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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 ↓
THE AGL ENGINEERING CENTRE by Precision AGL Academy · Electrical Systems

Airport Isolating Transformer

The link between the CCR series circuit and each airfield light: L-830 / L-831 types, 6.6 A and 20 A ratings, wattage selection, 5 kV insulation, construction, testing, failures and troubleshooting.

Start Learning L-830 Type Table
Overview

What Is an Airport Isolating Transformer?

The interface between the series primary circuit and the individual fixture.

An airport isolating transformer is a key component of a series-circuit Airfield Ground Lighting system. It electrically separates the primary series circuit from the individual light fixture while transferring the required power to it. It is used for runway, taxiway, approach and other airfield lighting.

In FAA terminology, series-to-series isolation transformers are classified L-830 for 60 Hz and L-831 for 50 Hz, with a numerical suffix identifying wattage and current arrangement. FAA AC 150/5345-47C (22 July 2011) is the specification, and the FAA lists it as Active.

CCR
Primary Cable
Primary Connector
Isolation Transformer
Secondary Connector
AGL Fixture

Electrical separation

Separate primary and secondary windings on a core.

Power transfer

Magnetically transfers power from the series circuit to the fixture.

Current matching

Matches series-circuit current to fixture-side requirements.

Robust outdoor design

Watertight, buried and outdoor service, unlike an ordinary building transformer.

Isolation Transformer Principle

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 1. Schematic of the isolation transformer principle.
AGL Visual Encyclopedia: Isolation Transformer Principle Transformer Circuit Position Electrical Isolation
Principle

Why Isolation, and How It Works

Fixtures cannot simply be connected across a long primary series circuit.

AC in primary winding
Alternating flux in core
Voltage induced in secondary
Power to the fixture

The primary winding is connected into the series circuit driven by the CCR; the secondary winding supplies the individual fixture. The windings stay electrically separate while energy transfers through the core. The primary side is part of the continuous series circuit, and every lighting point has its own transformer.

Primary side

  • CCR output and primary cable
  • Primary connectors
  • Series-circuit current
  • High-voltage insulation requirements

Secondary side

  • Secondary cable and connector
  • Light fixture
  • Fixture operating voltage / current
  • Fixture power

Under AC 150/5345-47C the primary winding is rated 5,000 V and the secondary winding 600 V.

CCR Working PrincipleSeries vs Parallel CircuitWhy Series Circuits Are Used

Lamp-Out Behaviour in a Series Circuit

1 · THE SERIES LOOP WITH ONE LAMP OUT 2 · WHAT THE CIRCUIT DOES CCRCONSTANTCURRENTPRIMARY IN THE SERIES LOOPLOOP CURRENT FLOWS THROUGH EVERY ONESECONDARY AND LAMP, ONE PER TRANSFORMERLAMP OUTSECONDARY OPENPRIMARY STILL CLOSES THE LOOPLOOP CLOSED · SET CURRENT HELD · OTHER LAMPS UNCHANGEDTHE CCR SEES ONLY A SLIGHTLY LOWER LOAD 1 · CURRENTHELD BY THE CCR 2 · OTHER LAMPSSAME BRIGHTNESS 3 · CCR LOADA LITTLE LOWER 4 · FAILED UNITOPEN SECONDARY A LAMP-OUT IS NOT AN OPEN CIRCUIT AN OPEN CIRCUIT BREAKS THE LOOP LAMP FAILS · SECONDARY OPENS · LOOP STAYS CLOSED · REPLACE THE LAMP PROMPTLY
Figure 2. Lamp-out behaviour in a series circuit.
AGL Visual Encyclopedia: Lamp-Out Behaviour in a Series Circuit Isolation Transformer Principle Film Cutout Operation
FAA Classification

L-830 / L-831 Transformer Types (AC 150/5345-47C)

Table 1 of the specification: the suffix identifies wattage and current rating. The same suffixes apply to L-831 at 50 Hz.

