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
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).
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.