For certain occupancies, where the fire response is NOT immediate and total evacuation (airport, high rise), the notification devices must remain operable for 2 hours, along with anything else that facilitates their operation (Pathway Survivability Level 2). Normally you'd use VITALink or a similar product. In NYC, none of those products meet some of the other requirements of Local Law 39, so the city has crafted this alternative: Install the circuits as Class A AND separate not by 4 feet but by 15 feet, and we'll consider it the equal of Pathway Survivability Level 2. The exception is NOT supposed to use 15 foot separation and a 2-hour rated cable.