Patent GB-749442-A (GB)
Title: Improvements in or relating to apparatus for indicating abnormal operation of one or more electrical devices within a group of such devices
IPC: H04M3/08
Filed: 1953-01-21
Abstract: 749,442. Unattended repeaters. TELEFONAKTIEBOLAGET L.M. ERICSSON. Jan. 21, 1953 [Jan. 21, 1952], No. 1769/53. Class 40 (4). [Also in Group XXXVII] Apparatus for indicating abnormal operation of one or more electrical devices within a group of such devices comprises indicating equipment IND responsive to changes in the impedance between control wires m, n which terminate in a fixed impedance Z and to which control means K individual to each device APP are connected in series and in parallel. The control means K have non-linear impedance characteristics and are arranged so that each control means connected in series with the wires m, n presents a very high impedance to the control wires in response to an abnormal current or voltage condition in the associated device, while each control means K connected in parallel presents a very low impedance to response to such a condition. The apparatus may be used for supervising unattended repeater stations. Each control means K may consist of an indirectlyheated thermistor having its heater energized by the current to be supervised. The control means may also comprise saturated reactors, Figs. 6 to 8 (not shown), having their D.C. saturating windings energized by the current to be supervised and their A.C. windings connected in series with, or in shunt to, the control wires. By providing a constantly energized bias winding, indication may be obtained for variations of the supervised currents above and below a normal value. In further arrangements, the non-linear properties of rectifiers (e.g. crystal diodes, hot cathode tubes and copper oxide or selenium rectifiers) are used. In Fig. 11 the terminals 3, 4 are connected to the supervised device and the terminals 1, 2 to the control circuit. When the voltage U across the terminals 3, 4 ceases to exceed that, Eo, of a fixed biasing source, the rectifier L becomes conducting and presents a low impedance to the control circuit. If the polarity of the rectifier is reversed and a suitable biasing potential is used, then the rectifier L becomes conducting when the voltage of the supervised device exceeds a normal value. By using two oppositely poled rectifiers the control means may present a low impedance to the control circuit should the voltage of the supervised device vary above or below a normal value, Figs. 12, 15, 16 and 17 (not shown). Figs. 18 and 19 (not shown) illustrate arrangements in which more than one voltage of a supervised device may be supervised. The indicating equipment, Fig. 20 (not shown) may comprise an oscillator which supplies an alarm device under the control of an attenuator connected to the control circuit, the impedance of the control circuit determining the extent of the attenuation. The oscillator may provide signals of several frequencies in which case a number of series-connected frequencydependent attenuators may be used, each attenuator being connected to a control circuit, Fig. 21 (not shown). Figs. 22 to 25 (not shown) illustrate various arrangements in which the indicating equipment is associated with line amplifiers.