GB977385A - Improvements in or relating to electrical warning systems - Google Patents

Abstract

977,385. Bridge measurements. ELLIOTT BROS. (LONDON) Ltd. Aug. 23, 1961 [May 23, 1960], No. 18169/60. Heading G1U. A Wheatstone bridge, particularly for measuring temperature, is first balanced as the temperature reaches a predetermined level; it is then unbalanced at this level and a first warning lamp operated, the bridge being again balanced when the temperature reaches a second level a fixed amount higher than the first and a second warning lamp is then operated. The bridge arms comprise fixed resistors 4, 5, 6, a variable resistor 3, a resistance thermometer R T and a relay coil A1, the coil M of a movable coil instrument 9 being connected across the measuring diagonal. On the temperature reaching the first level, pointer contact 10 of instrument 9 engages and is magnetically latched to a fixed contact 12 which energizes relays a relay B which locks over its contact B/2 primes a relay coil C at B/1, discharges capacitor 23 over B/3 through a coil 22 to provide a pulse to release contact 10, lights a warning lamp L 2 over B/4, and unbalances the bridge by disconnecting resistor 5 at B/5 from one arm. If the temperature now rises to the second level, the contacts 10 and 12 will again engage to operate relay C, which at contact C 1 will light a further warning lamp L 3 . A fault occuring in R T will cause an unbalance in the current through the coils A1, A2 of a double wound normally balanced relay. The relay contact A/1 will then operate to energize a relay D which locks over its contacts D/1, energizes a further warning lamp L 1 over D/3 and at D/2, prevents operation of lamps L 2 , L 3 . Keys SB1, SB2, SC, SA are provided for pulsing the release coil 22 and resetting the system to normal. In a similar embodiment the moving coil M is protected by being short circuited under fault conditions, the resistance thermometer R T , is connected in circuit through leads which compensate for temperature variations, and the keys SB1 &c., are connected in the circuit at different points.