UFC 3-535-01
17 November 2005
isolation transformer. The 20A output regulator is slightly more expensive than the 6.6A
output regulator. Therefore, the cost of a 20A system for any particular circuit will be
greater than the cost of a 6.6A system for that circuit. Proper consideration of actual
operating requirements in the design may show that a 30KW regulator with a 6.6A
output will provide 25KW, which will be sufficient to power a circuit up to 2,440 meters
(8000 feet) with runway centerline lights, edge lights, or touchdown zone lights.
However, if more than one 6.6A circuit is required to power a particular airfield lighting
system, for example, a 3,660 meter (12,000 foot) runway edge lighting system, two or
three 6.6A systems would be required, which would have higher initial cost than a single
20A system. Cost should, however, be considered over the operating life of a system.
One factor is the energy costs, which would generally be higher for the 20A system than
for the 6.6A system.
15-12.6
It is clear that there is no simple process for the selection of which system
to use in any particular case. No compromises should be made when it comes to safety.
However, the designer needs to consider the operational requirements of the airbase
and the airfield lighting systems, the geometrical layout of the proposed circuits, and the
soil conditions for the cable installations. The economic evaluation should include
installation costs, operating costs, and life expectancy.
15-13
CONSIDERATIONS FOR CIRCUIT DESIGN
Two major considerations in designing an airfield lighting series circuit are: maximum
allowable voltage on the primary series cable (discussed in paragraph 15-12 in the
selection of 6.6A or 20A circuit); and the insulation resistance of the entire primary
circuit.
15-13.1
FAA has established a minimum insulation resistance of 50 megohms for
new installations. Experience from many projects has shown that this value is
achievable. However, insulation resistance depends on the length of the primary series
cable, the number of splices on the circuit, soil conditions, and primarily the number of
isolation transformers of the circuit. The designer should therefore consider all these
factors in configuring a lighting circuit.
15-13.2
From the standpoint of economy, life expectancy and reliability, it may be
practical to use 50 megohms as the minimum insulation resistance for. The selected
value depends on the specific configuration of the particular system being designed. It is
recommended the following criteria be applied in optimizing design of new airfield
lighting circuits:
15-13.2.1
Limit the voltage to 2400 volts, at least for 90 percent of the operational
time.
15-13.2.2
Limit the number of isolation transformers on any one primary circuit to not
more than 130.
15-13.2.3
Limit the number of splices on any one primary circuit (not including those
connecting isolation transformers) to not more than 8.
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