It's explained in the CE Code Handbook. I'll copy it word for word if you don't have the book:
"If an exit sign is required by a building code and is electrically illuminated, Rule 46-400 requires that the power for the sign be provided by
- a branch circuit that supplies only other exit signs; or
- a branch circuit that supplies emergency lighting.
However, in an area where exit signs are installed and where emergency lighting is required by the NBCC or local building codes, exit signs must be illuminated by an emergency power supply only. This ensures that the exit sign provides light whenever the emergency lighting is energized."
What we've done in the past is connect the combo packs (exit/emerg. lights) to their own circuit and run the circuit through NO contacts on a definite purpose contactor. The contactor coil is held in by the lighting circuit (before any switching). If the power goes off or the breaker for the lighting trips, it will drop out the contactor and the emergency/exit lights will come on.
By installing it this way, you're able to keep the exit signs on their own circuit yet have them activated by any loss of power to the lighting circuit. Hope this helps.