UFC 3-535-01
17 November 2005
intensity level system, although it can be used on circuits which operate on three
intensity levels, provided that the three levels are the same as used on the 6.6 amp
system (4.8A, 5.5A and 6.6A), as measured on the secondary side of the isolation
transformer. Nevertheless, the two systems have somewhat different characteristics
and the preference for one or the other depends on the specific operational application
and the geometric configuration of the airfield lighting circuits. To determine the
preferred system for a particular design application, it is necessary to evaluate the
system characteristics of each system from the standpoint of safety, reliability and
economics.
15-12.1
The 6.6A primary series circuit generally consists of No. 8AWG 5kV non-
shielded cable, connected in series with isolation transformers having a 1:1 operating
ratio. This provides 6.6A on the secondary to the lighting component from 6.6A on the
primary side. The 20A primary series circuit consists of No. 6AWG 5kV non-shielded
cable, connected in series with isolation transformers having an operating ratio of about
3:1. This provides 6.6A on the secondary to the lighting component from 20A on the
primary side.
15-12.2
Based on the standard 5kV rating of the primary cable, it can be seen that
the 6.6A system can be powered by a regulator with a rated capacity of not more than
30KW. (30KW/ 6.6A = 4545 volts) The 20A system can be powered by a regulator with
a rated capacity of 70KW, the largest regulator currently manufactured, without
constraints (70KW/ 20A = 3500 volts). Cable losses (amperes2 x cable conductor
resistance) for the cable typically used in a 20A system will be as much as four times
higher than those for the cable used in a 6.6A system (approximately 164 watts/1000
feet compared with 37 watts/1000 feet).
15-12.3
For a given load, the regulator output voltage for the 6.6A system will be
more than 3 times higher than the output voltage for a 20A system. In the example
above for a 30KW load, the output voltage for a 6.6A system is about 4545V. For the
same load on a 20A system, the voltage is only 1500V. The stress on the primary cable
insulation as well as the cable connectors will be greater when the voltage is higher,
resulting in earlier cable deterioration and greater susceptibility to failure. With voltages
above 2400V, the electric field may not be entirely confined within the cable insulation
when unshielded cable is used. This can lead to potential problems such as surface
discharge and surface cracking at terminations. Such surface discharge and cracking
can lead to further deterioration and possible cable failure. These problems are
particularly likely to arise when the cable is installed in wet or contaminated locations.
15-12.4
It is recognized that most of the system operation, whether 6.6 or 20
amps, is not usually at the highest intensity level, thus not at highest loading. Most
systems are typically operated at mid-level intensities (level 2 for 3-step systems or
level 3 for 5-step systems - about 4.1A). Thus, a 5-step circuit with a load of 25KW at
full intensity will only be loaded to about 9.6KW at intensity step 3. At 9.6KW output at
4.1A, only 2353V is imposed on the primary cable, well below the rated capacity.
15-12.5
In a cost comparison, No. 6 AWG cable is more expensive than No. 8
AWG. The 20A:6.6A isolation transformer is more expensive than the 6.6A:6.6A
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