Home/ Knowledge Hub/ The Physics of AGL
Safety-critical technical reference — read before use. Independent educational guidance for qualified AGL and aerodrome personnel. It is not an approved procedure, design, specification or regulatory instruction and does not replace your aerodrome’s permit-to-work, isolation/LOTO procedures, the approved design or the manufacturer’s instructions. Series-circuit AGL equipment can carry lethal voltages. Read the full disclaimer ↓
PRECISION AGL ACADEMY · Electrical Fundamentals

The Physics of AGL

Resistance, reactance and regulation — the electrical behaviour behind an airfield series circuit. CCRs, cables, isolating transformers, connectors and fixtures are the components you can see. Underneath every one of them is a set of electrical principles that decides how current, resistance, reactance, impedance, voltage and power interact — and why an AGL circuit behaves the way it does.

Start Learning Review AGL Fundamentals
Current Impedance Voltage Regulation Light Output
Overview

What Is the Physics of AGL?

The physics of AGL is the study of how electrical energy behaves as it travels through an airport lighting system. An AGL series circuit is not simply a wire carrying electricity — it is an electrical system containing conductors, cable resistance, cable inductive reactance, isolation transformers, fixture loads, connector interfaces, distributed impedance, CCR regulation, and ground and insulation characteristics.

Understanding these principles helps an engineer explain why CCR output voltage changes with load, why cable losses occur, why transformer behaviour changes with loading, why series-circuit current must be controlled, why open circuits create high-voltage conditions, why faults affect circuit behaviour, and why electrical measurements do not always directly indicate optical performance.

A System, Not a Wire

Resistance, reactance, transformers, fixtures and connectors all combine to determine how the circuit actually behaves.

Electrical → Optical

The fundamental chain: Electrical Input → Series Circuit → Transformer → Fixture → Optical Output.

Foundation, Not Trivia

These aren't abstract formulas — they're the reasoning an engineer uses to explain and troubleshoot real circuit behaviour.

Series Circuit Architecture

Why AGL Uses a Series Circuit

Traditional airport lighting commonly uses a constant-current series circuit. Instead of supplying each fixture from an individual parallel voltage source, the lighting loads are connected in series.

CCR

Regulates circuit current

Cable

Carries controlled current

Isolation Transformer

Interfaces the fixture

Fixture

Converts energy to light

Return Path

Back through cable to CCR

The lighting system is designed around controlled current rather than simply maintaining a fixed voltage at every fixture. In an ideal series circuit, the same current flows through every point in the series path: I₁ = I₂ = I₃ = … = Iₙ. This is one of the most important differences between AGL series circuits and conventional parallel lighting systems.

Current Is the Same at Every Point in the Loop

Voltage varies fixture-to-fixture across the loop, but a healthy series circuit reads the same current everywhere it's measured

CCR T1 6.6A T2 6.6A Tn 6.6A T3 6.6A Every ammeter around the loop reads the same current — I₁ = I₂ = I₃ = … = Iₙ
SERIES LOOP · THE PRINCIPLE ONE LOOP · ONE CURRENT · EQUAL LIGHT CCR I FIXED OUT RTN I I I T1 SAME LIGHT T2 SAME LIGHT T3 SAME LIGHT T4 SAME LIGHT 1 THE CCR SENDS ONE CURRENT AROUND THE LOOP 2 THE SAME CURRENT PASSES EVERY LIGHT IN TURN 3 SAME CURRENT = SAME BRIGHTNESS THE PRINCIPLE ONE LOOP, ONE CURRENT LIGHTS CONNECTED END TO END THE RESULT EQUAL BRIGHTNESS WHATEVER THE POSITION OR CABLE LENGTH THE TRADE-OFF ONE BREAK STOPS THE LOOP SO THE LOOP MUST STAY COMPLETE SCHEMATIC · LOOP SHORTENED · NOT TO SCALE
Figure 1. Lights connected end to end on one loop with one current — so every light gives the same output.AGL Visual Encyclopedia: What is the AGL series circuit principle?
Interactive · Foundation

Ohm's Law — The Starting Point

The starting point for understanding AGL electrical behaviour is Ohm's law. Enter any two values and see the third calculated. In a purely resistive circuit, increasing resistance at a fixed voltage reduces current — but an AGL series circuit is generally an AC impedance system, so resistance alone doesn't tell the whole story (more on that below).

