Both the tower and vault control panels are wired into a double-throw "transfer-relay cabinet" located in the vault. That is shown in figure 6-34 with a single line representing the control cable. The transfer relay can connect either control panel to the pilot-relay cabinet. It can switch the system control from the tower to the vault or from the vault to the tower. The transfer relay has an eight-pole, double-throw, transfer-relay assembly unit. This unit is actuated by a toggleswitch.

The low-burden pilot relay is designed to operate at a wide range of voltages lower than the designed l20-volt ac rating. The pilot relay can be actuated at voltages from 50 to 90 volts ac.

The standard control cable is a No. 7 conductor, 600-volt, insulated, polychloroprene-sheathed cable. One conductor (black) is a No. 12 American Wire Gauge (AWG), and the remaining conductors are No. 16 AWG. The No. 12 conductor is the hot lead, and the No. 16, the "switch legs."

LIGHTING CIRCUITS

Several different lighting circuits are used on air- fields: runway edge lighting circuits, taxiway lighting circuits, approach lighting circuits, obstruction ligh- ting circuits, beacon lighting circuits, and the

Figure 6-34. - Airfield lighting control system