AGL Field Course

The runway at night is an electrical system

Airfield Ground Lighting (AGL) is every light, sign and guidance aid on the movement area, plus the regulators, transformers and cable that keep each one at the right brightness. This course covers what pilots see, why the circuits are built the way they are, and how to size, maintain and fault-find them.

DANGER

Series circuits run at kilovolts. A constant current regulator will raise its output voltage to whatever it takes to push the set current, up to its rated maximum (about 4.5 kV for a 30 kVA, 6.6 A unit). Opening a primary circuit or an isolation transformer secondary while energised produces dangerous voltages.

Always isolate the regulator, open the series cutout, lock out, tag out and prove dead before work. This course is training material. It does not replace your aerodrome's procedures, the manufacturer's instructions or the current edition of the standards.

Why AGL uses series circuits

A runway edge circuit can be 5 to 10 km of cable with a hundred lights strung along it. On a parallel (constant voltage) feed, the lights at the far end would see a lower voltage and burn dimmer. Pilots judge distance and alignment from light patterns, so uneven brightness is not acceptable.

In a series circuit the same current flows through every light. A constant current regulator (CCR) holds that current at a set value, typically 6.6 A at full brightness. Every lamp sees the same current, so every lamp has the same intensity regardless of where it sits on the loop.

Each light connects to the loop through its own series isolation transformer. The primary winding is in series with the loop; the secondary feeds the lamp. When a lamp fails, the transformer keeps the loop closed and the rest stay lit.

The circuit, in four parts

CCR	Converts the 400 V / 480 V supply into a regulated current (2.8 to 6.6 A) at whatever voltage the loop needs.
Primary cable	5 kV class single-core cable running from the vault to every light base and back.
Isolation transformer	One per light. Keeps the loop closed when a lamp fails and isolates the lamp from the high-voltage primary.
Fixture	Halogen lamp or LED engine, elevated on a frangible coupling or inset flush with the pavement.

Pick your track

Electrician / AGL technician

Keep it lit, safely

Airfield lighting map: name every light by colour and position
Series circuit simulator: what lamp failures and earth faults look like
Maintenance & fault finding: LOTO, meggering, sectionalising
Knowledge check, electrician questions

Electrical engineer

Understand the equipment

Series circuit simulator: current, voltage, load
CCRs, transformers, cable: types, protections, ratings
Design calculators: CCR loading, insulation trends
Fault finding: double earth faults, open circuits

System designer

Lay out compliant systems

Airfield lighting map: CAT I vs CAT II/III patterns
System design: circuits, interleaving, switch-over times
CCR sizing calculator
Knowledge check, designer questions

Runway 09, from the approach to the apron

Select any light, or a system from the list under the map, to see its colour, spacing, purpose and how it is wired. Switch between CAT I and CAT II/III to see which systems low-visibility operations add. Lateral distances are exaggerated so the patterns stay readable; longitudinal distances are to scale.

Approach category Brightness B5 View

Swipe the diagram sideways to see all of it.

Try this

Select the runway centreline lights. How does their colour tell the pilot how much runway is left?
Switch to CAT I. Which systems disappear?
Choose the Circuits view, then Lose circuit A. Is the runway still outlined?
Select the PAPI and fly the approach angle from 2° to 4°.

One current, eight lights, one regulator

This loop has a CCR, 4 km of primary cable and eight fixtures, each on its own isolation transformer. Change the brightness step, then break things: fail a lamp by selecting it, cut the cable, add earth faults. Watch which readings move and which stay put.

Brightness step Fixtures

Swipe the diagram sideways to see all of it.

What just happened

Try this

Step from B1 to B5. The current changes, and all lamps change together.
Select lamp 3 to fail it. Which readings move, and which stay put?
Add one earth fault, then the second. Watch the lamps between them.
Cut the primary cable and read what the regulator does.

Model assumptions

Lamp loads are modelled as fixed resistances reflected through transformers of 85 % efficiency. An open-secondary transformer is modelled as a reactive impedance 2.5 times its loaded value, standing in for the saturated magnetising branch. The earth path between two faults is taken as 8 Ω. Real values depend on the transformer, the soil and the regulator, so treat the numbers as trends, not predictions.