Solve for Voltage (V)
Solve for Current (I)
Solve for Resistance (R)
V = I × R → I = V / R
OHM’S LAW IN AGL · V = I × R THE CCR FIXES I · THE LOOP RESISTANCE SETS V V I R V = I × R  ·  I = V ÷ R R = V ÷ I CCR HOLDS I CONSTANT CABLE TRANSFORMER PRIMARIES V 1CCR HOLDS I CONSTANT 2R = WHOLE LOOP ADDED UP 3V = I × R · OUTPUT VOLTAGE FOLLOWS R MORE LOAD ON THE LOOP R UP · V UP · I HELD AT THE CCR RATING V CAPPED · I FALLS ILLUSTRATIVE EXAMPLE 6.6 A × 10 Ω = 66 V SCHEMATIC · SIMPLIFIED · NOT TO SCALE
Figure 2. V = I × R on a series lighting circuit — the CCR holds the current, and the resistance of the loop decides the voltage it must supply.AGL Visual Encyclopedia: What is Ohm’s law in AGL?
Resistance

Resistance in the AGL Circuit

Resistance is the opposition to current caused by the resistive properties of an electrical element. In an AGL system it exists in cable conductors, transformer windings, connector contacts, fixture wiring, terminations and joints — and it produces electrical losses.

Resistive Power Loss

P = I²R

Circuit current is intentionally held within a controlled operating range, so this relationship matters throughout the circuit.

Conductor Resistance

R = ρL / A

Longer cable → higher resistance. Larger conductor area → lower resistance.

Temperature Effect

Rₜ = R₀[1 + α(T − T₀)]

Resistance of most conductors rises with temperature — consider test temperature before calling a reading abnormal.

Total Circuit Resistance

A simplified total is Rtotal = R₁ + R₂ + R₃ + … + Rₙ. A small resistance at one connector may look insignificant, but a high-resistance connection can produce local heating, voltage loss, power loss and reliability problems.

Contact Resistance

A healthy connection has very low contact resistance. Poor mating, contamination, corrosion, mechanical damage, improper termination, water ingress or a loose connection can all increase it — and even a relatively small increase can produce significant I²R heating when current is high.

AC Behaviour

Reactance & Impedance

Resistance is only one part of AC circuit opposition. The other major concept is reactance — opposition to AC caused by inductance or capacitance. Resistance and reactance combine to form impedance (Z), a far more useful description of an AC AGL circuit than resistance alone.

Inductive Reactance

Xₗ = 2πfL

Increases with frequency and inductance. Relevant to transformer windings, cable characteristics, magnetic equipment and certain AGL loads.

Capacitive Reactance

Xc = 1/(2πfC)

Decreases as frequency increases. In long cable runs, distributed capacitance can influence circuit behaviour.

Impedance

|Z| = √(R² + X²)

For a series R-L circuit, Z = R + jX. This is what the CCR actually "sees" — not a single resistor.

ParameterResistance (R)Reactance (X)
Opposition to currentYesYes
Produces real power lossYesIdeally no
Associated withResistive elementsInductors / capacitors
Depends on frequencyGenerally not directlyYes
Phase shiftNoYes
Real AGL circuits contain both. A simplified circuit is: Cable Impedance + Transformer Impedance + Fixture-Related Reflected Impedance + Other Circuit Effects.

Phase Angle

For an AC impedance Z = R + jX, the phase angle is φ = tan⁻¹(X/R) — it describes the relationship between voltage and current, and it's the underlying reason phase angle helps explain reactive power, power factor, transformer behaviour and AC circuit performance generally.

The Impedance Triangle

R and X combine as perpendicular vectors, not as a simple sum — Z is the hypotenuse, and φ is the phase angle between voltage and current

R (resistance) X (reactance) Z (impedance) φ |Z| = √(R² + X²) φ = tan⁻¹(X/R) Z = R + jX A larger φ means more of the circuit's opposition is reactive, not resistive
Common Misunderstanding

Why CCR Output Voltage Changes

A constant-current regulator maintains the required circuit current — it does not maintain a fixed voltage. If circuit impedance changes, the voltage required to maintain that current also changes: V ≈ I × |Z|. Try it below.

