to high temperatures, the second one during the brazing operation. The leakage is expressed in terms of the quantity of gas (air) that flows through the leak during a particular time unit for a defined pressure difference from one side to another. The accepted unit for the size of a leak is mbarl/s. Usually, helium gas is used for such testing be-cause it has the smallest molecules and is not explosive. Gas-arrester feedthroughs are leak detected using a helium mass spectrometer, i.e. using a residual gas analyzer adjusted for the mass of helium. The sample has to be put in a suitable tool that is connected to the instrument. The space under the sample is then evacuated and on the upper side a narrow jet of helium gas is admitted to the critical places (Fig. 5). If the instrument senses a flow of test gas, it means that in the location of the jet there is a leak. For finished cells only one side of the joints is accessible. Therefore, another method is chosen, known as bombing. In this case a sample cell must Sl. 5. Načelo iskanja netesnosti s helijevim masnim spektrometrom Fig. 5. Principle of leak detection using a helium mass spectrometer | gnnaajMiBBgi stran 123 03-2 MSgTFIRDDBS Pregelj A., Brecelj F., Pirih A., Murko V.: Tehnolo{ke zahtevnosti - A Precision Technique Sl. 6. Prvi del “bombanja” Fig. 6. First step of bombing procedure primeru namreč v primerni bombici (sl. 6) izpostavimo preizkušanec tlaku helija (npr. 5 bar, dve uri), potem preskušano celico vzamemo ven, da se na okolnem zraku razplinijo zunanje površine (zračenje 1 do 2 uri), na koncu pa jo namestimo v komoro testirne naprave, ki zaznava helij. Če naprava ugotovi helij, je to znak, da le-ta prihaja skozi steno iz celice ali z drugo besedo, da je v steni netesnost. 2.3 Zaprtje plina v celico Končno zapiranje z zajetjem želenega plina v celico izvedemo v vakuumski peči, katero lahko prepihujemo oz. napolnimo z različnimi plini. Pri be exposed (in a suitable bomb, Fig. 6) to a pressure of helium (e.g. 5 bar, 2 hours). Then the sample has to be ventilated (e.g. 1 to 2 hours) and after this it is placed in an evacuated vessel - a part of the leak detector that senses the helium. If the presence of the test gas is established, it means that the gas is coming from the cell through a gap, and a leak is present in the envelope. 2.3 Gas encapsulation The final manufacturing operation is closing the cell, which has to ensure the hermeticity. This procedure is usually realized in a vacuum furnace vpuščanje plinov gases supply vrata peči z oknom door with a window merilniki tlaka pressure gauges Bour Pir Pen {J- -M- r~-SA $A :r :: Ar H. retorta ' retort grelnik peči furnace heater visokovakuumski črp. sistem high vacuum pumping system i___nnnn i DOE vzorci samples hladilna voda cooling water Sl. 7. Inkapsulacija plina v poskusne odvodnike s spajkanjem v vakuumski peči Fig. 7. Gas encapsulation in samples by hard brazing in a vacuum furnace VH^tTPsDDIK stran 124 Pregelj A., Brecelj F., Pirih A., Murko V.: Tehnolo{ke zahtevnosti - A Precision Technique našem razvoju smo uporabili retortno peč (sl. 7), connected with a gas-supply unit. In our case we ki je togo zvezana z visokovakuumskim sistemom used a retort furnace connected to a transportable na vozičku, tako da jo lahko kot celoto high-vacuum pumping station (Fig. 7), so it can be pomaknemo v grelno območje, ali pa jo potegnemo moved into or out of the heating zone. iz njega. The cleaned components of the gas arrester are Elemente odvodnika (lonček, tableto klorida, precisely put together on a suitable boat and are pushed vtalek in spajko) pravilno zložimo na podstavku na into a defined location in the retort. After that the furnace is “ladjici”, ki jo porinemo na določeno mesto v retorti; closed and evacuated until a pressure lower than 9x10-5 retorto nato zapremo in izpraznimo do tlaka < 9 x 105 mbar is achieved. During the heating the retort is still mbar; med segrevanjem še vedno črpamo tako, da pumped so that the pressure due to temperature degassing tlak pri tem ne naraste nad 2.104 mbar. Ko dosežemo does not increase over 2x10-4 mbar during the whole time. 500 °C, prekinemo črpanje in vpustimo okoli 800 mbar At 500°C the evacuation is stopped and argon (or another argona (ali izbrane plinske mešanice). Nato suitable gas mixture) is