Questions and Answers on Overcurrent Control for Vista® UDS

The overcurrent control for Vista® Underground Distribution Switchgear is a full-featured control offers exceptional flexibility and reliability, and includes features unavailable in other manufacturers’ devices — or available only at substantial cost. This article answers commonly asked questions about this overcurrent control.

Can Time Current Characteristic Curves (TCCs) below 25E be provided?
The Vista overcurrent control is self-powered by the current sensed from the current transformers installed internal to the Vista tank. The overcurrent control requires a minimum of 50 Amps of current (either continuous current or fault current) to power the control and provide a trip signal. Below a 25E curve, the control will never see above the minimum current of 50 Amps. However, the external trip provision allows the use of customer-supplied relays, remote power, and current transformers to expand the available TCC curves and relaying schemes.
Is there a way to verify the actual curve shapes and response times of the Vista TCC curves?
It is possible to verify these curves by removing the overcurrent control from the Vista unit and using a relay test set. However, it is emphasized that this is of little value since S&C has already verified the performance of the relay and Vista Overcurrent Control.
Can the overcurrent control be programmed when Vista UDS is de-energized?
Yes. The Vista Overcurrent Control will receive power from the PC in this instance.
Can actual phase currents be displayed?
Yes. The personal computer used to program the overcurrent control will display the actual phase current values at the instant the control is interrogated. Continuously updated real-time values are not provided. Incidentally, in the event of a fault, the event log will record the currents on all phases at the time of the fault.
Can the current transformers for the overcurrent control be used for SCADA applications?
No. They can only be used to power the overcurrent control and provide the sensing input for tripping the fault interrupters. SCADA applications require additional Current Transformers (CTs) or current sensors.
Can the overcurrent control be moved to a different location in the vault or pit in subsurface applications?
Yes, the control can be removed from the tank. The standard wire pass is 6 feet long, allowing the control to be moved up to six feet from the Vista tank. A longer wire pass can be provided, however, the desired length will have to be evaluated on a case by case basis due to the voltage drop considerations between the control and the Vista tank.
Is there a “trip button” on the overcurrent control for manually tripping the interrupters?
The fault interrupters can be manually opened using the operating handle. In addition, a “test trip” feature is available when a personal computer is connected to the overcurrent control for programming. If there’s requirement for manually tripping open the fault interrupters from a remote location (i.e. a “fireman’s switch”), it can be accommodated with the external trip option and a user-supplied trip signal.

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