Circuit Current
6.6 A
Total Impedance
— Ω
Required CCR Voltage
— V
vs Baseline
—
Required CCR output voltage relative to a light, short baseline circuit (2 transformers, 0.5 km).
Nothing has "failed" here. Current is held constant at 6.6 A regardless of the sliders — the simulator only changes circuit impedance. Adding transformers or lengthening cable increases the required CCR voltage; removing loads or shortening cable decreases it. This is normal series-circuit physics, not a fault.

Adding a Load

An additional compatible load increases total circuit impedance. If current is held constant, V = I × Z means the CCR must produce a higher voltage. This is normal series-circuit physics, not a defect.

Removing a Load

Removing a transformer or fixture reduces total circuit impedance, so the CCR may settle at a lower output voltage while still holding the same regulated current.

CURRENT VS CIRCUIT LOAD · WHAT CONSTANT-CURRENT REGULATION DOESCURRENTCIRCUIT LOAD (more lights / longer cable)CONSTANT-CURRENT REGULATOR: CURRENT HELDFIXED-VOLTAGE SUPPLY: CURRENT FALLS AS LOAD GROWSRATED LOAD LIMITbeyond limit,current not heldThe CCR varies its output voltage (V = I × R) to keep the current constant.1SAME CURRENT EVERYWHEREone series loop, one current2VOLTAGE FOLLOWS LOADV = I × R; more load needs more volts3REGULATIONcurrent held through supply and load changes4EQUAL BRIGHTNESSevery light gets the same current5RATED LOAD RANGElimit of load the CCR can drive6OPEN-CIRCUIT HAZARDoutput voltage can rise if loop is broken1 · Same current everywhere2 · Voltage follows load3 · Regulation4 · Equal brightness5 · Rated load range6 · Open-circuit hazard
Figure 3. Holding the current in an airfield series circuit at a set value while the voltage rises or falls with the load, so every light gets the same current.AGL Visual Encyclopedia: What is constant-current regulation?
Fault Physics

What Happens During a Fault?

Different fault types create different electrical signatures. Click a fault to see how it behaves.

Open Circuit

Interrupted Path

  • Series current path is broken — CCR → Cable → OPEN
  • Circuit current falls toward zero
  • CCR may enter a protective or open-circuit response state
  • Requires controlled, appropriate testing procedures

Short Circuit

Very Low Impedance

  • Z drops toward a very low value
  • A fixed-voltage source could see very high current
  • A CCR responds differently — it's current-regulated with its own protection
  • Never assume every CCR behaves identically here

Earth Fault

Unintended Path to Earth

  • Depends on fault impedance, insulation, cable construction
  • Also depends on CCR design, earthing arrangement, monitoring
  • Can cause abnormal current distribution
  • Can cause insulation degradation or protective action

High-Resistance Connection

Additional Local Impedance

  • Produces I²R heating at the joint itself
  • Connection can progressively get hotter as current flows
  • May lead to deterioration, intermittent operation, eventual open circuit
  • Visual inspection alone may not catch it

Water Ingress

Insulation & Leakage Change

  • Affects insulation resistance and leakage current
  • Affects connector and cable condition
  • Can affect transformer insulation
  • Environmental sealing is an electrical reliability issue, not just mechanical

Transformer Failure

Impedance & Load Change

  • Circuit impedance can shift
  • Load behaviour on the series circuit can change
  • Can influence required CCR output voltage
  • Should be investigated together with circuit history
SERIES LOOP · WHERE DOES THE CURRENT GO? ISOLATE & LOCK OUT BEFORE TOUCHING CABLE ENDS A OPEN CIRCUIT T1 T2 T3 CCR PATH NONE · LOOP BROKEN LAMPS ALL OUT TESTER OVER-RANGE (OL) B SHORT CIRCUIT T1 T2 T3 CCR PATH SHORTCUT, REST SKIPPED LAMPS BEYOND SHORT OUT TESTER FAR BELOW BASELINE C EARTH FAULT T1 T2 T3 CCR PATH LOOP INTACT + LEAK LAMPS STAY ON INSUL. LOW TO EARTH A SECOND EARTH FAULT ELSEWHERE BEHAVES LIKE B SCHEMATIC · LOOP SHORTENED · NOT TO SCALE
Figure 4. Where the current actually goes once a fault changes the loop — nowhere, through a shortcut, or round the loop with a leak to earth.AGL Visual Encyclopedia: What is the series circuit fault path?
Cable Physics

Why Cable Length & Size Matter

Cable resistance produces loss according to P = I²R — so cable losses scale with the square of current, not linearly with it.

