1. - ericssonhistory.com

Fig. 2 X 6975IO-pair strips(From left) connecting strip NEL 1100, protectorstrip NFL 3111, and protector stripNFL 3211conditions it may be advisable to place over-voltage arresters in the protectorstrip at the exchange. A new over-voltage arrester has been designed. Itconsists of two metal electrodes pressed into a plastic tube to providethe proper spark gap. The 3-amp. fuses employed up to now, the main objectof which was to prevent overheating of the over-voltage arrester, have been discarded.The new over-voltage arrester cannot be overheated since, whenheated up, the plastic tube softens and the electrodes are forced together bythe mounting springs, so providing a direct circuit to earth. The overvoltagearrester is described in greater detail below.Opinions in regard to the proper protection for exchange equipment will,110 doubt, always be divided. However, the present trend is towards a reductionin the number of protector elements at exchanges served by cables. Thereare already exchanges at which all protection has been discarded, or atwhich an ordinary 0.5 amp. fuse alone is retained. The new strip has thereforebeen designed for use either with or without different combinations ofprotectors.Connecting and Protector StripsThe new connecting and protector strips are made up in 10-pair units. The10-pair strips are assembled on a common mounting (fig. 3), which willcarry 10, 20 or 50 pairs. Several 50-pair mountings can be placed one abovethe other in the M.D.F., as previously.5>The 10-pair strips are made in various forms to cater for different combinationsof protector elements. Consequently different categories of lines,even in one exchange, can be protected in different ways. These differenttypes of 10-pair strips are interchangeable without reforming of the cables.Similarly, they are capable of replacement by other not yet existing types, asnew ideas and designs of protector are produced. In the new strip the sectionsfor alarm and interception circuits have been omitted. The alarm device wasconsidered unnecessary since it functioned only on operation of the heat coil.Trouble of this kind represents only a very small fraction of the total interruptionsto telephone service. The interception circuits are now terminated onthe test jack strip. A main distribution frame is shown in fig. 1 with two50-pair strips of the old type alongside three 50-pair strips of the new type.Fig- 350-pair protector mountingAt present five different 10-pair strips are being manufactured, as listed intable 1. They have soldering tags on the line side and screw terminals for thejumper wires.15