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SAFETY FOR LIFE

S260-75-1

The actuating level multiplier for the inrush restraint feature for phase faults is set by a jumper wire connected to a tab on the circuit card. Tabs are provided for X1, X2, X4, X6, X8 and BLOCK settings. The duration of the raised level is set by a knob on the top of the timer (Figure 18). The timer is infinitely adjustable 0.1 to 5.0 seconds. No settings are required for ground inrush since it is automatically blocked for the duration the phase level is raised.

Operating Instructions

The FAULT BLOCK OPERATED lamp on the front panel of the control, Figure 19, will light when the fault block accessory is activated. To reset the accessory:

1. Move the OPERATION SELECTOR switch (S3) to

MANUAL.

2.Depress and hold the RESET switch (S7) until the lamp goes out, (approximately 6-10 seconds) indicat- ing the accessory has reset.

3.To restore service, return the OPERATION SELEC- TOR switch (S3) to AUTOMATIC; the preferred source high voltage switch will close.

OR

Operate either the MANUAL OPER. SOURCE I switch (S1) or the MANUAL OPER. SOURCE II Switch (S2), as applicable, to restore service manually.

Note: Instructions for operating the fault block accessory also are printed on the front panel of the control (Figure 19).

PHASE CURRENT LEVEL

MULTIPLIER SETTING FOR

INRUSH CURRENT RESTRAINT.

GROUND-

ACTUATING

CURRENT

RESISTOR

CLIPS TO THE

PHASE CARD

Figure 20.

020078KM

 

Phase fault current actuating level printed circuit card.

Theory of Operation

—Fault Block Accessory

The fault block accessory prevents automatic closure into the alternate source when loss of preferred source results from a fault on the load tap. The schematic diagram for the fault block accessory, Figure 21, is used as reference throughout this discussion.

Overcurrent Sensing

The output of bushing current transformers in both the Source I and Source II high voltage switches are applied to the fault block accessory via terminals lA, IB, IC, IN and IX, IY, IZ, IN respectively. When the load current through the switch exceeds the phase or ground actuating level selected, the relay contacts in the fault block accessory (contacts 7-8 and 10-11 of relay RY1-B), located in the closing circuits of both high voltage switch- es, are opened. Subsequent closing of either switch is blocked in either the automatic or manual mode of the S control until the fault block accessory circuitry is reset.

Phase Fault Operation

Referring to the accessory schematic diagram, Figure 21, the phase currents are rectified by the diode bridges D103 through D116. Current out of the positive side of the bridge passes through diode D127 and divides into two parallel circuits. One circuit consists of zener D123 and the minimum phase actuating resistor R301 to the other side of the bridge. The other circuit consists of normally closed contact (1516) of latching relay RY1, diode D201, and capacitor C201 to the other side of the bridge. C201, charged by load or fault current, provides tripping energy for RY1.

The current through R301 produces a voltage drop proportional to the line current which is impressed across the series resistor string R101, R102, and R103. A portion of this voltage is applied through diode D120 to the base of transistor Q201.

When this voltage exceeds the break-down voltage of zener D212 and the base emitter junctions of Q201 and Q202, Q201 will conduct causing Q204 to conduct. In turn, Q204 provides the gating current to turn on the SCR (Q206) to discharge capacitor C201 through the trip coil of the latching relay RY1. Contacts 10-11 and 7-8 of RY1 (in series with the close coils of the high voltage switch- es) open to prevent the high voltage switches from closing until the fault block circuitry is reset.

An important feature of this accessory is that a trip signal cannot be initiated until there is sufficient charge on C201 to activate the trip coil of RY1. Transistors Q201 and Q204, effectively in series across C201, cannot conduct until the voltage across C201 is sufficient to break-down zener diodes D211 and D212.

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