Longer Cable

Higher resistance → higher voltage drop → greater resistive loss. Cable route and circuit length are a core AGL design input.

Larger Conductor

Larger cross-sectional area → lower resistance → lower resistive voltage drop, within the applicable cable specification.

Current Doubles → Loss ×4

If current increases by a factor of two, I² becomes 4I² — so resistive loss becomes roughly four times greater at the same resistance.

A Cable Is Not Just a Conductor

Long AGL cables have distributed electrical properties — resistance, inductance and capacitance. Long series circuits can therefore exhibit behaviour that simple DC resistance calculations alone cannot fully explain, including resonance effects in systems containing both inductance and capacitance (where Xₗ = Xc).

VOLTAGE DROP · ALONG THE LOOP CCR V OUT RETURN PATH CABLE −2 V CONNECTOR −1 V CABLE −3 V TRANSFORMER −4 V CABLE −2 V VOLTAGE LOST AT EACH STAGE (ILLUSTRATIVE) VOLTAGE AVAILABLE FALLS STEP BY STEP 1 CURRENT (I) IS THE SAME IN EVERY SECTION 2 DROP = I × R OF THAT SECTION THE RULE V = I × R LOSS ACROSS ONE SECTION THE SUM V OUT = ALL DROPS CCR SUPPLIES THE TOTAL WATCH FOR RISING DROP CORROSION · BAD CONNECTORS
Figure 5. The voltage lost as current passes through the cable, connectors and transformers of a loop — and the reason the CCR must supply more than the lamps alone need.AGL Visual Encyclopedia: What is voltage drop?
Power

Power & Power Factor

AC systems require separating real power, reactive power and apparent power — and AGL systems containing magnetic components such as transformers can exhibit reactive behaviour.

Apparent Power

S² = P² + Q²

P = real power (W), Q = reactive power (VAR), S = apparent power (VA).

Power Factor

PF = P/S = cos φ

Indicates how effectively apparent power converts into real power.

Transformer Ratio

V₁/V₂ = N₁/N₂

Ideal turns ratio; real transformers differ due to losses and non-ideal characteristics.

Input currentEquipment loadingPower consumption Electrical infrastructureGenerator capacityDistribution equipment
The Power Triangle

Real power (P) does the useful work of lighting the fixture; reactive power (Q) is exchanged with magnetic components; apparent power (S) is what the supply and generator must actually be sized for

P — Real Power (W) Q — Reactive Power (VAR) S — Apparent Power (VA) φ S² = P² + Q² PF = P/S = cos φ Power factor closer to 1 means apparent power is being used more effectively as real power
Transformers & Fixtures

Transformer & Fixture Load Physics

An AGL isolating transformer is not an ideal transformer — it has winding resistance, leakage reactance, magnetizing characteristics, core losses and its own insulation system, and it contributes to overall series-circuit impedance.

CCR → Transformer → Fixture

The transformer transfers energy from the series circuit to the fixture — it does not behave like a conventional voltage transformer connected to a fixed-voltage supply. Its role is specifically adapted to the series-circuit architecture.

  • P₁ ≈ P₂ for an ideal transformer
  • P₂ < P₁ for a real transformer (copper losses, core losses, leakage)
  • Efficiency: η = Pout / Pin

Incandescent vs LED Loads

An incandescent lamp is primarily a resistive load, though its resistance changes substantially with temperature. An LED system may include electronic components and may not behave electrically like a simple resistor.

  • Fixture compatibility with the series circuit must be engineered
  • Never assumed from lamp wattage alone
SERIES LOOP SIDE FITTING SIDE CONSTANT-CURRENT LOOP 6.6 A (EXAMPLE) HIGH-VOLTAGE LOOP MAGNETIC CORE MAGNETIC FLUX CARRIES THE ENERGY PRIMARY SECONDARY LIGHT FITTING EARTH FAULT AT FITTING NO ELECTRICAL CONNECTION BETWEEN THE WINDINGS ENERGY CROSSES THROUGH THE CORE ONLY ILLUSTRATIVE · CORE AND WINDING LAYOUT VARIES BY DESIGN · NOT TO SCALE
Figure 6. The loop current drives the primary, a magnetic core carries the energy across, and the secondary feeds the fitting. With no electrical connection between the windings, a failed lamp does not break the loop.AGL Visual Encyclopedia: How does an isolation transformer work?
Why It Matters

Regulation vs Optical Output

A CCR can regulate current correctly while a lighting system still has an optical problem. Correct electrical regulation does not automatically prove correct photometric performance.

