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Connecting and Protector Strips for MainDistribution FramesE NYLUND, TELEFONAKTIEBOLAGET LM ERICSSON, STOCKHOLMU.D.C. 621.395,669The connecting and protector strips made by L M Ericsson have undergonevirtually no change for many years. They are rather bulky, however, and thismay prove a serious inconvenience especially in rapidly growing telephonesystems. A new connecting and protector strip with pairs on 10 mm centreswas therefore designed, occupying only two-thirds of the vertical spacerequired by the previous strip. The strip is composed of standard elements,so is simpler to manufacture, pack and install. New and more efficientexcess current cut-outs and over-voltage arresters have been designed at thesame time.Protector Unit CombinationsThe most common method of protection used in telephone exchangeshitherto consisted of a combination of three protector elements placed in thefollowing sequence: nearest the line a fuse which usually blows at 3 amps.;thereafter an over-voltage arrester, normally consisting of carbon blocks withmica separator, for protection against atmospheric discharges; and, on theexchange side, a heat coil which operates to long-duration "sneak" currents ofmin. 0.15 amp. This form of protection was originally designed since theexternal plant consisted largely of open wire lines.Many areas are nowadays served entirely by metal sheathed cables and donot require so high a measure of protection; the over-voltage arrester at anyrate must be considered unnecessary in such areas. If there is any risk ofsneak currents or contact with power lines, however, excess current cut-outsshould be installed. Sneak currents, if of sufficient duration, may unduly heatthe coils of line relays. The current at which the heat coil operates must beabove the transmitter current at the lowest line resistance, but below thesneak current that would cause overheating of the line relays.L M Ericsson has designed a new fuse which is a combination of ordinaryfuse and heat coil. In this combined fuse the current passes through a smallheater element soldered to a straight fuse wire. At sneak currents exceeding0.15 amp.—the minimum fusing current—the solder melts and disconnectiontakes place with roughly the same time lag as in the old heat coil. At currentsabove 2—3 amps, the straight wire fuses first, and very quickly, as shown infig. 7. The combined fuse is described in greater detail below.x2147Fig. iMain distribution frame type BAB 12showing old connecting and protector strips(left) and the new designs (right)On the fringes of an exchange area long stretches of line may consist ofbare wire or self-supporting insulated wire. Over-voltage protection will berequired at the distribution points between bare wire and cable. Hazardousvoltages caused by atmospheric discharges or by contact between the barewire and a power line cannot enter the telephone cable or the exchange ifthe over-voltage protector operates in the required manner. It may happen,however, that part of the voltage wave from the overhead line continues tothe exchange owing to high resistance to earth or to a fault in the protectorelements. But this wave cannot attain higher amplitude than the breakdownvoltage of the cable, which is usually limited to 2,000 volts. Under such14