Systems, commissioning and power quality
A constant current regulator drives a series lighting circuit, such as airfield lighting, at a steady current whatever the load. Output testing proves the current at each brightness step, the control response, the input-interruption behavior and the monitor functions before the regulator is accepted for service.
A constant current regulator is a power conversion device that supplies a series circuit with a controlled alternating current regardless of how many lamps or isolating transformers are connected, within its rating. It is best known from airport lighting, where long series loops of runway, taxiway and approach lights are fed from a regulator in a vault, but the same principle serves other series circuits. The regulator offers a small number of brightness steps, each defined by a target output current, and it is built to hold each step steady as lamps age, fail or are replaced. The output test confirms what the regulator actually delivers: the current at each step, how quickly and smoothly the control moves from step to step, what happens when the input power is interrupted and restored, and how the built-in monitor detects and reports abnormal conditions. Efficiency, power factor and the shape of the output waveform are also examined. The test is performed on new regulators at acceptance, whatever the style or class of the unit and whether it is a stand-alone cabinet or part of a larger lighting vault.
In a series circuit every lamp carries the same current, so brightness depends on that current and nothing else. A regulator that delivers too little leaves lights dim and out of specification for pilots, while too much shortens lamp life sharply and can overheat isolation transformers and cable. An error that is small in percentage terms still matters, since lamp output changes steeply with current. A regulator that responds slowly or unevenly to a step change produces flicker or visible disturbances, and one that restarts badly after a power interruption may come back at the wrong brightness or not at all. The monitor is a safety function as much as a convenience: open-circuit and overcurrent protection, ground fault detection and output limits protect the regulator, the cable and the people working on the loop. Because a lighting system supports aircraft operations, a failure found in the field is far more costly than one found at acceptance, and the readings taken at commissioning become the reference for later maintenance.
The regulator works by controlling current instead of voltage. A transformer with a controlled element, commonly a saturable reactor, thyristors or a similar arrangement, adjusts its output voltage automatically so that the current in the series loop stays at the selected target. When the load grows or shrinks, the control senses the loop current through a measuring element and corrects the output until the current returns to the target. Each brightness step corresponds to a different target current. Because the output is alternating, the waveform matters as well. A regulator that controls with phase-angle devices can produce a distorted waveform, so the standard asks how efficient the unit is, how much of the input power is usable, and how distorted the output is, quantities summarized by efficiency, power factor and crest factor. The monitor circuits act as a second set of eyes, comparing the loop condition with set points for current, open circuit and fault conditions, and timing their response so that a momentary disturbance does not trigger a nuisance shutdown while a sustained fault does.
The regulator is de-energized, isolated and locked out, and the inspection covers the cabinet, wiring, the control and monitor boards, the input and output terminals and the grounding. A load representing the series circuit is connected, either the actual field loop or a resistive or transformer-based load bank sized to the regulator rating, and calibrated true-RMS instruments are placed to read the output current, voltage and power. The unit is then energized and operated through every brightness step in turn, the output current being read at each step and at the loads called for by the procedure, such as a full load and a reduced load. Control response is observed while stepping up and down, noting the smoothness of the change and the time the output takes to settle. The input power is interrupted and restored to see how the unit restarts and which step it returns to. The monitor is exercised by simulating an open circuit, an overcurrent and the other conditions it is designed to detect, and its set points and delay times are checked against the specification. Efficiency, power factor and crest factor are derived from the measured power, voltage and current at the loads the procedure specifies. For regulators of a type not covered by the usual classes, the manufacturer's published criteria are used and the same sequence applies.
The record identifies the regulator, its style and class, rating, input supply and the circuit or load that was connected. It holds the current delivered at each brightness step and load condition, the observed behavior when stepping between brightness levels, the response to input interruption and restoration, the results of the monitor functional checks with the set points and timing that were verified, and the efficiency, power factor and waveform quantities derived from the measurements. The instruments used and their calibration status are noted, along with ambient conditions and the as-found state of the equipment and any adjustments made.
Results are judged against several shapes of criteria rather than a single number. Output current at each brightness step is compared with the target for that step in the governing document, with an allowed tolerance that depends on the class and style of the regulator. Control response is judged on stability and on how the output behaves as the step changes, and the behavior after an input interruption is judged on whether the regulator returns to a safe, defined state. Monitor functions are judged on whether each protective condition is detected and on whether set points and timing match the specified values. Efficiency, power factor and waveform shape are compared with published minimum or maximum expectations for the unit. Regulators outside the standard classes are judged against the manufacturer's own criteria. The governing document is FAA AC 150/5345-10H, which sets the requirements for constant current regulators and regulator monitors; use your own copy of it for the values, tolerances and tables, which remain the authority.
The class and style of the regulator change what is measured. Classes differ in the circuit current the regulator is built to supply, and styles differ in how many brightness steps it offers, so the list of steps to be read and the target values change from one design to another. Regulators for other series circuits that fall outside the standard classes are tested the same way but against the manufacturer's stated output, which must be obtained before the test. Acceptance testing establishes the baseline and exercises every function, while later maintenance checks usually concentrate on output current at the steps in actual use, the monitor and the condition of the output transformer and cable. Units with a remote or computerized control interface add checks on the control command path and on how commands and status are reported. A regulator that is fed from a standby generator adds the question of how it behaves on source transfer, which ties the test to the system-level checks described under energization and phasing.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
In a series lighting circuit every lamp carries the same current, so brightness follows current. Holding the current steady keeps all lamps at the intended brightness even as lamps age, burn out or are replaced, which a fixed voltage would not do.
They are the discrete settings, each tied to a target output current, that let the lights be run dimmer or brighter for different conditions. The style of the regulator sets how many steps it has, and every step is checked at acceptance.
It watches the loop for abnormal conditions such as open circuit, overcurrent and ground fault, and reports or shuts the unit down after a set delay. The test simulates each condition and checks that the set points and delay times behave as specified.
In FAA AC 150/5345-10H, which covers constant current regulators and regulator monitors, along with the manufacturer's data for units outside its classes. Use your licensed copy for the targets, tolerances and tables.
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