CCR Current Correct
Transformer Operating
Fixture Energized
Alignment/Optics May Still Be Wrong
MeasurementWhat It Tells You
Circuit currentElectrical operating condition
CCR voltageCircuit electrical demand
Cable resistanceConductor condition
Insulation resistanceInsulation condition
Transformer resistanceWinding condition
ContinuityPresence of an electrical path only
PowerElectrical energy consumption
PhotometryOptical performance
Colour measurementLight colour performance
AlignmentFixture aiming
No single measurement completely describes AGL performance — that's why current, voltage and photometry are analysed together, not in isolation.
FIXTURE OPTICAL SYSTEM SIDE SECTION OF THE LIGHT PATH SOURCECOLOUR FILTERLENS OR PRISM REFLECTOR OR OPTICHOUSING BEAMOPTICAL AXIS SPREAD HOW THE OPTICS SHAPE THE LIGHT 1 SourceLamp or LEDs make the light 2 Optic gathersReflector or lens collects it 3 Filter coloursFilter, or coloured LEDs 4 Lens aims itSets direction and spread The source only makes light. These parts decide where it goes, how wide it spreads and what colour it is. SCHEMATIC · LAYOUT DIFFERS BETWEEN PRODUCTS · RAY PATHS ILLUSTRATIVE · NOT TO SCALE
Figure 7. The source only makes light. The optical system is everything after it: the reflector or lens, the colour filter and the front prism that turn that light into a beam with the right direction, spread and colour.AGL Visual Encyclopedia: What is a fixture&rsquo;s optical system?
Applied Physics

Physics in Design, Commissioning & Maintenance

These electrical principles aren't just theory — they shape how an AGL circuit is designed, how it's baselined at commissioning, and how it's maintained afterward.

Design

Electrical design must consider circuit length, cable characteristics, transformer characteristics, fixture load, CCR rating, voltage capability, circuit impedance, environmental conditions, protection and testing requirements — so the circuit operates reliably within equipment and system limits.

Commissioning

During commissioning the engineer should establish an electrical baseline — CCR current, CCR output voltage, circuit resistance, insulation resistance, transformer measurements, equipment status, functional response and photometric performance — as reference data for future troubleshooting.

Maintenance

Maintenance should identify changes in system behaviour, not just failed components — e.g. a gradual CCR voltage increase with no obvious fixture failure should prompt investigation of circuit impedance. This is a more predictive approach to AGL maintenance.

Physics of AGL Reliability

Reliability is influenced by the complete engineering chain, not electrical physics alone: electrical design, component quality, installation quality, environmental protection, electrical regulation, mechanical integrity, maintenance and photometric verification. The physics provides the foundation — system reliability depends on all of it together.

Diagnostic Model

A Physics-Based Troubleshooting Sequence

A useful troubleshooting philosophy is to not ask only "which component failed?" — ask "what changed in the electrical behaviour of the system?" This avoids replacing components based on a single measurement alone.

Confirm CCR operating step

Establish the intended operating point before judging any reading against it.

Verify actual circuit current

Current is the controlled quantity — confirm it's actually where it should be.

Record CCR output voltage

Voltage is the circuit's response to impedance, not a fixed target.

Compare with historical & design values

A trend against baseline is far more informative than a single snapshot.

Confirm circuit configuration

Check for added, removed or reconfigured loads before assuming a fault.

Check transformer population

Every series-connected transformer contributes to total circuit impedance.

Inspect connectors & joints

High-resistance connections are a common, easily-missed contributor.

Investigate cable condition

Consider insulation resistance and physical cable condition, not just continuity.

Perform component-level testing

Isolate individual transformers, connectors or cable sections as needed.

Verify fixture performance

Confirm the fixture itself is operating as expected electrically.

Perform photometry where required

Electrical health does not by itself confirm optical performance.

