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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 trans-
former TR1, proportional to the ground current. This volt-
age is stepped-up, rectified, and impressed across the
series resistor string R104, R105, R106 and R107. A por-
tion of this voltage is applied to the base of Q201 through
diode D119. From this point, ground and phase opera-
tions 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 oper-
ation 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 back-
up 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 actu-
ate Q201. Contact 5-8 of RY2 closes to short-out the sec-
ondary 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.
Type S Automatic Load-Transfer Control Installation, Operation, and Maintenance Instructions
32
Figure 22.
Test circuit for testing and troubleshooting the fault block accessory.