Relative intensity per step follows a typical halogen curve: B1 ≈ 0.15 %, B2 ≈ 1.2 %, B3 ≈ 5 %, B4 ≈ 25 %, B5 = 100 %.

The hardware between the switchboard and the lamp

Each piece of equipment in a series circuit has one job and a known failure mode. Knowing both is what lets you read a fault from the vault.

Constant current regulator (CCR)

The CCR takes a constant-voltage supply and delivers a constant current to the loop. It measures output current continuously and adjusts output voltage so the current stays at the selected brightness step, typically within about ±0.1 A at 6.6 A. Load changes (a lamp failing, the cable warming up) change the voltage, never the current.

Rating. A CCR's kVA rating is its rated current times its maximum output voltage. A 20 kVA, 6.6 A regulator can drive up to about 3 030 V. If the loop needs more than that, the CCR can no longer hold the current and the lights dim.

Measuring output. Thyristor CCRs produce a chopped waveform at low steps. Measure output current with a true-RMS clamp meter; an average-responding meter will read wrong.

Step	5-step current	Relative intensity*
B1	2.8 A	≈ 0.15 %
B2	3.4 A	≈ 1.2 %
B3	4.1 A	≈ 5 %
B4	5.2 A	≈ 25 %
B5	6.6 A	100 %

3-step regulators (common on taxiways) use 4.8, 5.5 and 6.6 A. *Intensity for a typical halogen lamp; LED fixtures are driven to their own curve.

CCR types and protections

Technology	How it regulates	Strengths	Watch for
Thyristor (SCR)	Phase-controlled thyristors feed an output transformer	Robust, well understood, widely installed	Low power factor and high harmonics at low steps; true-RMS metering essential
Ferroresonant	Saturating resonant transformer with capacitors	Very rugged, tolerant of surges	Heavy, noisy, sensitive to supply frequency; mostly legacy
IGBT / PWM	Switch-mode inverter synthesises a sinusoidal output	Near-unity power factor, low harmonics, compact, good with LED loads	Electronics need clean cooling air; check firmware for monitoring features

Open-circuit protection

If the loop opens, current falls to zero and the CCR would drive full voltage into the break. Protection shuts the output down within a few seconds and latches until reset.

Overcurrent protection

Trips if output current exceeds the set step by a set margin, protecting lamps and transformers from a regulation failure.

Insulation monitoring

An insulation resistance monitor (IRMS / earth fault monitor) injects a DC test voltage between the loop and earth and reports the insulation resistance continuously.

Series isolation transformer

HIGH V

Never open a secondary on a live circuit. With the lamp removed, the secondary terminals of a saturated transformer carry high peak voltages. Replace lamps only with the circuit isolated, or where your procedures and equipment (for example IEC 62870 secondary circuit safety devices) explicitly allow live lamp changes.

Each fixture has a transformer with its primary in series with the loop and a secondary feeding the lamp. Typical units are 6.6 A / 6.6 A, sized by lamp load: 10/15, 20/25, 30/45, 65, 100, 150, 200 and 300 W are common ratings.

Lamp failure. When the lamp opens, the secondary has no load. The transformer core saturates, the voltage across its primary rises, and the loop stays closed. The CCR sees a small rise in reactive load and keeps the current where it is. Every other lamp stays lit.

Sizing. Pick a rating at or above the lamp wattage. An overloaded transformer cannot deliver full secondary current, so the lamp runs dim. A very oversized one wastes magnetising current.

Cable, connectors and light bases

Primary cable is single-core 5 kV class (FAA L-824), commonly 8 AWG (8.4 mm²) or 6 mm² copper, shielded or unshielded according to the design. It runs in duct or direct-buried, looping in and out of each light base.

Connectors (FAA L-823) are moulded plug-and-receptacle kits on the transformer leads and cable ends. Field-attached kits must be made up exactly as specified and sealed with heat-shrink; a badly made connector is the most common source of water ingress and earth faults.

Light bases (cans) house the transformer and connectors. FAA L-867 bases suit elevated lights in non-load-bearing areas; L-868 bases are load-bearing for inset lights in pavement.

Lightning protection. A bare copper counterpoise is laid above the duct or cable and bonded to each light base, so lightning currents travel in the counterpoise rather than through the cable insulation.

Records. Label every cable at every pit and keep an as-built record of circuit routing. Fault finding on an unrecorded circuit takes days instead of hours.