FAULT-ISOLATION FLOWCHART · SYMPTOM, TYPE, SECTIONFIRSTTELL ATC · SWITCH OFF AND ISOLATE THE CCR · LOCK OFF · AUTHORISED PERSONNEL ONLYALL LIGHTS OUTCCR open-circuit alarm, no currentOPEN CIRCUITa break in the loopTEST CONTINUITY BY SECTIONFault is where it failsCCR TRIPSEarth-fault or overcurrent alarmEARTH FAULTinsulation to earth is lowTEST INSULATION RESISTANCE BY SECTIONLow reading marks the sectionSOME LIGHTS OUTCCR normal, circuit runningLOCAL FAULTfixture, connector or transformerCHECK THE DEVICE ITSELFLight, secondary connector, transformerNARROW DOWN BY HALF-SPLITTEST: GOOD · SET ASIDEBAD · SPLIT AGAINfault sectionTHEN1 Repair the faulty part2 Retest section and circuit3 Restore, run, confirm status4 Record results, tell ATC
Figure 8. Make the circuit safe, read the symptom at the CCR, then narrow the fault down section by section until one part remains.AGL Visual Encyclopedia: How is a fault isolated in a series circuit?
Test Yourself

Common Misunderstandings — Quick Check

These are recurring misunderstandings in AGL electrical work. Pick the correct answer for each.

1. "The CCR should always produce the same voltage."

True — voltage should be fixed regardless of circuit
False — voltage varies with circuit impedance at constant current

2. "Continuity means the circuit is healthy."

True — a continuous path guarantees healthy performance
False — continuity only proves an electrical path exists

3. "Low resistance always means good."

True — lower is always better in every case
False — measured resistance must be interpreted against expected values and test conditions

4. "Correct current means correct light output."

True — correct current guarantees correct photometry
False — optical performance requires separate photometric verification

5. "A cable is just a conductor."

True — only resistance matters for a cable
False — an AGL cable has resistance, inductance and capacitance, especially over length
Synthesis

What Every AGL Engineer Should Understand

Beyond individual equations, an AGL engineer should be able to explain each of the following in plain terms:

Why AGL uses a series circuit Why current is regulated Why CCR voltage changes with circuit loading Why cable resistance matters Why impedance beats resistance alone for AC systems How transformer impedance affects the circuit Why high-resistance connections create heating Why insulation resistance differs from conductor resistance Why power factor matters Why electrical regulation doesn't guarantee photometric compliance
Reference

Key Equations for AGL Engineers

Ohm's Law

V = IR

Resistance

R = ρL/A

Resistive Power Loss

P = I²R

Inductive Reactance

Xₗ = 2πfL

Capacitive Reactance

Xc = 1/(2πfC)

Impedance Magnitude

|Z| = √(R² + X²)

Apparent Power

S² = P² + Q²

Power Factor

PF = P/S

Transformer Ratio

V₁/V₂ = N₁/N₂
Checklist

Physics-Based AGL Engineering Checklist

A working checklist across the four areas an AGL engineer needs to hold together. Click items to track your own review.

Electrical Fundamentals

Ohm's law understood
Resistance understood
Reactance understood
Impedance understood
Power relationships understood
Power factor understood
0 / 6 complete

Series Circuit

Series-current principle understood
Cable resistance considered
Cable impedance considered
Transformer impedance considered
Circuit loading evaluated
0 / 5 complete

CCR Behaviour

Current regulation understood
Output voltage behaviour understood
Open-circuit behaviour understood
Fault response understood
Operating limits checked
0 / 5 complete

Field Equipment

Transformer characteristics considered
Connector resistance considered
Cable condition considered
Fixture load considered
Environmental effects considered
0 / 5 complete

Verification

Current measured
Voltage measured
Resistance tested where applicable
Insulation resistance tested
Historical data reviewed
Photometry performed where required
0 / 6 complete
Comparison

AGL Physics vs Conventional Electrical Systems

Conventional SystemAGL Series System
Usually voltage-controlledCurrent-controlled
Parallel loads commonSeries circuit common
Fixed supply voltage often centralCCR output voltage varies with load
Individual loads directly suppliedIsolation transformers interface fixtures
Voltage is primary operating parameterCircuit current is a key controlled parameter
Load changes mainly affect currentLoad changes influence required CCR voltage
This difference is essential for engineers transitioning from conventional electrical systems into AGL work.
Standards & References

Where These Principles Meet Real Requirements

The equations on this page are fundamental electrical engineering concepts. Every AGL application must still be checked against the latest applicable standards and project documentation — exact operating values, tolerances, test voltages and acceptance criteria should never be assumed from generic equations alone.

