Patent GB-1504595-A (GB)
Title: Ground fault detection apparatus
IPC: H02H3/33
Filed: 1975-04-10
Abstract: 1504595 Phase-sensitive detectors; transistor switching circuits RCA CORPORATION 10 April 1975 [15 April 1974] 14748/75 Headings H3A and H3T [Also in Division H2] Phase-sensitive detector.-A phase-sensitive detector (left half of Fig. 1), providing a D.C. output current proportional to the amplitude of that component of an input current I SEC  which is in phase with a reference voltage, has a push-pull configuration arranged to provide zero input impedance. The standing currents maintained through diodes 51a, 61a are equal and sufficiently large relative to input currents I SEC  to be detected (applied between the anodes of the diodes) that the voltages across these diodes always remain substantially equal, thus effectively presenting zero impedance to I SEC . Any input current will, however, alter the conductivity of transistors 51b, 61b in opposite senses, and these in turn each control one transistor of the pairs 75, 77 and 76, 78. Square wave gating pulses in phase and 180 degrees out of phase with the reference voltage are applied to transistors 73b, 74b respectively so that in, say, positive half cycles of this voltage only transistor 73b, and hence the pair 75, 77, is conductive. During these half cycles inverter network 80 supplies to an output terminal 48 a current equal to the collector current of transistor 75, whilst, transistor 79 draws from this terminal a current equal to the collector current of transistor 77. Thus the net output current at any instant is proportional to the unbalance between transistors 75, 77, and hence to the instantaneous valve of I SEC . The mean current over the half cycle will be zero for any component of I SEC 90 degrees out of phase with the reference voltage, but will be non-zero for any in-phase component and proportional thereto (and positive or negative depending on whether the phase relationship is 0 or 180 degrees). During negative half cycles of the reference voltage transistor pair 75, 77 is inoperative but the pair 76, 78 is conductive, and since the connections between the latter and network 80/transistor 79 are interchanged any mean output current will be of the same sign as during the positive half cycle. Current threshold detector.-A current threshold detector (right half of Fig. 1), providing an output when the absolute value of an input current, (which may be positive or negative) exceeds a threshold, utilizes two transistors 101, 102, whose emitter currents are maintained constant and whose c-b junctions tend to be forward biased in the absence of any input, to prevent the charging of a capacitor 24 by the input until the threshold is exceeded. Constant current sources 105, 106 maintain the emitter currents constant and equal. A bias network 130 maintains the bases of transistors 101, 102 and one terminal of the capacitor 24 at half the line (123) to earth potential. The collectors of transistors 101, 102 are interconnected by seriesconnected diodes 111, 112 whose junction 48 constitutes the input terminal. With no charge on capacitor 24 and no input current one or both transistors 101, 102 will be in, or nearly in, saturation. When an input current occurs there will initially be a slight increase in voltage across the capacitor 24 which will forward bias the c-b junction of one of the transistors 101, 102 still further, thereby reducing its collector current. The other transistor will now be biased into normal operation but will still be demanding a collector current equal to its emitter current, which it will derive from the input current. Only when the input current exceeds this demand will capacitor 24 charge further. Once the capacitor charges transistor pair 113, 115 or 114, 116 will switch on to cause transistor 119 to supply a firing current to a thyristor 25.