Ground Fault Operation
The ground current (the vector sum of the phase cur- rents) flows through the ground actuating resistor, R306, to produce a voltage across the primary of input transformer TR1, proportional to the ground current. This voltage is stepped-up, rectified, and impressed across the series resistor string R104, R105, R106 and R107. A portion of this voltage is applied to the base of Q201 through diode D119. From this point, ground and phase operations use common circuitry described in the preceding “Phase Fault operation” discussion.
Reset Circuits
Once actuated, the fault block accessory can be reset in either of two ways.
•Manually— Actuating the Reset Switch, S7, on the front panel of the S control, will apply 120 Vac to ter- minal G of the fault block accessory tie board. This voltage is rectified by diode D205 and applied to the reset timing circuit, which consists of R201, C206, and the programmable uni-junction (PUT) Q205. This voltage is also applied to capacitor C202, which pro- vides energy to the reset coil of latching relay RY1. When the voltage across C206 reaches the break- down voltage of the PUT, transistor Q203 is then turned on causing C202 to discharge through the reset coil of RY1. The relay transfers and the fault block accessory is now reset.
Note: S7 is energized only when the Operation Selector switch is in MANUAL.
•Automatically— by load current below the minimum actuating level flowing through the high- voltage switch. The voltage developed by the load current is applied through contact 16-17 of RY1 to the reset timing circuit R207, C206, and Q205.When the voltage across C206 reach- es the breakdown voltage of the PUT, Q203 is then turned on causing C202 to discharge through the reset coil of RY1. The latching relay transfers and the fault block accessory is reset.
Note: If the load current is above the actuating level of the accessory, both manual and automatic reset is defeated by Q202 which conducts to short-out the reset timing capacitor C206 through resistor R205.
Inrush Restraint Feature
The inrush restraint feature raises the actuating level of the fault block accessory by a multiple (or blocks its operation entirely) for a period following restoration of source voltage or an automatic open transition transfer operation, to prevent inrush currents from activating the fault block.
Relay RY2 is normally energized by the phase B or phase Y voltages from the S control via the “a” auxiliary contacts in the two high-voltage switches. When both B and Y phase voltages are lost due to operation of a backup device or opening of the high-voltage switches, the coil of RY2 is de-energized, closing its contacts. Contact 1-4 closes to insert a paralleling resistor across the phase actuating resistor R301, reducing its effective value and increasing the line current necessary to actuate Q201. Contact 5-8 of RY2 closes to short-out the secondary of TR1, rendering the ground section operative.
Upon re-energization of either Phase B or Phase Y through closing of the backup devices or the closing of the high-voltage transfer switches, relay RY2 is energized. RY2 has a built-in time delay programmable by means of a knob in the top of the relay case. After the selected time delay has elapsed, N.C. contacts 1-4 and 5-6 open, returning the fauIt block accessory to normal operation.
