1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 + TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU02/4%#4)/.G0G0!).34G0G0).4%2 modified at Malaga-Torremolinos, 1984)In certain unfavourable circumstances, sudden transient voltages of exceptionally high instantaneous amplitude,of the order of 1 kV for examp
2、le, may occur across a telephone set which is normally connected to a metal wire line, asa result of electromagnetic disturbances affecting the line.If such voltages occur during a telephone call, they are liable to cause, through the earphone, such strong soundpressure as to endanger the human ear
3、and the nervous system.Such bursts are most likely to occur when lightning protectors are inserted in the two conductors of a telephoneline and do not function simultaneously, so that a compensating current flows through the telephone. The CCITTtherefore recommends the use, particularly on lines equ
4、ipped with vacuum lightning protectors, of protection devicesagainst acoustic shock arising from inadmissibly high induced voltages (see Chapter I/6 of the Directives, page 16).Such devices consist, for example, of two rectifiers, in parallel and with opposite polarities, or of othersemiconductor co
5、mponents connected directly in parallel to the telephone receiver.For telephone sets of more recent design, sudden voltage bursts liable to occur in the receiver may beeliminated by ensuring that the electrical circuits between the access to the line where dangerous voltages originate andthe earphon
6、e itself have suitable characteristics.It is also recommended that the proposed provisions should limit the aural discomfort which might be causedby abnormal electrical signals applied to subscriber systems as a result of erroneous operation or unwanted actuation ofthe equipments to which subscriber
7、 systems are connected.The provisions adopted to provide protection against acoustic shock should: be compatible with the technical requirements applicable to the equipment; facilitate performance checks; not noticeably impair telephone transmission quality.For this purpose, it is particularly recom
8、mended that:1) with regard to specific devices, their dimensions should be such that they occupy a small space, so that theycan be placed in the case of the subscribers or operators telephone receiver;2) the electrical characteristics should not show significant changes under the temperature and humidityconditions to which the device is subjected in service;3) effectiveness should be checked in conformity with the provisions of CCITT Recommendation P.36.
