Type S Automatic Load-Transfer Control Installation, Operation, and Maintenance Instructions

FAULT BLOCK ACCESSORY

(Applicable to VR, VLR, VRV, TSC, CS, and PST-6 switchgear.)

The fault block accessory is mounted in the upper right corner of the back panel of the control, Figure 18. Controls and instructions for operating the accessory are located on the front panel above the input fuses, Figure 19.

General Description

In response to fault current above preselected phase or ground levels, the accessory is activated and latched to disable both high-voltage switches. Thus a load-side fault followed by loss of potential will result in partial load transfer opening the connected source but not closing the faulted load into the alternate feeder.

The accessory must be manually reset at the control panel to restore service to the load. If activation of the fault block accessory is due to a temporary fault (pre- ferred source voltage is restored before the transfer delay timer runs out), line current of 5 Amps, or more, flowing through the high-voltage switch will automatically reset the accessory in approximately 10 to 15 seconds.

To prevent the fault block accessory from being activated due to inrush current that may occur as a result of the backup opening and closing, an inrush restraint feature is built into the accessory logic. Upon loss of the preferred source voltage, the inrush restraint operates to increase the phase fault actuating level by a predetermined multiple for a predetermined time after voltage is restored. Simultaneously, ground fault current detection is blocked completely for the same time interval. When the time runs out, both the phase and ground current actuating levels return to their normal values.

To operate the fault block accessory, the Source I and Source II high-voltage switches must be equipped with 1000:1 ratio current transformers to monitor the magnitude of the line current and an auxiliary “a” contact. The bushing current transformers and associated wiring are provided as a factory-installed accessory to the high-voltage switch.

ACTUATING LEVEL PRINTED CIRCUIT CARD (FIGURE 18)

TERMINAL BLOCK FOR EXTERNAL CONNECTIONS

RAISED FAULT LEVEL DURATION TIMER

Accessory Settings

The selected phase and ground fault actuating levels should be greater than any peak load phase current or zero-sequence (ground) current, but less than the trip setting of the backup protective device. A general recommendation is to set the phase and ground actuating levels to approximately 80% of the phase and ground trip levels of the backup.

The phase fault current minimum actuating level is determined by a plug-in circuit card, Figure 20. Cards are available for 80, 112, 160, 224, 320 and 448 Amp actuating levels. The ground fault current minimum actuating level is determined by a resistor cartridge that clips on to the phase card. Cartridges are available for 10, 14,

Figure 18.

020080KM

20, 28, 40, 56, 80, 112, 160, 224,

Fault block accessory mounted in upper-right corner of the back panel.

INSTRUCTION LABEL Provides instructions for resetting the fault block accessory.

RESET SWITCH (S7) Reactivates the S control after a fault block operation.

Figure 19.

Fault block accessory controls and operating instructions plate.

320 and 448 Amp actuating levels.

FAULT BLOCK OPERATED SWITCH Indicates that the fault block accessory has operated.

020077KM

28