LED retrofits. LED fixtures draw far less power but present a non-linear load. Confirm the CCR is compatible, check its minimum-load requirement, and re-check lamp-failure detection, which relies on load changes that are much smaller with LEDs.

From operating category to circuit schedule

The operating category the runway must support sets the lighting systems, their serviceability levels and how fast they must recover from a power failure. Circuit design follows from that.

Design sequence

Operational requirement. Runway type, approach category (non-precision, CAT I, II, III), lowest RVR, low-visibility taxi routes, and which rule set applies (ICAO, EASA, FAA or national).
Lighting systems. Choose systems and patterns: approach lighting, PAPI, edge, threshold, end, centreline, TDZ, taxiway centreline or edge, stop bars, runway guard lights, signs.
Fixtures. Select certified fixtures for each position and photometric class. Record wattage and transformer rating for each.
Circuits. Group fixtures into circuits by switching need and integrity. Interleave critical systems. Check every circuit's load and voltage against the CCR rating and cable class.
Power supply. Size the AGL substation and secondary power so each system meets its switch-over time.
Control and monitoring. Specify the airfield lighting control and monitoring system (ALCMS), including individual lamp monitoring where serviceability rules demand it.

Circuit design rules of thumb

Interleave critical systems

Feed alternate lights of the approach, runway edge, centreline and TDZ from two circuits on separate CCRs. If one circuit fails, the pattern stays uniform at half density instead of showing a dark gap. Try Lose circuit A on the map.

One CCR per switching function

Lights that must be switched or set to a brightness step together share a circuit. Systems switched independently (each stop bar, each taxi route segment) need their own circuit or addressable switching on the circuit.

Keep within voltage and load

Load the CCR to roughly 60 to 90 % of rating, leaving room for extensions without running it so lightly that power factor suffers. Keep the step-5 output voltage within the cable's voltage class.

Route for fault finding

Bring circuits back to accessible pits so they can be split in half for sectionalising. Avoid running both interleaved circuits through the same duct where one dig-in could cut both.

Secondary power switch-over times

Maximum time for the light to return after primary power fails. Summarised from ICAO Annex 14, Vol I, Table 8-1; check the current edition.

Runway	Lighting aid	Switch-over
Non-instrument / non-precision	Approach, runway edge, threshold, end, obstacle lights	15 s
Precision CAT I	Approach, runway edge, PAPI, threshold, end, essential taxiway, obstacle	15 s
Precision CAT II/III	Inner 300 m of approach lighting	1 s
	Runway centreline, TDZ, threshold, end, all stop bars	1 s
	Other approach lighting, runway edge	15 s
	Essential taxiway, obstacle lights	15 s

A 1 s switch-over is normally met with a no-break supply (UPS or a continuously running source) or by running the critical CCRs from secondary power during low-visibility operations, since a standby generator start takes longer.

Serviceability objectives (CAT II/III)

Minimum percentage of lights that must be serviceable, summarised from ICAO Annex 14 Ch. 10. A light is unserviceable when its average intensity falls below 50 % of specification.

System	Serviceable
Approach lighting, inner 450 m	95 %
Runway centreline, threshold, runway edge	95 %
Touchdown zone	90 %
Approach lighting beyond 450 m	85 %
Runway end	75 %

There must also be no two adjacent unserviceable lights in most of these systems. This is why CAT II/III installations use individual lamp monitoring: a CCR alone can count failed lamps but cannot tell you whether two of them are next to each other.

Size the regulator, track the insulation

Each calculator opens with an example circuit so you can see how it works. Replace the values with your own.

CCR load and sizing

Fixtures on the circuit

Cable and margins

Loop cable length (m) Conductor Ground temperature (°C) Load power factor Spare capacity (%)

Recommended regulator

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Transformer efficiency defaults by rating are typical figures. Use the manufacturer's data sheet values for a real design, and confirm the regulator's own sizing method.

Insulation resistance trend

Record circuit-to-earth insulation resistance from your periodic megger tests. The trend matters more than any one reading: a steady fall points to a failing connector or cable long before it causes an outage.

Example circuit: taxiway A edge

Bands are example thresholds for a 5 kV circuit tested at its specified DC voltage. Use the acceptance and alarm values your authority or IEC 61821 maintenance plan sets.

