UFC 3-535-01
17 November 2005
15-11.3
Figure 15-5 shows that measuring the voltage across the secondary circuit
for a 100W light fixture will yield 15.15V. Similarly, the voltage will be 30.3V with a 200W
light, and only 6.8V with a 45W light. This illustrates the much lower voltage that is
present on the secondary side of a lighting circuit. From the examples it can be seen
that:
15-11.3.1
The current is constant regardless of load (within limits), while the voltage
is additive and will vary with load.
15-11.3.2
The low voltage on the secondary side is isolated from the high voltage
primary side by the isolation transformer.
15-11.4
When more than one light is connected to a secondary circuit, they are
connected in series. An isolation transformer is designed to provide a proper variable
operating voltage on its secondary. This forces a constant load current to flow so that
each light in the secondary will receive the same current regardless of the wattage of
the lamp or the quantity of lamps (within the design limits of the transformer). This
approach may be seen on light bars for approach lighting systems, on runway centerline
lights configured as smaller segments with several lights on a single transformer, and
other applications.
15-11.5
It is important to recognize that the current transformer operates differently
than the standard voltage transformer. The current will remain constant even when the
secondary side is shorted. The output terminals of the CCR are not grounded, hence
the airfield lighting series circuit is an ungrounded system. The result is that one short-
to-ground in the primary circuit will not shut down the system. When the secondary side
is open (for example a burned out lamp), the instantaneous voltage present at the lamp
socket can be very high, presenting a potential hazard for maintenance personnel. This
can be the condition when changing lamps and a short occurs between the socket
terminals and the body of the light fixture.
263
Previous Page
