693,838. Protective cut-out arrangements. BRILL, E. C., and SCHNELL, J. G. Aug. 28, 1950, No. 21208/50. Class 38 (v) [Also in Group XL (c)] A system for the protection of an apparatus normally functioning at a constant load provides adequate compensation for changes in current due to variations in the supply voltage by comparing linearly two voltage components dependent on the line current and the line voltage respectively, one component varying hyperbolically with respect to the line voltage or line current from which it is derived. For example, neglecting changes in efficiency, the fine current taken by a motor under a constant load, i.e. having a constant power consumption, will vary hyperbolically with respect to the line voltage, so that if one voltage component varies in proportion to the line current, the other voltage component is varied hyperbolically with respect to the line voltage so that a direct linear comparison of the resulting voltage components may be made to determine when the line current and line voltage depart from their normal hyperbolic relationship. This enables the system to respond to small increases in power consumption without responding to increased line current solely due to reduced line voltage. In Fig. 1 the hyperbolic relationship between the voltage component derived from the line voltage and the line voltage is obtained in the transformer stage of the system. The component dependent on the line current is produced across a resistor 16 or 17 by a current transformer 10. The component dependent on the line voltage is produced by the opposing series connected secondary windings of transformers 11, 12 which have primary windings 22, 23 and 24, 25 respectively connected in series across the supply. Though the transformer 11 is a step down transformer while the transformer 12 is a step up transformer, the transformer 11 produces the predominating secondary voltage as the high impedance of the primary winding 22, 23 limits the current through the primary winding 24, 25 so that the flux of transformer 12 is small and the secondary voltage produced is low. As the core of the transformer 12 is working on the lowest part of its magnetisation curve, the secondary voltage increases at an increasing rate so that by the choice of suitable circuit constants, as described in detail in the specification, the resultant voltage across the secondary windings of the transformers 11, 12 varies hyperbolically with respect to the line voltage. The two voltage components are rectified by diodes 38, 30 and are applied across potentiometers 40, 33 which provide grid bias potentials, for amplifying triodes 101, 116. The anode circuit of the triode 101 is arranged to apply a negative bias to the screen grid 97 of a thyratron 90, the bias decreasing in a linear relationship with respect to the increase in the line current. The anode circuit of the triode 116 is arranged to apply a positive grid bias to the control grid 98 of the thyratron 90, the bias decreasing in a linear relationship with respect to the decrease in the voltage component which varies hyperbolically with respect to the line voltage. When the motor is under its normal constant load, the thyratron 90 is in a non-conducting state, as an increase in line current which results in a decrease in the negative bias of screen grid 97 is counterbalanced by a proportional decrease in the positive bias of the control grid 98 due to the drop in line voltage giving rise to the increased current. If, however, the power consumption increases, the decrease in the negative bias due to the increased current is not counterbalanced by a suitable decrease in the positive bias so that the thyratron 11 discharges and energises the coil 87 of a relay 82. The relay moves into its other position from that shown and completes a circuit from a supply terminal 75, through switch 81, relay 82, fuse 79, unit C to supply terminal 77 to energise the unit C which may be a circuit breaker, clutch or brake etc. to protect the motor or driven apparatus. To reset the system after operation, a remote reset switch E or an adjacent push switch 126 is operated to energise the coil 125 of a relay which moves its contacts 88 and 140 to insert a high resistance into the anode circuit of the thyratron and to increase the negative bias of the thyratron, so that the thyratron ceases to discharge. By depressing a push button switch 81 and applying an overload to the motor the system may be checked without operation of the unit C. A remote tripping or control device D may be used to operate the unit C. A transformer 45, double diode 46, capacitor 47 and glow tube voltage regulators 49, 50 in conjunction with the resistors and potentiometers shown provide the operating potentials for the valves 30, 38, 90, 101, 116. In a modification, Fig. 2, (not shown) the hyperbolic relationship between the voltage component derived from the line voltage and the line voltage is produced by operating the amplifier 116 on the non- linear part of its anode current-grid voltage curve.so that its anode current varies hyperbolically with respect to its grid voltage. In this case conventional current and voltage transformers are used. Reference is also made to the use of current, voltage or temperature sensitive resistance materials to supplement the action of the transformers or amplifying valves or to impart the desired characteristics thereto.