ICAO Annex 14

International aerodrome design & operations standard

FAA AC 150/5340

Airport lighting & CCR equipment guidance

IEC Standards

Applicable cable, connector & equipment standards

DGCA / National Regs

Local regulatory & airport-specific requirements

The Complete Model

The Physics-Based AGL Performance Chain

The final engineering principle: an AGL series circuit is a controlled electrical system, not simply a collection of lights connected by cable.

RESISTANCE
REACTANCE
IMPEDANCE
CURRENT
VOLTAGE
REGULATION
POWER
TRANSFORMER BEHAVIOUR
FIXTURE PERFORMANCE
LIGHT OUTPUT
PHOTOMETRY

Resistance determines losses. Reactance influences AC behaviour. Impedance determines the electrical demand of the circuit. The CCR regulates current. Circuit voltage responds to the connected impedance. The transformer transfers energy to the fixture while providing isolation. The fixture converts electrical energy into light. Photometry verifies the resulting optical performance.

Measure the circuit. Understand the physics. Interpret the behaviour. Verify the light.

Knowledge Hub

Explore Related Topics

AGL Design & Electrical Engineering

Apply electrical principles to complete AGL system design.

View Guide

AGL Cables & Connectors

Resistance, impedance, installation and connection behaviour in depth.

View Guide

AGL Photometry

Connect electrical performance with actual optical output.

View Guide

Take the Physics Into the Field

See how these principles apply across runway, taxiway, approach and PAPI systems.

Runway Lighting PAPI Engineering Guide
AGL Engineering Hub

Explore the AGL Engineering Knowledge Hub

Fundamentals

All About AGL How Airfield Lighting Works Physics of AGL AGL Components Airfield Lighting Colour Codes LED Airfield Ground Lighting LED Benefits, Lifespan & Energy Savings Solar AGL (SAGL) Portable AGL (PAGL) Advantages & Disadvantages by System Type Sustainability in AGL

Standards & Compliance

ICAO Annex 14 AGL Requirements ICAO Aerodrome Lighting Requirements FAA AC 150/5340 Series EASA CS-ADR-DSN Compliance NOTAMs

Runway, Taxiway & Approach Lighting

Runway Lighting Systems Taxiway Lighting Systems Airport Approach Lighting Systems Approach Lighting Systems Apron Lighting & Stand Guidance Apron Floodlighting Design & Lux Levels VDGS: Visual Docking Guidance Obstruction Lighting & Marking Airfield Signage & Guidance Signs Markings & Signage Integration

Surface Movement & Safety

Follow the Greens Runway Guard Lights & Stop Bars Runway Incursion Prevention & SMGCS A-SMGCS ALCMS

PAPI

How Does PAPI Work? PAPI Engineering Guide ICAO PAPI Requirements PAPI Installation & Aiming PAPI Calibration & Alignment PAPI Alignment & Maintenance PAPI Maintenance Checklist

Electrical Engineering & Power

AGL Design & Electrical Engineering Diagrams, Schematics & Wiring CCR Working Principle CCR Deep Dive AGL Isolating Transformers Airport Isolating Transformer Junction Box & Transformer Housing Cables & Connector Engineering Electrical Architecture & Redundancy Emergency Backup Power Emergency Power & Lighting Systems Surge Protection & Lightning Mitigation Lightning & Surge Protection for Circuits

Photometry & Testing

AGL Photometry Photometric Testing Guide Photometric Testing Procedure Runway Lighting Photometry Taxiway Lighting Photometry AGL Testing & Inspection Installation & Commissioning Testing & Commissioning Guide

Maintenance, Inspection & Safety

Preventive & Corrective Maintenance Maintenance Schedule & Checklist Inspection, Maintenance & Audit Checklists How to Inspect & Replace Airfield Lights Common Failure Causes & Fixes Outage Response & Runway Closure Maintenance Work Safety: LOTO, Permits, PPE

Resources, Documentation & Careers

Datasheets, Catalogues & PDF Guides Project Documentation: ITP, As-Built, O&M Engineer & Technician Job Description / Salary
FAQ

Frequently Asked Questions

Q1What is the physics of AGL?

