Figure 6-31. - Vault grounding arrangement.

be talking primarily about high-voltage systems most of the time, the functions of the components will apply to either system.

In the 2,400/4,160-volt system, the four-wire wye primary source is usually from the base electrical system by means of either an overhead or an underground line. Inside the vault, the lines are connected to a suitable switch, then to a bus system consisting of heavy metal bars that are supported on insulators. This bus system may be mounted on either the wall or the ceiling.

The bus is divided into a high-voltage (2,400-volt) bus and a low-voltage (120/240-volt) bus for service as follows: the 2,400-volt bus supplies all of the 2,400- volt regulators and one or more distribution type of transformers. The distribution transformers supply 240 volts to the low-voltage bus that is connected to the regulators operating from this lower voltage as well as for light and power inside the lighting vault.

Where an emergency power supply is available for airfield lighting, a changeover switch makes the primary connection to the bus. This changeover switch in its normal position connects the bus to the base power source. Changing the switch to the emergency operation position connects the bus to the emergency power and, at the same time, disconnects the base power source.

Emergency power can be supplied by a completely automatic engine-driven generator; for example, failure of the base power causes the engine to start. In a matter of seconds, the changeover switch automatically shifts to the emergency position, connecting the generator to the airfield lighting bus.

At many advance bases, this automatic feature may not exist. You would have to hook up the proper sized generator manually. The generator should have a kilowatt (kW) rating capable of handling the airfield lighting systems, runway edge lights, threshold lights, approach lights, distance markers, optical launching system (OLS), and other circuits that may be used. The generator is three phase; its voltage output varies from 120/240 volts delta or 120/208 volts wye to 2,400/4,160 volts, and it has to be capable of being operated at frequencies of 50 or 60 hertz (Hz).

Constant-Current Regulator

Runway lighting systems are supplied from series circuits served by constant-current regulators (CCRs). Each lighting circuit on the airfield has a separate regulator. The CCRs maintain the output current throughout its rated output value, depending on the load. Some of the regulators are equipped with brightness controls. These brightness controls adjust the brightness of the lamps in the lighting system to compensate for visibility conditions.

The CCR uses solid-state devices to maintain a constant-current level in its respective lighting system. The regulators are silicon-controlled rectifiers (SCRs) in a feedback circuit to obtain a constant-current output instead of resonant circuits, moving transformer elements, or saturable reactors. The SCRs are controlled to vary the part of a cycle during which the current is permitted to flow into the load circuit. In the load circuit, the current is maintained constant at any value preset with the brightness control by means of a feedback circuit as the load resistance is varied from maximum to zero. The block diagram (fig. 6-32) shows the elements constituting the regulator. Load current is measured by the current transformer and the Hall unit, or multiplier unit, that has an output voltage proportional to the square of the load current. The Hall unit, or multiplier, output is filtered and fed into the input of an amplifier and compared with an input from a brightness control potentiometer. The output voltage is a function of the difference between the two inputs.