Cut=out circuit for three=phase transformer protection - has three transformers with resistors and thermistor associated with capacitor and solenoid

摘要

The cut-out circuit is for contactors esp. those providing protection for the low voltage side of polyphase transformers used in electrical power distribution and particularly those employed in rural areas. The cut-out also minimises heating at the contactor proper. Current level and balance in each phase is detected by a current transformer (2, 3, 4) within an insulated housing (1). Each transformer secondary is shunted by a low value resistor (8, 9, 10). The three resistors are spaced symmetrically around a single central heat sensitive element (11). This forms one part of a potentiometer (27, 11) controlling the gate bias of a threshold detecting thyristor or similar semiconductor. The voltage from each resistor also determines the charge on a capacitor (19) via rectifying diodes (13-18) and a further resistive, zener diode potentiometer (20-23) controlling the supply voltage across the thyristor.