Electrical signaling system



Oct. 11, 1949. w. SAVILLE 2,484,188 ELECTRICAL SIGNALING SYSTEM Filed Oct. 8, 1945 V ON WA 22419 Jaw'ZZe Patented Oct. 11, 1949 ELECTRICAL SIGNALING SYSTEM Willie Saville, Liverpool, England, assignor to Automatic Telephone & Electric Company Limited, Liverpool, England, a British company Application October 8, 1945, Serial No. 620,982 In Great Britain December 29, 1944 4 Claims. The present invention relates to electrical signalling systems and is more particularly concerned with systems for signalling over electric supply mains by means of superimposed currents. For convenience these signalling currents will be referred to as high frequency currents but it will 1 be understood that this merely means high compared with the supply frequency and in practice a frequency within the audio range may conveniently be employed. One of the difficulties which has to be guarded against in systems of this type is that there may be a'tendency for signals to be simulated by parasitic currents including the signalling fre quency or, particularly where different signals are sent in the form of characteristic rhythms, parasitic currents may have the effect of mutilating the rhythms and thus causing incorrect operation of the responding equipment. It is the chief object 'of the invention to ensure that signalling cannot be initiated in the presence of parasitic currents likely to cause interference so that the reliability of transmission is increased. According to the invention, in an electrical signalling system having signalling effected by high frequency currents superimposed on electric supply mains, immediately prior to a signalling operation a test is made to ascertain whether currents of the signalling frequency are present on the mains and if such currents are detected the initiation of the signalling operation is delayed. Conveniently in this case a visual and/or audible alarm is given and the application of the signalling currents to the network is preferably prevented until the interfering currents are no longer present. Once the signalling operation has actually commenced the parasitic current detection equipment is prevented from interfering with the superimposition of signalling currents, either by direct disablement such as disconnection from the network or by disablement of the control which it would normally effect on the signal superimposition equipment. One method of carrying the invention into effect will now be described with reference to the accompanying drawing which shows only sufiicient equipment to enable the invention to be understood. It is assumed that power is supplied to the system over the three-phase high tension conductors HTI, I-ITZ and H'I3 which are delta-connected to the primary of the power transformer T. The secondary of this transformer is star-connected to the low tension leads I, 2 and 3 and the neutral conductor N is connected to the star point. It is assumed also 2 that the various responding devices are connected between any one of the phase leads and the neutral conductor and that any known or suit-v able system of injection is employed which results in the signalling currents appearing at comparable amplitude on all three phases so that the responding units may be connected indiscriminately to any phase conductor. The actual injection equipment is indicated at IE and it is assumed that following known practice injection is made onto the low tension network comprising the leads l, 2, 3 and N and that'the signalling currents find their way via the high tension network conductors HTl, HTZ and HT3 to other low tension networks fed therefrom. An important element of the testing equipment shown is a three-electrode cold-cathode gas discharge tube GOT to the grid or starter electrode of which parasitic currents of the same frequency as the signal frequency are adapted to be applied and if of suificient amplitude cause the tube to strike. The main discharge circuit includes a relay TR which operates when the tube strikes and controls contacts TR! which prevent the initiation of the injection operation and may also give a visual and/or audible alarm to indicate to the control-officer that signalling to bring about the necessary switching operation is not taking place. The tube OCT is normally primed by way of the potential divider arrangements PA. and PB which for operation from 230 volt A; C. mains are preferably set so that during alternate half cycles of the supply current RMS voltages of the order of 120 volts and 60 volts positive with respect to the neutral lead N are applied to the anode and grid respectively of the tube COT, the cathode being connected to the neutral lead. Under these conditions the tube will not flash but it is suitably primed in readiness for the reception of any parasitic currents of the signal frequency. The supply to the tube CCT is shown extending over contacts 18] and 182 of an isolating switch which serves to disable the testing equipment whenever its use is not required and could conveniently be closed immediately prior to a signalling operation for instance by interlocking with the signal control switch. Assuming now that it is desired to effect a particular switching operation at the