Circuitry, apparatus and installations for controlling the supply of electrical power to a plurality of downstream loads having time graded protection are described. The output of an uninterruptable power supply (UPS) is connected to an automatic by-pass contactor. An AC-DC converter or rectifier is connected to the UPS output and is connected to an under voltage monitoring relay by a voltage limiting circuit arranged to limit the voltage applied to the under voltage monitoring relay. An operating coil of the bypass contactor is connected across a battery. When the UPS supply voltage falls to a level corresponding to a downstream short circuit, the under voltage monitoring relay allows under voltage monitoring relay contacts to operate and continue to allow the operating coil of the contactor to be connected to the battery thereby maintaining the contactor as connecting the UPS output to the outgoing power bus-bar. The voltage limiting circuit may comprise a Zener diode in parallel and a resistor in series. The device prevents rapid switching between the UPS and by-pass circuit. The system may be used to provide uninterrupted power to, for example, railway signals, motorway signs or hospital equipment.
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