џWPCL ћџ2BJ|xа HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаа АА X агга ХА6p&А6p&Х аб cмˆ4 PŽТ б вЦ‚HјР!Цв‡аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаааб cмˆ4 PŽТ бIMPORT R:\\ART\\WMF\\ITU.WMF \* mergeformatУ Уб cмˆ4 PŽТ б аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHP И XА`ИhР!Р!џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ Ф б cмˆ4 PŽТ бINTERNATIONAL TELECOMMUNICATION UNIONб cмˆ4 PŽТ бУ У а јА ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџјP И XА`ИhР!Р!џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаˆа HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјpи (#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџ ЬаТа ТТ№ ТТ№јТб cмˆ4 PŽТ бCCITTб cмˆ4 PŽТ бСHШ CСƒб cмˆ4 PŽТ бK.11б cмˆ4 PŽТ бЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјpи А"(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџ ЬаТа ТТ№ ТТ№јТ‚б cмˆ4 PŽТ бФ ФTHE INTERNATIONALЦЦ Та ТТ№ ТТ№јТTELEGRAPH AND TELEPHONEЦЦ Та ТТ№ ТТ№јТCONSULTATIVE COMMITTEEЦЦ Та ТТ№ ТТ№јТб cмˆ4 PŽТ бЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи А"(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјpи А"(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџ ЬаТа ТТ№ ТТ№јТб cмˆ4 PŽТ бУ УPROTECTION AGAINST INTERFERENCEЦЦ Та ТТ№ ТТ№јТЦЦ Та ТТ№ ТТ№јТб cмˆ4 PŽТ бЦЦ Та ТТ№ ТТ№јТЦЦ Та ТТ№ ТТ№јТPRINCIPLES OF PROTECTIONЦЦ Та ТТ№ ТТ№јТAGAINST OVERVOLTAGESЦЦ Та ТТ№ ТТ№јТAND OVERCURRENTSЦЦ Та ТТ№ ТТ№јТЦЦ Та ТТ№ ТТ№јТЦЦ Та ТТ№ ТТ№јТб cмˆ4 PŽТ бЦЦ Та ТТ№ ТТ№јТЦЦ Та ТТ№ ТТ№јТЦЦ Та ТТ№ ТТ№јТб cмˆ4 PŽТ бRecommendation K.11б cмˆ4 PŽТ бЦЦ Та ТТ№ ТТ№јТЦЦ Та ТТ№ ТТ№јТб cмˆ4 PŽТ бФ ФЦЦ Та ТТ№ ТТ№јТб cмˆ4 PŽТ бвЦ‚HјР!Цв‡аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјpи џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаIMPORT R:\\ART\\WMF\\CCITTRUF.WMF \* mergeformatб cмˆ4 PŽТ бУ УЦЦТа ТТ№ ТТ№јТ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP И XА`ИhР!Р!џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ ФGeneva, 1991б cмˆ4 PŽТ бУ УЦЦ а јА ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџјP И XА`ИhР!Р!џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаˆФ Фа HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи А"(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа‚ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ б ‚С`(#5СPrinted in Switzerland аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаУ У Та ТС€ HССр8NСFOREWORDФ ФЦЦ а H№ аС СThe CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СThe Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The approval of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988). а H аС СRecommendation K.11 was prepared by Study Group V and was approved under the Resolution No. 2 procedure on the 18th of March 1991. ‚Ср PС___________________  аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС‚Ср TСб cмˆ4 PŽТ бCCITT NOTEУ Уб cмˆ4 PŽТ бЦЦ б cмˆ4 PŽТ бФ ФС СIn this Recommendation, the expression Р"РAdministrationР"Р is used for conciseness to indicate both a telecommunication Administration and a recognized private operating agency. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа ‚Ср UСРMР  ITU  1991 а H аAll rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU.‚У У PAGE BLANCHE ггеŸI а Hx аУ У styleref head_footRecommendation K.11Ф ФPAGE11б cмˆ4 PŽТ бУ УŸее{† а HH аPAGE10б cмˆ4 PŽТ бУ У styleref head_footRecommendation K.11 {еа X Ш аRecommendation K.11 аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ ФRecommendation K.11 ‚Ср 7СУ Уб cмˆ4 PŽТ бPRINCIPLES OF Ф Ф У Уб cмˆ4 PŽТ бPROTECTION AGAINST OVERVOLTAGES AND OVERCURRENTS а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ ФУУ(Geneva, 1972; modified at MalagaЉTorremolinos, 1984 and at Melbourne, 1988, revised in 1990)ФФ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаУ УС СIntroductionФ Ф аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СCurrent CCITT documents recognize lightning and faults on nearby electrical installations as sources of dangerous disturbances in telecommunications lines, which may cause damage leading to interruptions in service and the need for repairs or even hazards to personnel. а H аС СThe object of the present Recommendation is to set out principles which enable the frequency and seriousness of such disturbances to be limited to levels which take account of quality of service, operating costs and safety of personnel. These principles are applicable to all parts of a telecommunications system. More details on certain methods of protection and for certain parts of the system are given in the References and in the following Recommendations: K.5, K.6, K.9, K.12, K.15, K.16, K.17. Information about disturbing phenomena and protection techniques are given in [1] and [2] (see also Recommendation K.26). а H аС СThis Recommendation deals principally with the local exchange, local loop plant and subscribers' equipment, but its contents may have wider application. а H аС СУУNoteФФ РIР The disturbing phenomena, when they appear, are relatively rare or of very brief duration (usually of the order of a fraction of a second) and in framing the present Recommendation, consideration has not been given to methods of avoiding interruption of the functioning of equipment during an actual disturbance. The CCITT is pursuing the study of such methods. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа‚У У1ТX ТGeneral considerationsФ ФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС1.1С   СУУOrigin of dangerousФФб cмˆ4 PŽТ б УУovervoltages and overcurrentsФФЦЦ ТX  ТТX јТС€  СС€ HСб cмˆ4 PŽТ б1.1.1С јСУУDirectФФб cмˆ4 PŽТ б УУlightning strikesФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СSuch strikes may cause currents of some thousands of amperes to flow along wires or cables for some microseconds. Physical damage may occur and overvoltage surges of many kilovolts may apply stress to the dielectrics of line plant and terminal equipment. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа1.1.2С јСб cмˆ4 PŽТ бУУLightning strikes nearbyФФ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СLightning currents flowing from cloud to earth or cloud to cloud cause overvoltages in overhead or underground lines near to the strike. The area affected may be large in districts of high earth resistivity. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HСб cмˆ4 PŽТ б1.1.3С јСУУInduction from fault currents in power lines includingб cмˆ4 PŽТ б electric traction systemsФФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС  СEarth faultsй€Earth faultsйб cмˆ4 PŽТ б in power systems cause large unbalanced currents to flow along the power line inducing overvoltages into adjacent telecommunications lines which follow a parallel course. The overvoltages may rise to several kilovolts and have durations of 200 to 1000 ms (occasionally even longer) according to the fault clearing system used on the power line.аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ б Та Т1.1.4СP Сб cмˆ4 PŽТ бУУContacts with power linesФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СContacts may occur between power and telecommunication lines when local disasters, e.g. storms, fires, cause damage to both types of plant or when the normal safeguards of separation and insulation are not followed. Overvoltages rarely exceed 240 V a.c., r.m.s. above earth in countries where this is the normal distribution voltage but may continue for an indefinite period until observed. Where higher distribution voltages, e.g. 2 kV, are used the power line protection arrangements usually ensure that the voltage is removed in a short time if a fault occurs. The overvoltage may cause excessive currents to flow along the line to the exchange earth causing damage to equipment and danger to staff. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС1.1.5С јСУУRise ofб cмˆ4 PŽТ б earth potentialФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СEarth faults in power systems cause currents in the soil which raise the potential in the neighbourhood of the fault and of the power supply earth electrode. (See also Recommendation K.9.) These earth potentials may affect telecommunication plant in two ways: а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТh ТС€  СТh Тa)СјСTelecommunication signalling systems may malfunction if the signalling earth electrodeй" signalling earth electrode"йб cмˆ4 PŽТ б is in soil whose potential rises by as little as 5 V with respect to true earth. Such voltages may be caused by minor faults on the power system which may remain undetected for long periods.ЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТТ№ ТС€ Сb)СpСHigher rises of earth potential can cause danger to staff working in the affected area or, in extreme cases, may be sufficient to break down the insulation of the telecommunications cable causing extensive damage.ЦЦ а HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ б1.2Тh  ТУУMethods of protectionФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа1.2.1С СSome of the protective measures for lines which are described in РSР 2 have the effect of reducing overvoltages and overcurrents at their source and so reduce the risk of damage to all parts of the system. а H а1.2.2С СOther protective measures which may be applied to specific parts of the system as indicated in РSРS 2, 3 and 4 fall broadly into 2 classes: а H аТа ТРIРТ№ Тthe use of protective devices which prevent excessive energy from reaching vulnerable parts either by diverting it (for example, spark gaps) or by disconnecting the line (for example, fuses);ЦЦ а H№ аТа ТРIРТ№ Тthe use of equipment with suitable dielectric strength, current carrying capacity and impedance so that it can withstand the conditions applied to it.ЦЦ а HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС1.3С   СУУTypes ofФФб cмˆ4 PŽТ б УУprotective devicesФФЦЦ ТX  ТТX јТС€  СС€ HСб cмˆ4 PŽТ б1.3.1С јСб cмˆ4 PŽТ бУУAirЉgap protectors with carbon or metallic electrodesФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СUsually connected between each wire of a line and earth, they limit the voltage which can appear between their electrodes. They are inexpensive but their insulation resistance can fall appreciably after repeated operation and they may require frequent replacement. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС1.3.2С јСб cмˆ4 PŽТ бУУGas discharge tubesФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СUsually connected between each wire of a line and earth or as 3Љelectrode units between a pair and earth. Their performance may be specified to precise limits to meet system requirements. The protectors are compact and will operate frequently without attention. а H аС СDetailed requirements for gas discharge tubes appear in Recommendation K.12.аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа Та Т1.3.3СP Сб cмˆ4 PŽТ бУУSemiЉconductor protective devicesФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СDevelopments in the technology of these devices have produced some units which may be used as primary protectors and others which should be used as secondary protectors only. а H аС СPrimary and secondary protection should always be coordinated correctly. Та ТТ№ ТС€ Сa)СpСУУPrimary protection devicesЦЦ а H аТа ТТ№ ТФФA widespread trial of these devices is taking place in which the units are being used to replace carbon electrode protectors and gas discharge tubes. The outcome of the trials may depend on both technical and commercial factors.ЦЦ а H аТа ТТ№ ТTypically, the benefits which may be provided over carbon electrodes may be the elimination of circuit noise and lower operating levels, while the disadvantages may be higher costs and selfЉcapacitance with lower current handling capability.ЦЦ а H№ аТа ТТ№ ТIn the case of gas discharge tubes, benefits may arise from the elimination of spark discharges while the higher selfЉcapacitance and the lower current handling capability may be considered to be disadvantageous.ЦЦ а H аТа ТТ№ ТThe trials are continuing and further developments of this technology may arise.ЦЦ Та ТТ№ ТС€ Сb)СpСУУSecondary protection devicesЦЦ а H аТа ТТ№ ТФФDepending on their location in a circuit, they may be designed to have overvoltage operation levels as low as 1 V. They are precise and fastЉacting, but may be damaged by excessive currents unless coordinated correctly.ЦЦ а HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС1.3.4С јСб cмˆ4 PŽТ бУУFusesФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СThese are connected in series with each wire of a line to disconnect when excessive current flows. Simple fuses have a uniform wire which melts. Slow а H аЉacting fuses have a uniform wire which melts quickly when a large current flows, and a springЉloaded fusible element which melts gradually and disconnects when lower currents flow for a prolonged time. High level currents of 2 A and prolonged currents of 250 mA are typical operating levels. Fuses should not sustain an arc after operation. Fuses do not give protection against lightning surges and in districts where such surges are common, fuses of a high rating (up to 20 A) may be necessary to avoid trouble from fuse failures. Such fuses may not give adequate protection against power line contacts. Fuses can also be a source of noise and disconnection faults. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС1.3.5С јСб cмˆ4 PŽТ бУУHeat coilsФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СFitted in series with each wire of a line, heat coils either disconnect the line, earth it, or do both, with the earth extended to line. Heat coils have some fusible component and operate when currents of, typically, 500 mA flow for some 200 s. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа1.3.6С јСб cмˆ4 PŽТ бУУSelfЉrestoring currentЉlimiting devicesФФ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СFuses and heat coils have the disadvantage that they permanently interrupt a circuit when operated and it is then necessary to replace them manually. Certain variable impedance devices are available which, when heated by overload currents, increase their electrical resistance to a very high value. The device will return to a normal low electrical resistance when the overload current is removed. Attention is drawn to the response time and voltage handling capabilities of these items.аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ б Та Т1.3.7СP СУУFusible linksЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаФФС СFuseless overЉvoltage protector assemblies installed on telecommunication lines can be protected against the risk of overheating in the event of a prolonged contact between the telecommunication line and a power distribution line by means of a fusible link. а H аС СA fusible link usually consists of insulated conductors in series with the telecommunication line and located between the exposure to the power line and the protector assembly. The conductors are usually at least two wire gauges smaller than the conductors terminated on the protector assembly and are of a suitable length to avoid a sustained arc if the power system does not deЉenergize promptly and the conductors fuse. If the fusible link or part of it is installed in a building or other location where a fire hazard might occur, it is enclosed within a cable sheath, splice enclosure, or other suitable enclosure to contain any arcing that may result if the conductors fuse. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа1.4УУТh  ТResidual effectsЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаФФС СThe essential purpose of protective measures is to ensure that the major part of the electrical energy arising from a disturbance is not dissipated in a vulnerable part of the installation and does not reach personnel. However, no device exists which has characteristics for suppressing ideally all voltages or currents connected with disturbances, for the following reasons: аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС1.4.1С јСб cмˆ4 PŽТ бУУResidual overvoltagesФФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СAccount should be taken of: а H аТа ТТ№ ТС€ Сa)СpСvoltages which are unaffected by the protective device because they are below its operating level;ЦЦ Та ТТ№ ТС€ Сb)СpСtransients which pass before the device operates;ЦЦ Та ТТ№ ТС€ Сc)СpСresiduals which are sustained after the device operates;ЦЦ Та ТТ№ ТС€ Сd)СpСtransients produced by the operation of the device.ЦЦ а HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС1.4.2С јСб cмˆ4 PŽТ бУУTransverse voltagesФФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СProtective devices on the two wires of a pair may not operate simultaneously and so a transverse pulse may be produced. Under certain conditions, particularly if the equipment to be protected has a low impedance, operation of one protective device may prevent the operation of the other one and a transverse voltage may remain as long as the longitudinal voltages are on the line. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС‚1.4.3С јСУУEffect on normal circuit operation РIР Coordinated designФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СSufficient separation should be allowed between the operating voltage of the protective devices and the highest voltage occurring on the line during normal operation. С СLikewise the normal characteristics (internal impedances) of the protective elements must be compatible with the normal functioning of the installations, which must take account of their possible presence. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС1.4.4С јСУУModifying effectsФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СA protective device may safeguard one part of a line at the expense of another, e.g. if a main distribution frame (MDF) fuse operates due to a power line contact, the voltage on the line may rise to full power line voltage when the fuse disconnects the telecommunication's earth. а H аС СLikewise the operation of a protector may greatly reduce the equivalent internal impedance of a circuit relative to equipment connected to it, thus permitting the circulation of currents which may cause damage.аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа Та Т1.4.5УУСP СCoordination of primary and secondary protectionФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СFor the protection of sensitive equipment it is sometimes necessary to use more than one protective device, e.g. a fastЉoperating, lowЉcurrent device such as a semiconductor and a slowerЉoperating, highЉcurrent device such as a gasЉdischarge tube. In such cases steps must be taken to ensure that in the event of a sustained overvoltage, the lowЉcurrent device does not prevent the operation of the highЉcurrent device since, if this happens, the smaller device may be damaged, or the interconnecting wiring may conduct excessive current. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС1.4.6С јСУУTemperature riseФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СProtective components should be designed and positioned in such a way that the rise in temperature which occurs when they operate is unlikely to cause damage to property or danger to people. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС1.4.7С јСУУCircuit availabilityФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СThe circuit being protected may be temporarily or permanently put out of service when a protective device operates. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС1.4.8С јСУУFault liabilityФФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СThe use of protective devices may cause maintenance problems due to unreliability. They may also prevent some line and equipment testing procedures. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа1.5Тh  ТУУAssessment of riskФФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа1.5.1С СThe performance of a telecommunications system with respect to overvoltages depends on: а H аТа ТРIРТ№ Тthe environment, i.e. the magnitude and probability of overvoltages occurring in the line network associated with the system;ЦЦ Та ТРIРТ№ Тthe construction methods used in the line network, see РSР 2;ЦЦ а HX аТа ТРIРТ№ Тthe resistibility of equipment in the system to overvoltages;ЦЦ Та ТРIРТ№ Тthe provision of protective devices;ЦЦ а Hр аТа ТРIРТ№ Тthe quality of the earth system provided for operation of the protective devices.