admitted into the retort with a nadaljujemo segrevanje do tališča spajke in ob njenem pressure of approximately 800 mbar; then the heating is stečenju ostane plin ujet v celici. S tem je naš cilj continued of a faster rate. When the braze ring melts (780°C) dosežen. the gas is captured inside the cell and the heating is stopped. Postopek je treba izpeljati tako, da se ne The procedure needs a lot of experiences and pokvari spoj steklo-kovina in da je spajkani spoj has to be undertaken very carefully, so that the glass- tesen. Pri tem je nemalo težav, ki jih odpravljamo z to-metal joint remains undamaged and the brazed izkušnjami, pridobljenimi pri poskusih. connection is sealed hermetically. 3 SKLEP 3 CONCLUSION Energetski plinski odvodnik je pomemben The power or so-called “heavy duty” gas element v prodajnem programu tovarne Iskra arrester is an important product of Iskra Protections. Zaščite. Da bi uvoz nadomestili z domačo celico, In an attempt to substitute for imports by developing so bile sprožene razvojne dejavnosti, ki sta jih new types or domestic cell, several activities were podprli tudi ministrstvi za znanost in za started. The development is also supported by the gospodarstvo. Slovenian economy and science ministries. V prispevku je na kratko opisano delovanje The working principles, design and manufac- odvodnika ter konstrukcija z izdelavno tehnologijo. turing technology of gas arresters are briefly treated in Pri tem so podrobneje predstavljeni trije manj this paper. More detailed information is presented about običajni in dokaj zahtevni koraki v tehnološkem the three basic steps of the manufacturing process: postopku, in sicer: problematika spoja steklo-kovina making glass-to-metal joints for the feedthrough, leak- pri izdelavi vtalka, preverjanje tesnosti spojev in detection of the arrester cells for checking the tightness zaprtje plina v celico. V sedanjem stanju razvoja and gas encapsulation by hard brazing. At the moment vlagamo največ naporov v odpravljanje netesnosti, great efforts are being invested in leakage elimination, v postavitev merilnih metod za testiranje električnih in the establishment of the testing procedures for con- sposobnosti celice, v projekt skrajševanja postopka trolling the cell’s electrical performance, in shortening zaprtja (peči s posebnim grafitnim grelnikom) ter v the encapsulation process (special furnaces with graph- razvoj oblikovno novih (večpolnih) in zato ite heaters) and in the development of new shapes of zmožnejših odvodnikov. gas arresters with more poles. 4 LITERATURA 4 REFERENCES [1] Bernat, D. (2001) Prenapetostna zaščita z uporabo odvodnikov prenapetosti razreda B, Ljubljana, Delo + varnost, 46, 4. [2] Meppelink, J., C. Drilling, M. Droldner, E.G. Jordan, J. Trinkwald (2000) Lightning arresters with spark gaps. Requirements and future trends of development and application, Proceedings of ICPL 2000, Rhodes, Greece. [3] Reece, M.P. (1963) Properties of the vacuum arc (part 1 of »The vacuum arc«), Proceedings IEEE, vol. 110, No 4. [4] Benson, A., P.M. Chalmers (1958) Effects of argon content on the characteristics of neon-argon glow-discharge reference tubes, The Institution of Electrical Engineers, Monograph No321 R. [5] Hering, E. (2001) Trennfunkenstrecken ffir den Blitzschutz-Potentialausgleich, Elektropraktiker, Berlin, 55, 5. [6] Altmaier, H., K.Scheibe (1997) Netzfolgestromunterbrechung von Funkenstrecken, ETZ, Heft 7. [7] Standard IEC 60151-17 (1969-01): Measurements of the electrical properties of electronic tubes and valves. Part 17: Methods of measurement of gas-filled tubes and valves. | lgfinHi(s)bJ][M]lfi[j;?n 03-2_____ stran 125 I^BSSIfTMlGC Pregelj A., Brecelj F., Pirih A., Murko V.: Tehnolo{ke zahtevnosti - A Precision Technique Naslovi avtorjev: mag. Andrej Pregelj France Brecelj Tehnološki center za vakuumsko tehniko Teslova 30 1000 Ljubljana mag. Andrej Pirih mag. Vladimir Murko Iskra Zaščite Stegne 35 1000 Ljubljana Authors’ Addresses: Mag. Andrej Pregelj France Brecelj Tehnološki center za vakuumsko tehniko Teslova 30 1000 Ljubljana, Slovenia Mag. Andrej Pirih Mag. Vladimir Murko Iskra Protections Stegne 35 1000 Ljubljana, Slovenia Prejeto: Received: 8.7.2002 Sprejeto: Accepted: 29.5.2003 Odprt za diskusijo: 1 leto Open for discussion: 1 year VBgfFMK stran 126