In Airfield Ground Lighting (AGL), consistency is everything. Pilots depend on uniform brightness from runway, taxiway, and approach lights, and that consistency is achieved using Constant Current Regulators (CCR) — the backbone of AGL power systems.
Unlike conventional electrical systems that supply constant voltage, AGL systems operate on constant current. This is essential to maintain uniform light output across long series circuits, and it is what makes the CCR one of the most important pieces of equipment on an airfield.
- 01 Why Constant Current Is Required
- 02 Basic Working Principle of a CCR
- 03 Series Circuit in AGL
- 04 Brightness Control – Step Control
- 05 Key Components of a CCR
- 06 Protection and Safety Features
- 07 Role in Photometric Performance
- 08 Practical Field Considerations
- 09 Modern Advancements in CCR Technology
- 10 Why CCR Performance Matters to the Entire System
- 11 Key Takeaways
- 12 Frequently Asked Questions
Voltage is variable, but current is constant. A CCR holds output current at the selected brightness step (6.6 A at full brightness) regardless of load, cable resistance, or environmental conditions, within its rated output, which is what allows every fitting on a series circuit to deliver the same, predictable light output.
Why Constant Current Is Required
AGL circuits are series circuits, where multiple lights are connected in a loop through isolating transformers. In such a system, the same current flows through every fitting, while voltage varies depending on load and cable length.
If current fluctuates, the consequences show up directly in the field:
- Light intensity becomes inconsistent across the circuit
- Some lights may appear dim or overly bright relative to others
- Pilot visual guidance can be affected
The CCR exists to prevent exactly this: it ensures that current remains constant regardless of load variations, which is fundamental to maintaining predictable lighting performance across the airfield.
Basic Working Principle of a CCR
A CCR is designed to hold the output current at the selected setpoint (6.6 A at full brightness) irrespective of the number of connected lights, cable resistance, or environmental conditions, so long as the circuit stays within the CCR’s rated output. The working logic behind that is straightforward and continuous.
In simple terms: voltage is variable, but current is constant. This is what allows the AGL series circuit to maintain consistent light output despite changing circuit conditions.
Series Circuit in AGL
In an AGL system, the primary circuit is a high-voltage series loop, and each light is connected into that loop through an isolating transformer. The key electrical relationships are consistent across the circuit:
- Primary current — constant, controlled by the CCR
- Secondary output — supplies the lamp or LED fitting
- Isolation — provides safety and allows easier replacement
Even if one fitting fails, the circuit continues to operate (fittings should still be serviced only with the circuit isolated and locked out). This series-circuit architecture, combined with the isolating transformer arrangement, allows the AGL system to maintain circuit continuity while individual fittings are serviced or replaced.
Brightness Control – Step Control
CCR systems provide multiple brightness steps to adjust light intensity according to visibility conditions. Brightness is selected by changing the regulated current level, not by setting a voltage: a 5-step CCR typically uses 2.8, 3.4, 4.1, 5.2 and 6.6 A, while a 3-step CCR uses 4.8, 5.5 and 6.6 A. The CCR adjusts its output voltage as needed to hold whichever current is selected.
| Step (5-step CCR) | Current / Intensity | Typical Use (indicative) |
|---|---|---|
| Steps 1–2 | 2.8 A – 3.4 A · low | Clear night, good visibility |
| Steps 3–4 | 4.1 A – 5.2 A · medium | Dusk/dawn, reduced visibility, moderate traffic |
| Step 5 | 6.6 A · full | Daylight, fog, low visibility, peak demand |
Actual step selection follows ATC and aerodrome procedures for the runway category and prevailing visibility.
Brightness can be controlled from the CCR panel directly, or through an integrated control system such as SCADA/PLC, which allows Air Traffic Control (ATC) to adjust lighting according to:
- Day / night conditions
- Fog / low visibility
- Traffic conditions
The objective throughout is the same: provide the appropriate lighting intensity for the prevailing operational environment.
Key Components of a CCR
A CCR system typically brings together a defined set of electrical and control components, each with a specific role in holding the output current stable:
Modern CCRs are typically microprocessor-controlled, with precise control and monitoring, and are specified to FAA AC 150/5345-10 (L-828/L-829) and IEC 61822. The combination of current sensing, control electronics, and protection systems enables the CCR to maintain stable operation while responding to changes in the AGL circuit.
Protection and Safety Features
CCR systems include multiple protection functions designed to catch abnormal circuit conditions before they escalate:
- Open circuit protection (an open series loop would otherwise drive the output voltage dangerously high)
- Overcurrent and short-circuit protection
- Earth fault detection
- Surge protection
In the event of a fault, the CCR can isolate or shut down the circuit safely, and an alarm is generated for operator action.
These protection functions matter because AGL circuits operate in an environment where electrical reliability and operational continuity are directly connected to airfield safety.
Role in Photometric Performance
CCR performance directly affects photometric output. The relationship between electrical stability and optical performance is straightforward, and each type of CCR deviation points toward a specific downstream effect:
During photometric testing, the circuit current must be set and verified with calibrated instruments, and measurements are normally made at the rated 6.6 A (full brightness); other steps are checked where the specification requires. A stable CCR is essential for reliable photometric results — photometry measures the actual optical performance of AGL fittings, and if the electrical input to those fittings is unstable or incorrect, the resulting measurements can be affected too.
CCR performance is therefore an important part of achieving reliable and meaningful photometric testing results.
Practical Field Considerations
From field experience, several practical factors can influence AGL circuit and CCR performance:
During testing and maintenance, current should always be monitored, and coordination between the CCR room and the field team is essential.
Good coordination between the electrical/control-room team and field personnel is especially important during AGL testing activities — understanding what is happening at the CCR and what is happening in the field allows abnormal conditions to be identified more effectively.