ЦЦ а HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС1.5.2С јСУУThe environmentФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СIn assessing the environment, consideration should be given to the factors mentioned in РSР 1.1. а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС  СThe severity of overvoltages due to lightning varies widely in different localities. A high keraunic levelй: keraunic levelб cмˆ4 PŽТ б:й and a high soil resistivityй< soil resistivityб cмˆ4 PŽТ б<й increase the risk of direct and nearby lightning strokes and, since lightning is the cause of a large proportion of power system faults, induction and rise of earth potential effects are also increased. On the other hand buried metal plant such as water pipes, armoured cables, etc., screens telephone cables and greatly reduces overvoltages due to lightning or induction. а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТРIРТ№ ТIn city centres and in regions of low keraunic activity experience shows that overvoltages rarely exceed the residual voltages of protective devices and such environments may be classified as Р"РunexposedР"Р. Recommendations K.20 and K.21 specify the tests to be applied to equipment for use in unexposed environments without protection and these tests give an indication of the most severe environment which can be regarded as unexposed.ЦЦ а H аТа ТРIРТ№ ТAll other environments are classified as Р"РexposedР"Р but this, of course, covers a wide range of conditions including exceptionally exposed situations where a satisfactory service can only be achieved by the use of all available protective measures.ЦЦа HH а а Hр аС СIn the case of induced voltages and rise of earth potential the overvoltages can be calculated as indicated in [2] which also recommends the maximum values which may be permitted under various conditions. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС1.5.3С јСУУFault recordsФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СThe risk of overvoltages and overcurrents can only be properly assessed in the light of experience. It is recommended that fault statistics be kept in a form which is convenient for that purpose. Faults due to overvoltages or overcurrents and faults due to failures of protective components should be separated from each other and from other component faults. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа1.6Тh  ТУУDecision on protectionФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа1.6.1С СIn considering the degree to which a telecommunications network should withstand overvoltages, two classes of failure may be recognized: а H аТа ТРIРТ№ ТMinor failures affecting only small parts of the system. These may be allowed to occur at a level acceptable to the Administration.ЦЦ а H аТа ТРIРТ№ ТMajor breakdowns, fires, exchange failures, etc., which must, so far as possible, be avoided completely.ЦЦ а H аС СExamples of conditions which may be permitted to cause minor failures but not major breakdowns are given in Recommendation K.20. It is desirable also that failure of a single protective device should not cause a major breakdown. 1.6.2С СParticular attention should be given to overvoltage and overcurrent protection for new types of exchange or subscribers' equipment to ensure that the benefits of its improved facilities are not lost due to unacceptable failures arising from exposure to overvoltages or overcurrents. Such equipment may be inherently sensitive to these conditions and damage or malfunction may affect large parts of a system. а H а1.6.3С СIt should be noted that overЉprotection, by the provision of unnecessary protective devices, is not only uneconomic but may actually worsen system performance since the devices themselves may have some liability to cause failures. а H аС СTo avoid disturbances in telecommunication circuits caused by activated protective devices, the striking voltage values and the numbers of arrestors should be considered. а H а1.6.4С СIn the light of the above considerations and the assessment of risks in accordance with РSР 1.5, a decision should be made on the protection to be provided in all parts of the system. Account should be taken of commercial considerations such as the cost of protective measures, the cost of repairs, relations with customers and the probable frequency of faults due to overvoltage and overcurrent relative to the fault rate due to other causes. а H аС СThe responsibility for making this decision and for ensuring the provision of any protective devices needed to coordinate lines and equipment should be clearly laid down. а H аС СIt is necessary for manufacturers of equipment to know from the operating Administration the conditions the equipment will need to resist and for а H аline engineers to know the resistibility of the equipment which will be connected to the lines. The line engineer should also define the constraints which equipment connected to the line will encounter, depending on the standards of line protection provided. Where parts of the network, such as subscribers' apparatus, lines and switching centres may be under different ownership, this coordination may require formal procedures such as the production of local standards. Recommendations K.20 and K.21 give guidance for the preparation of these standards.аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа‚У У Та Т2СјСб cмˆ4 PŽТ бProtection of linesЦЦ а HH аб cмˆ4 PŽТ бФ Ф2.1Тh  ТУУProtective measures external to the conductors themselvesФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа2.1.1С СTelecommunication lines may be shielded from lightning to some extent by adjacent earthed metal structures, e.g. power lines or electric railway systems. Efficient metallic screens either in the form of cable sheaths, cable ducts or lightning guard wires, reduce the effects of lightning surges and power line induction. In areas with a high risk of lightning strikes special cables with multiple screens and high strength insulation are often used. Bonding all metal work gives useful protection. 2.1.2С СInduction from power lines may be minimized by coordinating the construction practices for the power and telecommunication lines. The level of induction may be reduced at its source by the installation of earth wires and current limiters in the power system. а H а2.1.3С СThe likelihood of contacts occurring between power and telecommunications lines is reduced if agreed standards of construction, separation and insulation are followed. Economic considerations arise but it is often possible to benefit from jointly using trenches, poles and ducts, providing suitable safe practices are adopted. (See Recommendations K.5 and K.6.) It is particularly important to avoid contacts with high voltage power lines by a high standard of construction since, if such contacts occur, it may be very difficult to avoid serious consequences. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС2.2С   Сб cмˆ4 PŽТ бУУSpecial cablesФФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СSpecial cables of high dielectric strength may be used where high overvoltages are likely to occur. а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС  СStandard plastic insulated and sheathed cables have a higher dielectric strength than paper insulated, leadЉsheathed cables and are suitable for most situations where cables with extra thick insulation were formerly used. The use of cables with strengthened insulation may be justified in situations where there is exceptional proximity or length of parallelism to power lines, high rise of earth potential in the immediate neighbourhood of power stations or extreme exposure to lightning due to high keraunic levelй keraunic levelйб cмˆ4 PŽТ б and low soil conductivityб cмˆ4 PŽТ бй soil conductivityйб cмˆ4 PŽТ б. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СOther examples of the use of special cables are: а H аТа Тб cмˆ4 PŽТ бРIРТ№ Тcables with metal sheaths which provide a good reduction factor to screen circuits within the cable;ЦЦ а H аТа ТРIРТ№ Тcables which carry circuits to exposed radio towers and which must be able to carry lightning discharge currents without damage;ЦЦ а H аТа ТРIРТ№ ТallЉdielectric (i.e. nonЉmetallic) optical fibre cables to effect isolation between conductive lengths of cable.ЦЦ а HH ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа2.3Тh  ТУУUse of protective devicesФФЦЦ а Hр ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СThe use of protective devices may be desirable in the following circumstances: а H а2.3.1С СThey may be more economical than the special construction described in РSРS 2.1 and 2.2. In this connection the cost of maintenance should not be overlooked since protective devices inevitably incur some maintenance expenditure whereas special cables, screening, etc., though initially expensive, usually incur no continuing costs. 2.3.2С СCables with extra thick insulation may themselves be undamaged by overvoltages or overcurrents but they can nevertheless conduct such conditions to other more vulnerable parts of the network. Extra protection is then required for the more vulnerable cables and is particularly important if these are large underground cables which are expensive to repair and affect service to many customers. а H а2.3.3С СInduced overvoltages from power or traction line faults may still exceed levels permitted by theУУ DirectivesФФ even after all practicable avoidance measures have been followed. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа2.4Тh  ТУУInstallation of protective devicesФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа2.4.1С јСTo protect conductor insulation it is beneficial to bond all metal sheaths, screens, etc., together, and to connect overvoltage protectors between the conductors and this bonded metal which should be connected to earth. This technique is particularly useful in districts of high soil resistivityй soil resistivityйб cмˆ4 PŽТ б as it avoids the need for expensive electrode systems for the protector earth connection. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа2.4.2С СWhere protectors are used to reduce high voltages appearing in telecommunication lines due to induction from power line fault currents, they should be fitted to all wires at suitable intervals and at both ends of the affected length of line, or as near to this as practicable. 2.4.3С СTo protect underground cables against lightning surges protective devices may be placed at the points of connection to overhead lines. The protective devices fitted at the MDF and at subscribers' terminals reduce the risk of damage to lines but their main function is to protect components having lower dielectric strength than the cables. See Recommendations K.20 and K.21. 2.4.4С СConnections for lines and earth to overvoltage protectors used against lightning should be as short as possible to minimize surge voltage levels between lines and the equipotential bond point. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ б2.5Тh  ТУУPlanning of worksФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СThe general considerations of РSРS 1.5 and 1.6 apply to the protection of lines. To the greatest extent possible it is recommended that the а H аprotective measures applied to the line should be decided at the outset of a project and should depend on the environment. It may be difficult and expensive to achieve a satisfactory standard of reliability from a line provided initially with insufficient protection. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа2.6Тh  ТУУRecommended policyФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СWhere lines in a telecommunications network are exposed to frequent or severe disturbances from power line faults or lightning, the voltage of these lines relative to local earth potential should be limited either by connecting protective devices between the line conductors and earth or by using appropriate construction methods for the line. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У3С  Сб cмˆ4 PŽТ бProtection of exchange and transmission equipmentЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ Ф3.1Тh  ТУУNeed for protection external to the equipmentФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СOperating organizations should take account of the possible need to fit protection external to the equipment, bearing in mind the following considerations: а H а3.1.1С СA telecommunication line will give some protection to equipment under certain conditions, e.g.: Та ТРIРТ№ Тa conductor may melt and disconnect an excessive current;ЦЦ а Hр аТа ТРIРТ№ Тconductor insulation may break down and reduce an overvoltage;ЦЦ а H аТа ТРIРТ№ ТairЉgaps in connection devices may break down and reduce overvoltages.ЦЦ а H а3.1.2С СThe increased robustness of plastic insulated cables has the effect of increasing the levels of overvoltages and overcurrents which can circulate in the lines and be applied to equipment. By contrast the use of miniature electronic components in exchange and transmission equipment tends to increase its vulnerability to electrical disturbances. С СFor these reasons, in districts exposed to frequent and serious disturbances (lightning, power lines, soil of low conductivity), it is usually necessary to interpose protective devices of the types described in РSР 1.3 between the cable conductors and the equipment to which they are connected, preferably on the MDF. This will prevent cables from the MDF to equipment from having to carry heavy overcurrents. а H аС СThe protective devices are fitted to the line side of the MDF to avoid the need to carry discharge currents in the MDF jumper field and to expose as little of the MDF wiring and terminal strips as possible to mains voltage in the event that a mains voltage line contact causes a series protective device to disconnect the line. 3.1.3С СIn less exposed locations it may be that disturbances (voltages and currents) have statistical characteristics of level and frequency so low that in practice the risks do not exceed those resulting from the residual effects indicated in РSР 1.4 for exposed regions. Protective devices then serve no purpose and are an unnecessary expense. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа3.2Тh  ТУУNeed for equipment to have a minimum level of electrical robustnessФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СIn locations where lines are exposed and protective devices are provided, the residual effects considered in РSР 1 can cause overvoltages and overcurrents to appear in the equipment. In less exposed environments the disturbances described in РSР 3.1.3 can cause similar effects. It is necessary for equipment to be designed to withstand these conditions and detailed recommendations on the resistibility which equipment should possess are given in Recommendation K.20. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа3.3Тh  ТУУEffect of switching conditionsФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СSince the configuration and interconnection of equipment connected to a given line is required to vary during the successive stages of connecting a call, it is important not to limit the study of protection solely to individual line equipments. Much equipment is common to all lines and can be exposed to disturbances when connected to a particular line. С СThe effectiveness of the protection provided can be influenced by the reduction in the probability of exposure if the effective duration of the connection to lines is short. On the other hand common equipment should be better protected since its failure risks more serious degradation in the performance of the exchange or the district. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У4С  Сб cмˆ4 PŽТ бProtection of subscribers' terminal equipmentЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ ФС СThe protection methods already set out for exchange equipment can often be usefully applied to subscribers' equipment. Detailed tests to determine the resistibility of subscriber equipment are given in Recommendation K.21. It is also appropriate to consider the specific aspects described below.аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа Та Т4.1СP Сб cмˆ4 PŽТ бУУDegree of exposureФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бС СLines to installations near exchanges in urban or industrial zones are usually little exposed to surges on account of the screening effect of numerous nearby metallic structures as described in РSР 2.1. а H аС СOn the other hand, lines to installations remote from builtЉup areas can be very exposed on account of their length, the absence of a protective environment, overhead construction at the subscriber's end and the high resistivity of the soil. The mechanical robustness of the overhead cables at the subscriber's end makes the effect of surges all the more serious since the line itself can carry higher voltages and currents. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа4.2Тh  ТУУDielectric strengthФФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СIt is desirable to have a high dielectric strength for the insulation between the conducting parts connected to the lines and all parts accessible to the user. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа4.3Тh  ТУУUse of protectorsФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СWhere telephone lines are exposed to frequent and severe disturbances from power line faults or lightning, the voltage of the lines relative to local earth potential should be limited by connecting protective devices of the types described in РSР 1.3 between the line conductors and the earth terminal. а H аС СThe terminal equipment dielectric strength should be chosen taking account of the breakdown voltage of the protective device and the impedance of the protectorЉline to earth connection. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа4.4Тh  ТУУCommon bondingФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СAt installations of subscriber terminal equipment a low resistance earth for overvoltage protectors may be unavailable, or the costs of procuring a suitable lowЉresistance earth may be excessive compared to other а H аinstallation costs. Furthermore, the terminal equipment may be located adjacent to earthed systems, such as water pipes, or may receive power from an electricity system. а H аС СTo minimize both equipment damage and exposure of the subscriber to high voltages, even if the earth resistance is not sufficiently low, all earthed systems, signalling earths and the power neutral should be bonded together either directly or by means of a spark gap. Although this bonding may be expensive it allows the difficulty of providing a low resistance earth to be resolved and is a technique widely used. In some countries connection to the electricity system neutral is governed by national regulations, so that agreement with the electrical Authority should be obtained. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа4.5Тh  ТУУNational regulationsФФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС СMany countries have national standards covering the protection of users of telecommunications equipment not only from the risks associated with connection to the electricity mains but also from conditions which may appear on the telephone line. 4.6С  СУУHigh cost of maintenance of subscribers' installationsФФ С СThe cost of repairs at exposed terminal installations may be high by reason of the distance from the maintenance centre, transport delays and, possibly, the seriousness of the damage. Moreover, insufficient protection is the cause of repeated interruptions of service which are particularly damaging to the quality of service and the satisfaction of the customer. This justifies the granting of special attention to protection measures.аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа‚У У ReferencesФ ФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа[1]Тh  ТCCITT Manual УУThe protection of telecommunication lines and equipment against lightning dischargesФФ, ITU, Geneva 1974, 1978.ЦЦ а H а[2]Тh  ТCCITT УУDirectives concerning the protection of telecommunications lines against harmful effects from electric power and electrified railway linesФФ, ITU, Geneva, 1988.ЦЦ