Abstract: A circuit arrangement for providing in a fail-safe manner an alternating output signal to a load, e.g. in a railway signalling system. The load is connected across a first winding of a transformer, to opposite ends of a second winding of which transformer is connected a bi-directional current switching circuit. First and second modulated signal sources are connected to the bi-directional switching circuit. Each of the signal sources modulates a carrier signal with a modulation signal, the outputs of the signal sources being out of phase with respect to each other. The bi-directional switching circuit is operative to switch current through the second winding alternately in opposite directions at the frequency of the modulation signal. The circuit arrangement is such that if any of its components fails to operate correctly, the output appearing across the first winding is no greater than a threshold level.

Abstract: A high-speed current limiting circuit breaker comprises a self-recovery type current limiting element for limiting a fault current, a parallel resistance connected in parallel with the self-recovery type current limiting element and a circuit breaker connected in series with the self-recovery type current limiting element, a fault voltage detector for detecting the voltage generated on the parallel resistance and thereby driving an electromagnetic repulsion coil for breaking the circuit breaker.

Abstract: An overload protection circuit is disclosed which limits the current supplied by a battery to a load. The circuit is particularly adapted for use in portable radio applications which must meet rigorous specifications of intrinsically safe equipment for use in hazardous environments. Load sensor 20 continuously monitors for the presence of load to enable power switch 16 via switch control 22. Current trip circuit 24 monitors the voltage developed across current sensor 18 to provide a disable signal to switch control 22 in the event of an overload condition. Current limit circuit 26 limits the drive signal to power switch 16. Voltage trip circuit 28 prevents overdissipation of the power switch device. Thus, intrinsically safe battery circuit 30 provides protection such that hazardous gases will not be ignited by sparks or thermal heating during overload conditions.

Abstract: A microprocessor based trip unit for a circuit breaker is disclosed. The instantaneous current magnitudes in each phase and in the neutral of the circuit being protected are simultaneously and sequentially sampled by respective sample and hold circuits. The measured values are then applied to a respective analog-to-digital converter and the digitized values are sequentially applied to a microprocessor. The microprocessor calculates true RMS current and sums the signals of all phases and neutral to determine presence and magnitude of a ground fault current. To permit use of an 8-bit microprocessor, while maintaining high resolution of measured current over a large range, novel automatic scale adjustment is provided. The memory of prior thermal history of the circuit breaker is provided electronically by a capacitor circuit which is discharged at a rate representative of the rate of cooling of the circuit breaker.