The B-Zone Power Grid was an apparent alternative power grid used in Haven City during Jak II. It was known to be inactive for years until Jak activated the power boxes guarded by Krimzon Guard turbo cannons in the mission "Turn on five power switches". It was also linked to other Haven City technology such as the Shield Wall indicator helix and resonant flux routes, and the GX75B, as mentioned by Vin.
The elevators to the Haven Palace support towers were powered by the B-Zone grid. If Jak wanted to access the Palace, he would have to sneak in via climbing the support towers, information given to him by Keira. Therefore, he asked Vin to help him in gaining access to the elevators. It is unknown if the B-Zone grid remained active after Jak's utilization of it, though it remains unlikely given its status as old and obsolete, and the eventual shutdown of the eco grid altogether.