GB950596A - Time delay device - Google Patents

Abstract

950,596. Protective cut-out arrangements. McGRAW-EDISON CO. March 15, 1960 [March 19, 1959], No. 9114/60. Heading H2K. [Also in Division H3] In an overload protective arrangement an inverse time delay is provided by a capacitor 29 which is shunted by a rectifier 66 and a resistor 63, the capacitor only being permitted to charge when the voltage drop across the resistor 63 is raised in response to the current in the protected system 20 reaching a predetermined minimum value. In the arrangement shown in Fig. 2, the current derived from a current transformer 22 and rectified by a rectifier 24 causes transistors 75 and 76 to conduct, so causing a junction point F to have a potential proportional to the system current. When this potential exceeds the fixed potential of a junction point G determined by a Zener diode 72, a transistor 70 conducts and causes a further transistor 94 in series with the resistor 63 to conduct. This increases the voltage drop across the resistor 63 which prevents the flow of current through the rectifier 66 and permits the charging of the capacitor 29 through a resistor 36, transistor 40 and a resistor 47. As the capacitor 29 charges, the voltage at a junction point D rises causing the emitter currents of transistors 100, 101 to rise, with the result that the potential at point M connected to the base of a transistor 98 rises. The potential at the emitter of this transistor is maintained constant by a Zener diode 110 so that when the capacitor 29 has charged sufficiently for the potential at point M to exceed that at point P, the transistor 98 conducts causing the energizing of the relay 120 and the tripping of a main circuit-breaker 130. The power supply for the transistors and the relay may be derived from a battery, or as shown from a capacitor 30 which is charged from the current transformer to a potential determined by a transistor 44 and a Zener diode 45. The inverse time delay characteristics of the arrangement may be varied by altering the values of series resistor 47, shunt resistor 48 and the capacitor 29 as in the automatic reclosing circuit-breaker arrangement of Fig. 10 in which a number of rapid tripping operations are followed by a number of slower tripping operations before lock-out occurs. In this arrangement a known hydraulic counting arrangement 172 moves a bar 214 step-by-step on successive closing operations of the circuit-breaker 130, the movement of the bar controlling the operation of switches 224 to connect or disconnect an additional capacitor 229 and additional resistors 247 and 248 to vary the time delay. Lock-out is effected when an arm 260 on the bar 214 releases a lock-out switch 174 to open contacts 186 in series with the closing coil 176 of the circuit-breaker.