The installation documentation for ArcSystem fixtures and drivers carries the following warning:
WARNING: Sense (normal) and Maintained (normal/emergency) feeds must have the same phase.
The Maintained and Sense lines must be provided by the same phase in order for a driver to properly track emergency state.
Consider the following scenario: an ArcSystem fixture's maintained line is powered from an SC1008 which handles switching the power from a normal source--on phase A--to an emergency source when normal power is lost. However, its normal sense line is powered from a breaker on phase B. In this case, if only phase A fails due to an errant rodent chewing through a feeder, the SC1008 will do its job and switch the power for the driver to the emergency source. If the normal phase B stays energized, the driver will detect that and remain in normal mode, which could present a life safety risk. The only way that the driver can detect a partial power failure which affects it is to have the sense line come from the same phase as its maintained line.