various responding devices, for instance the switching on of street lights, and that the necessary operation effected by a control officer also closes contacts ISI and 152, if parasitic currents are present on the mains the tuned circuit comprising the coupled inductors LI and L2 and capacitors Cl and C2 will permit a voltage to be applied to the grid of the tube CCT by way of the current limiting resistor YA. The comparatively small voltage between the neutral lead and the appropriate phase lead will be further increased by the step-up transformer action-of the coils LI and L2 and if the amplitude is sufficient, this voltage addedto the priming voltage will cause the tube CCT to flash on the first additive half cycle of the audio-fre-- quency parasitic current which occursduringa positive half cycle of the mains frequency supply. The tube thereupon remainssfiashed for the remainder of the positive half-cycle: of the mains frequency and operates relay TR; At the end of the positive half-cycle, the tube is extinguished and subsequently re-fiashes on the next-positive half-cycle and so on so long as the parasitic current persists. Relay TB. is shunted by metal rectifier MBA and therefore holds operated on the positive D. C. potential over contacts MSR3 to =2 operatethe lamp L and bell B to advise the control officer of the presence of parasitic currents at-this' time. Even if he should operate themotor'start switchMS however it will be ineffective owingto the opening of contacts TRI If switch MS'is operated inxthe absence of any parasitic currents at signalling frequency and above predetermined minimum amplitude, relay MSR'will operate and thereupon locks up over its contacts MSRI and contacts ON which are closed when the. motor M starts and remain closed until the end of the signalling cycle. The motor M is operated by the closing of contacts MSR2. Hence. once a signalling operation has been started the signal detection equipment is ineffective to disable its operation. As the isolating switch contacts'may remain closed during the signalling operation, relay MSR also at contacts MSR3 cuts outthe lamp L and bell'B so as to prevent an alarm being given by the operation-of the testing equipment in response to the signalling currents. It will be appreciated that thetube CCT itself canbe made to serve as. a visual indicator'of the presence of parasitic currents at'signal frequency. Potential divider PB could be calibrated in terms of either parasitic'or signal voltage; since the applied voltage which will flash tube CCT will depend on the normal priming voltage of the grid of this' tube. By means of changeover switches ll 3, which may be of the ganged multi-point type, different filter circuits can be interposed in the circuit of tube CCT to cater for circumstances where signalling involves differing frequencies. In place of the tube CCT use could also be made of a contact voltmeter connected to the network 4 in the manner described in Saville and Rosely U. S. application Serial No. 612,934, filed August 27, 1945, and arranged to operate a relay equivalent to TR when a predetermined pointer deflection is reached. I claim: 1 1. An electrical signalling system comprising a plurality of receiving devices connected to electric supply mains and responsive to currents of signalling frequency superimposed on said mains, asingle transmitter for generating currents of said signalling frequency, means for applying currents from said transmitter to said supply mains to effect the operation of said receiving devices, a three-electrode cold cathode gas discharge tube, meansfor subjecting the main electrodes of said tube to a suitable priming voltage immediately prior to a contemplated signalling operation, a circuittuned to the signalling frequency and connected to said supply mains, connections between saidtuned circuit andthe startingv electrodeiof said tube so that the output from said tuned circuit when currents of signalling frequency are: superimposed on said mains varies the potential. of said starting electrode to cause striking of said tube, a relay connectedin circuit with themainl electrodes of said tube and contacts controlled by' said relay for preventing a signalling operation from taking place when currents of signalling" frequency are present on said supply mainswhen; said signalling operation is initiated. 2. A signalling system according to claiml'inr: which said relay when operated. closes contacts; to complete a circuit forgiving an alarm. 3. A signalling system according: to claiml in: which if said relay is not operatedthe initiation: of a signalling operation completes .a' circuitior; a second relay which thereupon operates. and... completes a locking circuit for itself untilthev signalling operation is completed so as tube independent of the condition of said first relay' while the signalling operation is taking place: 4. A signalling system according to claim. 1 which the tube is mounted so that'when it strikes the glow is visible and serves as an alarm signal. WILLIE SAVILLE: REFERENCES CITED The following references are of record in the file of this patent: UNITED STATES PATENTS Lindsay et a1 Mar. 14, 1944'



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