60 Hz typeWattagePrimarySecondaryMin. efficiency50 Hz type
L-830-130/45 W6.6 A6.6 A80%L-831-1
L-830-230/45 W20.0 A6.6 A80%L-831-2
L-830-365 W6.6 A6.6 A80%L-831-3
L-830-4100 W6.6 A6.6 A85%L-831-4
L-830-5100 W20.0 A6.6 A85%L-831-5
L-830-6200 W6.6 A6.6 A90%L-831-6
L-830-7200 W20.0 A6.6 A90%L-831-7
L-830-8300 W6.6 A20.0 A90%L-831-8
L-830-9300 W20.0 A20.0 A90%L-831-9
L-830-10300 W6.6 A6.6 A90%L-831-10
L-830-11300 W20.0 A6.6 A90%L-831-11
L-830-12500 W6.6 A20.0 A90%L-831-12
L-830-13500 W20.0 A20.0 A90%L-831-13
L-830-14500 W6.6 A6.6 A90%L-831-14
L-830-15500 W20.0 A6.6 A90%L-831-15
L-830-1610/15 W6.6 A6.6 A70%L-831-16
L-830-1720/25 W6.6 A6.6 A70%L-831-17
L-830-18150 W6.6 A6.6 A85%L-831-18
L-830-19150 W20.0 A6.6 A85%L-831-19
Efficiency from Table 2 of the AC. Minimum power factor is 0.95 for every type. This is the FAA classification, not a universal product-selection table.

Identify the transformer type

Choose the three values from the nameplate or design data. Wattage alone never identifies a transformer.

Why wattage alone is not enough. Both L-830-4 and L-830-5 are 100 W, but one is 6.6 A primary and the other 20 A primary. Both L-830-8 and L-830-10 are 300 W, but their secondaries are 20 A and 6.6 A. Correct identification needs wattage + primary current + secondary current + frequency + type.

Different Transformers

FOUR TRANSFORMERS · SAME PRINCIPLE, DIFFERENT JOBS CCR TRANSFORMER at the power end CCR CABINET SUPPLY TO LOOP builds the loop voltage one per CCR, rated in kW a fault hits the whole circuit FIXTURE ISOLATING one light on the loop SERIES LOOP TO LAMP isolates one light below ground, in a can or pit open primary can open loop SIGN TRANSFORMER one sign on the loop SIGN SERIES LOOP TO SIGN feeds one sign only sized to the sign load plug-in sealed connectors APPROACH-LIGHT one light on a stem SERIES LOOP TO LAMP one per light, in a can lead up the frangible stem failed lamp darkens one light
Figure 3. Four AGL transformers in schematic: the CCR transformer inside the CCR cabinet at the power end of the circuit, the fixture isolating transformer in a can below an elevated light, the sign transformer in a can below a sign, and the approach-light transformer in a can beside a frangible stem.
AGL Visual Encyclopedia: Different Transformers CCR Transformer Sign Transformer Transformer Current
Current Ratings

6.6 A and 20 A Transformers

Match the circuit, the fixture and the transformer, not just the CCR label.

6.6 / 6.6 A types

L-830-1, 3, 4, 6, 10, 14, 16, 17, 18. 6.6 A is the most common series-circuit current. A "6.6 A CCR" still does not by itself determine the correct transformer — fixture wattage and type must match.

20 / 6.6 A types

L-830-2, 5, 7, 11, 15, 19. A 20 A primary supplies a 6.6 A fixture-side load, in 30/45, 100, 200, 300, 500 and 150 W classes.

20 A secondary types

L-830-8 and 12 (6.6 A primary) and L-830-9 and 13 (20 A primary) have a 20 A secondary.

Parameter6.6 A application20 A application
Series current6.6 A20 A
Primary currentTypically 6.6 ATypically 20 A
SecondaryDepends on transformer typeDepends on transformer type
WattageApplication dependentApplication dependent
Main considerationMatch 6.6 A circuit and fixtureMatch 20 A circuit and fixture

Using a transformer designed for a 20 A primary on a 6.6 A circuit, without confirming compatibility, would be an engineering error. CCR current, transformer primary rating and fixture load must be coordinated.

6.6 A Isolating Transformer20 A Isolating TransformerCCR Working Principle
Selection

Transformer Wattage Selection

Provide adequate capacity for the fixture and its operating characteristics.

Factors include fixture power, lamp aging, secondary lead losses, contact losses, transformer efficiency, power factor, operating temperature, fixture technology and manufacturer requirements. AC 150/5345-47C specifically requires enough capacity to allow for lamp aging, secondary lead losses and contact losses.

