Patent GB-654231-A (GB)
Title: Improvements in testing and protection of electrical distribution systems
IPC: H02H7/00
Filed: 1944-08-09
Abstract: 654,231. Resistance measurements. SCHEIRMANN, R. Aug. 9, 1944, No. 15165. Convention date, July 19, 1943. [Class 37] The condition of the contact between conducting parts is permanently or intermittently supervised, tested, indicated, or controlled while they are traversed by the working currents. Fig. 1 shows a system for testing, indicating, and controlling the variations in the contact resistance between the electrode 3 and its holder 2 in an electrical melting furnace. The primary winding 11 of a voltage transformer 10 is connected between points 6, 7 on the holder 2 and electrode 3, the secondary winding 12 being connected to opposite corners of a bridge rectifier 22 through contacts 147, 150 which are normally closed by contacts 15, 17 of a controller in its normal position A. The rectifier output terminals are connected through a contact 27 and a range-changing series resistance 29 to the voltage coil 25 of a cross-coil ohm-meter 26. In the supply lead 164 to the electrode 3 is the primary winding 33 of a current transformer 32, the secondary winding of which is connected through an ammeter 36 and rectifier 42 to a variable shunt resistance 48, across which is connected the current coil 53 of the ohmmeter 26. The ohmmeter measures the ratio of the potential across the contact between the electrode 3 and holder 2 and the current through the electrode and hence indicates the resistance of the contact. Should this become higher than a predetermined limit the pointer 161 of the ohmmeter closes the circuit of a relay 56 through contacts 54, 59, closing the relay contact 64 and actuating an acoustic or optical signalling device 66. A time relay, resistance, and circuit-breaker may be connected to the signalling device 66 by leads 67, 68 in order first to reduce and then to cut out the current supply. A relay 130 in parallel with the voltage coil 25 opens the contact 27 in the event of excess voltage. The controller has test positions B, C for testing the contacts of the testing circuit. A separate A.C. source feeds a low-tension transformer 70, the secondary winding 72 of which is connected through a regulating resistance 73 and ammeter 75 to controller contacts 155, 156 which in position B of the controller are connected to contacts 147, 148 so that the testing current is applied to the terminals of the transformer winding 12, thereby inducing current in the winding 11. The resistance 73 is adjusted to give a predetermined reading on a voltmeter 77 when a known reading should be obtained on the ammeter 75 if the contacts are in order. In position C of the controller the contacts 155, 156 are connected to the contacts 148, 150 and the testing voltage is applied to the rectifier 22, the deflection of the ohmmeter pointer 161 being noted. Modifications are described for testing the contact between bus-bars. In one of these the bus-bar carries D.C. and the testing device is traversed by the working current, a separate D.C. source being employed for testing the contacts of the test circuit. In another the bus-bar carries A.C. and a separate D.C. is employed for testing, choking coils being provided to prevent the A.C. drop of potential across the contact from influencing the testing apparatus. In a third modification the bus-bar carries D.C. and is tested from a separate A.C. source. Fig. 5 shows a modification for the protection of a double-pole D.C. circuit-breaker 511, in which an auxiliary switch in the potential circuit of the ohmmeter prevents the testing instruments from being subjected to the full working voltage before the circuit-breaker has been thrown in fully or to excess voltage when the breaker is thrown out. The circuit-breaker comprises switch blades 503, 508 fed from leads 501, 506, the output terminals being shown at 504, 509. Across the output terminals 504, 509 is connected a solenoid coil 516 in series with a normally closed contact 513. Closure of the circuit-breaker applies the full voltage between the terminals 504, 509 and excites the coil 516 which pulls down its plunger and closes an auxiliary switch 519. The rod 539 of the solenoid is suspended by a pull-spring 526. A pivoted catch 527 can move upwardly only, being prevented from turning downwardly by a stop 529. On the solenoid 516 being actuated, the catch 527 meets the arm 530 pivoted at 531 and after having been turned upwardly, moves underneath the arm 530. Closure of the switch 519 connects the potential coil 25, Fig. 1, across the input and output terminals of one pole of the circuit-breaker through leads 205, 207, the current coil 53 being connected across a shunt 523 in the output circuit through leads 219, 232. If great deterioration of the circuitbreaker contacts occurs the leads 67, 68 are energized, as shown in Fig. 1, and open the relay contact 513 and interrupt the circuit of the coil 516, the spring 526 then opening the switch 519 so that the connection to the ohmmeter voltage coil is the first to be interrupted. In its upward movement the catch 527 turns the arm 530 and causes it to engage a contact 534. This closes the circuit of a trip coil and opens the circuit - breaker. Manually - operated switches may be similarly protected, the auxiliary switch being operated manually or mechanically, as by forcing down an auxiliary switch by the lever switch bridge. A modification for three-phase current is shown in Fig. 6. Hightension leads 601, 602, 603 are connected to contacts 604, 609, 639. The primary winding 616 of a voltage transformer 614 is connected to the breaker terminals of two phases on the output side of the high-tension oil switch. The secondary winding 619 of this transformer feeds the auxiliary low-voltage switch 519. When the main high-tension oil breaker switch is thrown in, the primary 616 is excited from the breaker terminals 606, 611 and the secondary winding 619 feeds the solenoid coil 516 closing the switch 519. This closes a circuit from lowtension leads 621, 624 through a solenoid coil 623, which closes a switch 628 and thereby applies the voltage drop across the contacts 604, 606 to the primary winding 633 of a voltage transformer 632, the secondary winding 636 of which feeds the potential coil 25, Fig. 1, of the ohmmeter through leads 13, 14. A current transformer 639 in the output circuit of the high-tension system feeds the current coil of the ohmmeter through leads 35, 43. Fig. 7 shows a transformer for transforming the low voltage across the primary winding 633 to the secondary winding 636 without allowing any high voltage that may arise to reach this winding and also keeping the magnetizing current of the primary winding within predetermined limits. The transformer has four limbs 701, 702, 703, 704. The limb 702 carries the primary winding 633 while the limb 701, which has a very small cross-sectional area, carries the secondary winding 636. A third limb 703 having a large cross-section is formed with an air gap 707. The fourth limb 704 having a large cross-section is formed with a large air gap 714 which is adapted to be closed by a core 710 when a solenoid 713 is excited from leads 67, 68. With a normal primary voltage of a fraction of a volt the entire flux traverses the small limb 701 because the large limbs with their air gaps at so low a density offer too great a reluctance. The dimensions of the small limb 701 are so chosen that at a few volts full saturation is obtained, thereby limiting the voltage that may arise in the secondary winding 636. At a predetermined voltage the coil 713 is excited through leads 67, 68 from the signalling contact 66, Fig. 1, and causes the core 710 to close the gap 714 in the limb 704, which then takes the greater part of the flux and prevents rise of voltage in the secondary winding 636. Specifications 654,287 and 654,290, [Group XXXV], are referred to.