US 2212721 A
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g- 27, 1940- -r. B. D. TERRONI 2,212,721
ELECTRICAL S IGNALING SYSTEM Filed May 2, 1938 INVENTOR TESEO BRUNO DANTE TEPPON/ ATTY Patented Aug. 27, 1940 UNITED STATES ELECTRICAL SIGNALING SYSTEM Teseo Bruno Dante Terroni, Liverpool, England, assignor to Associated Telephone & Telegraph Company, Chicago, 111., a corporation of Delaware Application May 2, 1938, Serial No. 205,661 In Great Britain June 9, 1937 2 Claims.
The present invention relates to electrical signaling systems and particularly systems in which telephony and/or the performance of control operations are efiected by means of high frequency 5 carrier currents transmitted over the conductors of electric power distribution networks.
In such systems it is necessary to prevent the carrier currents from leaking away into undesired channels, for one reason in order that the efficiency of the arrangement may not be impaired and this is eiiected by the introduction of filter networks in the power line itself. For example, where it is desired to signal over a certain section of a power line, these networks are provided therein at the ends of the section so that the carrier currents applied thereto in well-known manner will be prevented from leaking away and interfering with adjacent sections and furthermore, so that substantially all the transmitted energy will be usefully employed in operating the receiving equipment at the opposite end of the section. These filter networks may consist in elementary form of simple resonant circuits made up of an inductance and condenser connected in parallel, the former generally comprising a helix of the power conductor itself. The inductance and capacity values are such that the network will act as a rejector circuit to carrier frequency currents and thus prevent them flowing into undesired channels while it will present negligible loss to the low frequency power currents which may be of the order of two hundred amperes or more.
Under conditions where heavy surge currents 35 are produced in the power line, due for instance to a lightning discharge in the neighborhood or to large scale switching operations, a high voltage will appear across the inductance and breakdown of the condenser is likely to result unless it is capable of withstanding high voltages.
It is the object of the invention to provide an arrangement whereby the expensive high voltage condensers used hitherto in such filter networks may be replaced by much cheaper low voltage 45 condensers without impairing the efliciency or safety of the system.
According to the invention, use is made of a unit of non-linear resistance material of the type which is substantially an insulator as long as the applied voltage does not exceed a certain value, but the resistance of which falls considerably when it is subjected to higher voltages. A suitably-dimensioned unit of this material is connected across the condenser so that if the voltage applied tends to rise due to abnormal conditions on the power line, the material will lower its resistance sufiiciently to absorb the excess voltage. By the term non-linear resistance material is meant material the resistance of which is substantially the same for both directions of current fiow but which decreases with increase of voltage.
One material having these properties comprises a mixture of silicon carbide with carbon and/r tungsten or molybdenum which is agglomerated under pressure and subsequently baked, the re sulting mass having a resistance-current characteristic which is substantially hyperbolic. The change in the resistance of this material is almost instantaneous and its recovery is equally rapid when the voltage falls so that it forms a highly efficient protective arrangement in the circumstances described.
The use of the invention is illustrated in the accompanying drawing in which it is assumed that carrier currents applied to the power conductors i, 2 and 3 are to be prevented from extending to the adjacent portions 4, 5, 6 and 7, 8, 9. For this purpose filter circuits are employed of which that shown between the two portion i and t is typical. This comprises an inductance iii shunted by a condenser H the values being so chosen that the combination offers a high impedance to currents of carrier frequency. For the protection of the condenser against transient high voltages, the unit of non-linear resistance material 12 is connected across its terminals and serves to absorb surges which would otherwise break down the condenser. Carrier current receiving and/or transmitting equipment is connected to the conductors i and 2 by way of the protective gear l3 and i4 and transformers l5 and it of which only the secondaries are shown. The carrier currents are confined to the section i, 2, 3 by the filter circuits and the condensers are protected by the non-linear resistance units.
1. In a filter circuit having parallel impedance and capacitance units connected in a circuit and tuned to bar alternating currents of particular frequency, a protective device comprising a non linear impedance unit connected in parallel with the impedance and capacitance which acts as a high resistance to low voltage currents in either direction and which decreases in resistance with increase in the voltage of currents passing in either direction thereover.
2. A filter circuit comprising parallel impedance and capacity tuned to prevent the flow of currents of certain frequencies, a non linear resistance unit shunted across the capacity to prevent breakdown of the capacity by high voltage currents passing in either direction thereover, said unit comprising silicon carbide mixed with carbon having a resistance which decreases rapidly with a rise in voltage and increases rapidly with a decrease in voltage.
TESEO BRUNO DANTE TERRONI.