With the START PROTECTION switch in the ON position, BATT RLY switches ON, and DC external power applied, the _______ light will extinguish indicating parallel operation.

• A. BATT RLY 1
• B. BATT RLY 2
• C. BTC CLOSED
• D. FAIL

Answer: (C)

Parallel operation happens when the battery and external DC sources are connected to the same bus, sharing the load. The Battery Tie Contactor (BTC) is the device that links the battery side to the external power side. When START PROTECTION is ON, the battery relays are energized, and DC external power is present, the BTC closes to tie the sources together. The indicator labeled BTC CLOSED goes out (extinguishes) to show that the BTC is closed and parallel operation is active. This makes BTC CLOSED the best indicator of parallel operation.

The other indicators don’t specifically signal that the sources are paralleled: BATT RLY 1 and BATT RLY 2 show the state of each individual battery relay, not the interconnection state, and the FAIL light would indicate a fault condition rather than normal paralleling.