Airport Lighting Current Calculation
In an AGL series circuit the CCR sets one primary current, and every isolating transformer in the loop carries that same current. The current is chosen by the brightness step, from approximately 2.8 A up to 6.6 A at full brightness. Because voltage can become very high in long circuits, the current and voltage for a circuit should be worked out together from the circuit design. As a rule of thumb, output voltage is the circuit load in VA divided by the current: for example, a 15 kVA load at 6.6 A needs about 2.3 kV. The load is the sum of the transformer and lamp loads plus cable losses, and it must stay within the CCR’s rated kVA.
Airport Lighting Current Test
During testing and maintenance, current should always be monitored. Read the output current at the CCR for each brightness step, and coordinate the CCR room with the field team so that what is seen at the CCR can be matched to what is happening in the field.
Airport Lighting Low Current
If the circuit current is lower than the selected step, light output falls with it. Check the input supply, whether the circuit load exceeds the CCR’s rated output, CCR calibration, and leakage or intermittent connections in the loop. Note that open-circuit protection acts if continuity of the series loop is lost.
Airport Lighting High Current
Current above the selected step is an abnormal condition. Overcurrent and short-circuit protection can isolate or shut down the circuit, and an alarm is generated for operator action. Verify CCR calibration and check for faults before returning the circuit to service.
Modern Advancements in CCR Technology
New-generation CCRs offer advanced monitoring and control capabilities that go well beyond simple current regulation:
- Remote monitoring through SCADA
- Data logging and diagnostics
- Predictive maintenance alerts
- Integration with smart AGL systems
These capabilities improve reliability and help reduce downtime. Digital monitoring also gives maintenance teams better visibility into system behavior and allows potential issues to be identified earlier.
Why CCR Performance Matters to the Entire AGL System
The CCR is not simply another electrical device in the AGL system. It controls the current delivered to the series circuit, and that current directly influences the light output of every connected fitting. A problem with CCR operation can therefore affect:
- Lighting intensity
- Lighting uniformity
- Photometric performance
- Operational reliability
- Maintenance activities
This makes CCR condition, calibration, and monitoring important considerations throughout the lifecycle of an AGL system.
Key Takeaways
- AGL runs on constant current, not constant voltage, so every fitting on a series circuit sees the same current
- The CCR holds output current at the selected step (6.6 A at full brightness) by continuously monitoring current and adjusting voltage
- Isolating transformers let a failed fitting drop out without breaking the series loop
- Step control adjusts brightness from low to full intensity to match visibility and traffic conditions
- CCR stability and calibration directly affect photometric compliance, so measurements are made with a stable, verified circuit current (normally 6.6 A)
- Modern digital CCRs add SCADA monitoring, data logging, and predictive maintenance on top of core regulation
CCR Control Loop at a Glance
Feedback → Adjustment → Constant Output
Conclusion
Constant Current Regulators are the heart of AGL electrical systems. They ensure that every light on the airfield delivers consistent and predictable output, regardless of circuit conditions.
Without a properly functioning CCR, maintaining uniformity in an AGL system is not possible. From safety to compliance, CCR performance directly impacts how pilots see and interpret the airfield.
In short, CCR controls the current, and current controls the visibility.
Frequently Asked Questions
Why do AGL systems use constant current instead of constant voltage?
AGL circuits are series circuits, where the same current flows through every fitting while voltage varies with load and cable length. Constant current keeps light output uniform across the circuit. If current fluctuates, intensity becomes inconsistent, some lights appear dim or overly bright and pilot visual guidance can be affected.
How does a Constant Current Regulator work?
A CCR continuously monitors output current, compares it with the fixed setpoint, raises voltage if current drops and reduces it if current rises, so output current is held constant. This holds the selected current, for example 6.6 A at full brightness, regardless of the number of lights or cable resistance, as long as the circuit stays within the CCR's rated output.
How does a CCR control brightness?
Brightness is selected by changing the regulated current level, not by setting a voltage. A 5-step CCR typically uses 2.8, 3.4, 4.1, 5.2 and 6.6 A, while a 3-step CCR uses 4.8, 5.5 and 6.6 A. Step selection follows ATC and aerodrome procedures, from the CCR panel or through an integrated SCADA/PLC control system.
What are the key components of a CCR?
A CCR typically includes an input transformer, thyristor or IGBT control, a feedback current sensor, a control panel for manual or SCADA-integrated operation and a protection system. Modern CCRs are typically microprocessor-controlled and specified to FAA AC 150/5345-10 (L-828/L-829) and IEC 61822.
What protection features do CCRs have?
CCRs include open circuit protection, since an open series loop would otherwise drive the output voltage dangerously high, overcurrent and short-circuit protection, earth fault detection and surge protection. In the event of a fault the CCR can isolate or shut down the circuit safely and generate an alarm for operator action.
How does CCR performance affect photometry?
Incorrect current results in incorrect light intensity, an unstable CCR produces fluctuating brightness and wrong calibration creates potential non-compliance with ICAO requirements. During photometric testing the circuit current must be set and verified with calibrated instruments, with measurements normally made at the rated 6.6 A.
What should be checked if the circuit current is low or high?
If current is lower than the selected step, light output falls with it, so check the input supply, whether the load exceeds the CCR's rated output, CCR calibration and leakage or intermittent connections. Current above the selected step is abnormal, because overcurrent and short-circuit protection can shut down the circuit, so verify calibration and check for faults before returning to service.
Is it safe to open an energised AGL series loop?
No. Opening an energised series loop is dangerous. Voltage can reach several kilovolts on large loops, so isolate, lock out and prove dead before touching the loop, and coordinate the CCR room with the field team during testing and maintenance.