Although some campus-type facilities use it, series lighting continues to dominate airfield lighting applications. But what exactly is it, and when should you use it?
Just imagine: You have a large number of small lighting loads scattered approximately 100 ft apart over a large area covering several square miles. But local utility service is only available at one location along the area's perimeter. Economically, how can you serve and control this lighting? One solution is to use series lighting circuitry. It's technically sound, simplistic, flexible, reliable, and it's cost effective.
Here's how it works. A series lighting circuit uses a constant current, variable voltage source to supply electrical energy to the loads connected to the circuit. This is in contrast to the more common use of constant voltage, variable current circuits to supply electrical energy in everyday applications. The constant current regulator, isolation transformer, and series circuit wiring are the three major components of this type of lighting circuitry.
Constant current regulator. This regulator supplies the electrical energy to the series circuits. It typically uses a ferroresonant type transformer and a solid-state control system. The regulator maintains the output current to within 1% of the nominal setting over a wide range of input voltage and load conditions. Manufacturers offer constant current regulators in both air- and oil-cooled varieties, with the latter generally used for the larger (50kW and 70kW) sizes and higher input voltage (2400V) types.
Isolation transformer. You connect series lighting loads through a series-to-series isolation transformer. This device transforms the current of the primary series circuit (either 6.6A or 20A) to a secondary current of 6.6A while limiting the secondary voltage to 135V or less. But remember one note of caution: You must match the lighting fixture lamps in these circuits. The 6.6A constant current and voltage range of each size (VA rating) or isolation transformer sets up the requirement for the fixture lamps.
Series circuit wiring. The loop formed by routing a single wire from the constant current regulator source to each load (one after the other) and returning to the source simplifies the series lighting circuit wiring. These circuits use underground construction methods; placing them in raceways or by direct burial. Series lighting cable systems generally have small conductor sizes (No. 8 AWG or No. 6 AWG). That's because of the normally low current ratings of the constant current regulator supply source, which are 6.6A or 20A. However, the voltage of these series lighting circuits can approach 5kV.
The selection of circuit ampere rating (6.6A or 20A) is a function of the maximum circuit voltage of the source at its rated current and load. For sources rated at or below 30kW, the maximum circuit voltage will be below 5000V. For sources rated above 30kW, you should select the 20A current rating to maintain circuit voltages below 5000V.
Using 5kV cable as the standard will limit single circuit load blocks to a maximum of 90kW. But don't be concerned about this limitation, since most load blocks fall below this level in practice due to the need for load segregation and control.
The pros and cons. Depending on the application, series lighting circuits have distinct advantages and disadvantages. However, the pros generally outweigh the cons.
Advantage: simplistic design of the circuitry. You use a single, relatively low-cost cable to serve each of the small widely scattered loads. For example, runway edge lighting design (a system of elevated lights used to outline the runway's edges and ends) allows you to route the supply and return cables together from the source to the runway. Then you loop the runway edges with a single cable to supply every light. Since runways can be more than two miles long, the advantage is apparent.
Disadvantage: increased installed cable cost. The increased cable insulation level (5kV) does add to material cost. However, if you consider the smaller conductor size, the overall cost is usually less. Installing, terminating, splicing, and maintaining the higher voltage cable does take a little more skill and care, but with some training and experience, installers can easily master it.
Advantage: control of lamp illumination level. Since the circuit has a constant current throughout, each load receives power equally, and each lamp operates at the same brightness. As a result, there is no lamp brightness variation due to voltage drop, as you'd expect with constant voltage circuits. To lower the brightness, simply lower the output current of the constant current regulator. In fact, you can get constant current regulators (used for airfield lighting applications) with three- and five-step brightness controls. The reason for lamp illumination control is to allow adjustments for both good and poor visibility conditions.
For example, when an aircraft pilot needs help identifying the airport runway, the illumination level needs to be high. But as the aircraft approaches the airport, the pilot may ask air traffic control to lower illumination to a more comfortable level for final approach and landing. When airports don't have a control tower, or if an air traffic controller isn't available, the pilot accomplishes the on-off and brightness control of lighting systems through air-to-ground radio.
Advantage: reliability. Series lighting systems can operate for long periods under adverse conditions. In fact, they can operate with a large number of lamps burned out, with badly degraded cable systems, and even with shorts and grounds. However, this is not advisable. You should always plan and perform preventive maintenance to keep the systems operating at peak performance.
Advantage: flexibility. You can easily add, move, and/or remove loads. And, the same cable system can supply a wide range of loads. Thus, you can modify the circuit loads with the only concern being the loading of the constant current regulator. However, major load changes may require an increase in circuit ampere rating. This, in turn, requires replacement of the isolation transformer.