A transformer secondary tie is a circuit opening at ___ volts or less between phases that connects two power sources or power supply points.

• A. 600
• B. 1000
• C. 12000
• D. 35000

Answer: (B)

The key idea is the voltage rating of the switch that opens the tie between two power sources feeding the transformer secondary. A transformer secondary tie is there to connect or isolate two sources, so the device that breaks that circuit must be able to interrupt the current up to a certain line-to-line voltage. In typical practice for this kind of tie, that interrupting capability is rated at up to 1000 volts between phases. That means the tie device is designed for systems where the phase-to-phase voltage does not exceed 1000 V; if higher voltages are involved, you’d need equipment rated for those higher voltages with appropriate clearances and protections. The other numbers correspond to voltages outside the standard low/medium-voltage range used for transformer secondary ties, so they don’t fit the intended rating.