It is the study of how electrical energy behaves as it travels through an airport lighting system. An AGL series circuit contains conductors, cable resistance and inductive reactance, isolation transformers, fixture loads, connector interfaces, distributed impedance, CCR regulation, and ground and insulation characteristics. Understanding this explains real circuit behaviour and supports troubleshooting.

Q2Why does AGL use a series circuit?

Traditional airport lighting commonly uses a constant-current series circuit, with loads connected in series instead of each fixture having its own parallel voltage source. In an ideal series circuit the same current flows through every point of the loop, so a healthy circuit reads the same current everywhere it is measured.

Q3Why does CCR output voltage change with the circuit?

A CCR maintains circuit current, not a fixed voltage. If circuit impedance changes, the voltage needed to maintain that current changes too, roughly V = I × |Z|. Adding transformers or lengthening cable raises the required voltage, and removing loads lowers it. This is normal series-circuit behaviour, not a fault.

Q4What is the difference between resistance, reactance and impedance?

Resistance opposes current and produces real power loss. Reactance is opposition to AC caused by inductance or capacitance and depends on frequency. Together they form impedance (Z), which describes an AC AGL circuit far better than resistance alone and is what the CCR actually sees.

Q5Why does a high-resistance connection matter?

Even a small increase in contact resistance can produce significant I²R heating when current is high. Causes include poor mating, contamination, corrosion, mechanical damage, improper termination, water ingress or loose connections. The connection can progressively get hotter and lead to deterioration, intermittent operation and eventual open circuit, and visual inspection alone may not catch it.

Q6How does temperature affect resistance readings?

Resistance of most conductors rises with temperature, so the test temperature should be considered before calling a reading abnormal.

Q7What happens electrically during an open circuit?

The series current path is broken and circuit current falls toward zero. The CCR may enter a protective or open-circuit response state, and the page notes this requires controlled, appropriate testing procedures.

Q8How does a CCR respond to a short circuit?

A fixed-voltage source could see very high current, but a CCR is current-regulated with its own protection and responds differently. The page cautions never to assume every CCR behaves identically.

Q9What affects an earth fault?

Fault behaviour depends on fault impedance, insulation and cable construction, and also on CCR design, earthing arrangement and monitoring. It can cause abnormal current distribution, insulation degradation or protective action.

Q10Why don't electrical measurements always show optical performance?

The chain runs from electrical input through the series circuit and transformer to the fixture and optical output, and healthy electrical readings do not by themselves prove the required optical performance. That is why photometry and inspection complement electrical testing.

Field Insight

Engineering & Expert Notes

RS
By Rahul Sharma
Airfield Ground Lighting Specialist
Ohm's Law Explains More Than You Expect
Basics

Voltage, current and resistance relationships help interpret many readings.

Reactance Matters in Long Circuits
Impedance

Cable length and transformers change the circuit's behaviour. Account for them in design.

Constant Current Changes How You Think About Faults
Series

A short or open circuit produces different symptoms from those in parallel systems.

Losses Appear as Heat
Power

Inefficiency heats equipment. Check ratings and cooling.

Electrical Regulation Is Not Optical Output
Optics

Constant current does not guarantee constant light. Optical condition matters too.

Use Physics to Diagnose
Troubleshooting

Reasoning from principles narrows the search before you open equipment.

Precision AGL Academy

Understand AGL. Build Your Engineering Skills.

You now understand the physics behind AGL circuits — resistance, reactance, regulation and how electrical theory governs constant current series lighting systems.

Take the next step with Precision AGL Academy.

Learn AGL Step by Step

Build your knowledge through structured technical learning covering:

AGL Fundamentals Components Electrical Systems Design Photometry Testing Troubleshooting Maintenance

Move from understanding how AGL works to developing the practical knowledge needed to analyse, operate and maintain complete AGL systems.

Explore AGL Academy Start Your Learning Journey
Understand the system. Build the knowledge. Apply the engineering.

Talk to Precision AGL Academy

Call+91 98214 96412
Emailprecisionaglacademy@gmail.com

Follow the Academy

YouTube@precisionaglacademy LinkedInPrecision AGL Academy Instagram@PrecisionAGLAcademy Facebook@PrecisionAGLAcademy