Test date Reading (MΩ)

Date	MΩ	Condition

Loop resistance check

Loop cable length (m) Conductor Number of transformers Primary winding resistance each (Ω) Cable temperature (°C) Measured loop resistance (Ω)

Measured vs expected

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Isolate, test, split, repair, prove

Most AGL faults are water, damaged connectors or failed lamps. The skill is finding which pit, quickly and safely, on a circuit that may be several kilometres long.

Safe isolation of a series circuit

Get the circuit released by air traffic control or the operations authority. Agree how the area is protected while it is dark.
Set the CCR to off and switch it to local control, so the ALCMS cannot turn it back on.
Open the CCR input supply and lock it out. Open the series cutout or output disconnect between the CCR and the field circuit.
Apply your personal lock and tag to both. Prove the output dead with a suitably rated detector, testing the detector before and after.
After any insulation test, discharge the cable to earth: a few kilometres of cable holds a charge that can bite.
Before re-energising: remove earths, run an insulation test, confirm all personnel are clear of the field circuit, then restore in reverse order.

Fault-finding scenarios

Work through each scenario. Choose the next step; the feedback explains why it is right or wrong.

Preventive maintenance at a glance

Typical interval	Task
Daily	Lights-out inspection drive, ALCMS alarm review, CCR operation at all steps
Weekly	Check output current at each step with a true-RMS meter; check PAPI and signs
Monthly	Insulation resistance test of each circuit; record and trend the results
Quarterly to semi-annual	Photometric measurement of inset lights; clean prisms and lenses; check PAPI aiming
Annually	Inspect light bases for water and corrosion; check torque on inset fixture bolts; CCR internal inspection

Intervals are examples. Set your own from the maintenance programme your authority requires (ICAO Annex 14 Ch. 10, FAA AC 150/5340-26, IEC 61821).

Test what you know

Questions are tagged by audience. Each answer comes with an explanation, so a wrong answer still teaches something.

Questions for

Terms of the trade

Search terms

Tables to look things up

Lookup tables used across the course. Values are typical; check the current edition of each standard.

Standards you will meet

Document	Covers
ICAO Annex 14, Vol I, Ch. 5, 8, 10	Visual aids (colours, spacing, patterns), electrical systems (secondary power, switch-over times), maintenance and serviceability
ICAO Doc 9157, Aerodrome Design Manual, Parts 4 and 5	Guidance on visual aids and on electrical system design
EASA CS-ADR-DSN	European certification specifications for aerodrome design, including lighting
FAA AC 150/5340-30	Design and installation details for airport visual aids
FAA AC 150/5340-26	Maintenance of airport visual aid facilities
FAA AC 150/5345-10 / -47 / -7 / -26 / -42	Equipment specifications: CCRs (L-828/L-829), isolation transformers (L-830), cable (L-824), connectors (L-823), light bases (L-867/L-868)
IEC 61820, 61821, 61822, 61823, 62870	Series circuit installation, maintenance, CCRs, series transformers, and safety of secondary circuits

Numerical values in this course are typical figures for training. Always design and maintain to the current edition of the standard your authority applies.

Colours and spacing at a glance

System	Colour	Typical spacing	Notes
Approach (precision)	White; red side rows CAT II/III	Barrettes every 30 m over 900 m	Crossbar at 300 m; extra crossbar at 150 m for CAT II/III
Threshold	Green	≤ 3 m across the runway	Unidirectional toward the approach
Runway edge	White; yellow last 600 m	≤ 60 m (instrument runway)	Yellow caution zone on instrument runways
Runway centreline	White; red/white 900–300 m from end; red last 300 m	15 m (30 m permitted in some CAT I cases)	Inset; required for CAT II/III
Touchdown zone	White barrettes	30 m or 60 m, over the first 900 m	CAT II/III
Runway end	Red	At least six lights	Unidirectional toward the runway
Taxiway centreline	Green; green/yellow near runway	≤ 15 m straight (low visibility)	Alternating green/yellow inside the ILS sensitive area or runway strip
Taxiway edge	Blue	≤ 60 m straight, closer on curves	Omnidirectional
Stop bar	Red	3 m across the taxiway	Required where RVR < 550 m operations are conducted
Runway guard lights	Flashing yellow	Pair each side (Config A)	Alternate flashing, 30–60 flashes per minute