What must the disconnecting means do for a multiwire branch circuit?

For a multiwire branch circuit, the disconnecting means must open all ungrounded conductors. This requirement is crucial for ensuring safety during maintenance and repairs. In a multiwire branch circuit, multiple ungrounded conductors share a neutral conductor. If only a single ungrounded conductor is disconnected while others remain energized, there is a risk of restoring power to an energized circuit unexpectedly, which could create hazardous conditions.

Opening all ungrounded conductors ensures that the circuit is completely de-energized, allowing maintenance personnel to work safely. This practice is in line with the National Electrical Code (NEC) standards that govern electrical installations and safety practices, emphasizing the importance of ensuring that no live parts are exposed during service.