Volume Switches are provided on the stair side of the lobby door at each level. The switches are MK retractable and when pressed allow the fans to be switched between low and high speeds. The switches are connected to the interface units at each level, which are in turn connected to the fan control panel.
These are provided on the Lobby side of the Stair door at each level. They are mounted in the ceiling void in their own protective box. The function is to turn the fans on or off to maintain a lower pressure in the lobby. This in turn prevents the smoke logging of the escape stair. The pressure switches are connected together all the way up the building and link back into the fan control panel.