The disconnecting means for ESS installed in a separate room must be lockable in the __ position.

• A. Closed
• B. Open
• C. Middle
• D. Neither

Answer: (B)

The safety principle here is lockout of the disconnecting means that isolates an emergency electrical system during maintenance. When the disconnect is in a separate room, locking it in the open position guarantees the system stays de-energized and cannot be re-energized accidentally while workers are servicing it. The open position is the defined state that represents isolation, so a lock placed there prevents closing the switch without removing the lock. Locking in any other position would not reliably keep the ESS de-energized, and the middle position isn’t a standard, valid isolation state for most disconnects. Therefore, open is the correct requirement.