NEC rules on Disconnect Requirements for Ungrounded Conductors
Q. What is the Code rule regarding disconnects for ungrounded conductors of multi-wire branch circuits?
A. Each multiwire branch circuit must have a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates [210.4(B)].
Note: Individual single-pole circuit breakers with handle ties identified for the purpose can be used for this application [240.15(B)(1)].
Caution: This rule is intended to prevent people from working on energized circuits they thought were disconnected.