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A stabilised voltage is applied to a resistance/capacitance circuit. The potential developed across the capacitor is applied to a transistor switching circuit which, in turn, operates an attracted Alstom Definite Time Delay Relay when the potential reaches the trigger level. The time delay is varied by altering the resistance value in the resistance/ capacitance circuit by a potentiometer calibrated in seconds. A circuit is incorporated to protect the transistors from damage due to high peak transients in the supply voltage. A diode is also fitted in the circuitry to take care of inadvertent connection of supply with reversed polarity. Type VTT 11 units have a delayed pickup, i.e., timing is started by closing an initiating contact and the output relay is energised at the end of the set time. The unit resets when the initiating contact is reopened. Type VTT 12 units have a delayed dropoff, i.e., the output relay is normally energised. Timing is started by opening an initiating contact and the output relay is deenergised at the end of the set time. The unit resets when the initiating contact is reclosed.