15 W25 W45 W65 W100 W150 W200 W300 W500 WTransformer VA SelectionTransformer Load Calculator

Transformer Wattage Ratings

SAME 45 W FIXTURE · FOUR TRANSFORMER RATINGS 30 W RATING 45 W FIXTURE = 150% OVERLOADED RUNS HOT · LAMP DIMS 65 W RATING 45 W FIXTURE = 69% WELL MATCHED EFFICIENT · HEADROOM 100 W RATING 45 W FIXTURE = 45% LIGHTLY LOADED LOWER EFFICIENCY 200 W RATING 45 W FIXTURE = 23% MUCH TOO LARGE MAGNETISING LOSS FIXTURE LOADPOWER IT DRAWSWHAT IT NEEDS RATINGON THE NAMEPLATETHE CEILING LOADINGSHARE OF RATINGCLOSE, NOT OVER CIRCUIT TOTALLOADS PLUS LOSSESADDS UP CCRSUPPLIES THE SUMMUST COVER IT THE RATING IS A CEILING, NOT THE LOAD · CHOOSE THE NEAREST STANDARD SIZE AT OR ABOVE THE REAL DRAW
Figure 4. Four isolation transformers with ratings of 30, 65, 100 and 200 watts, each feeding the same 45 watt fixture.
AGL Visual Encyclopedia: Transformer Wattage Ratings Transformer Sizing Transformer Nameplate
Selection Guide

Airport Transformer Selection Guide

Step 1

Identify the series circuit: 6.6 A, 20 A or project-specific current.

Step 2

Identify the fixture: type, manufacturer, lamp/LED technology, rated power, required secondary characteristics.

Step 3

Determine the secondary requirement: fixture-side current and voltage.

Step 4

Select transformer wattage with sufficient capacity.

Step 5

Confirm frequency: 60 Hz (L-830) or 50 Hz (L-831).

Step 6

Check installation conditions: burial, water, temperature, deicing chemicals, soil, cable routing, connectors.

Step 7

Verify standards: FAA specification, ICAO, national authority, airport specification, drawings, manufacturer data.

ParameterCheck
Transformer typeL-830 / L-831 or project equivalent
Frequency50 Hz / 60 Hz
Primary current6.6 A / 20 A / required value
Secondary currentMatch fixture
WattageMatch fixture and load
Primary / secondary ratingApplicable requirement
Efficiency and power factorVerify specification
Impedance characteristicsVerify manufacturer data
ConnectorApproved type
InstallationBuried / outdoor suitability
EnvironmentalTemperature, deicing, water
CertificationRequired project / authority approval

Transformer Selection

SELECTION FLOWCHART · FOUR DECISIONS IN ORDER 1LOAD What must it carry? A single fixture → Rating to fixture watts Sign or several loads → Rating to total watts 2CIRCUIT What circuit current? 6.6 A series circuit → 6.6 A primary Other circuit current → Primary to match it 3SECONDARY What does it feed? Lamp (halogen) load → Secondary suits lamp LED load → Secondary suits LED 4LOCATION Where will it sit? Below ground, pit or can → Sealed, water-resistant In a sign or housing → Fits the housing CONFIRM BEFORE ORDERING rating and primary current · insulation level · connectors · standard compliance FLOWCHART · THE MANUFACTURER DATA SHEET AND STANDARD DECIDE THE FINAL CHOICE
Figure 5. Flowchart of four decisions for selecting an AGL isolating transformer: load (fixture or sign), circuit current (6.6 A or other), secondary load (lamp or LED) and location (below ground or in a housing), followed by a confirmation of rating, insulation, connectors and standard compliance.
AGL Visual Encyclopedia: Transformer Selection Transformer Sizing Transformer Nameplate
Insulation

5 kV Primary Insulation

Why AGL transformers need purpose-designed construction.

5,000 V primary

Primary winding voltage rating; primary leads use cable insulated for not less than 5,000 V.

600 V secondary

Secondary winding voltage rating.

Buried and submerged

Must operate buried and submerged with up to 5,000 V between primary winding and ground.

Do not apply an arbitrary megger voltage because a transformer is described as "5 kV". The voltage rating and a field insulation-resistance test voltage are different parameters; the approved test procedure sets the field test voltage.

For context, the AC's qualification and production tests use DC for 1 minute between each winding and ground, with the transformer heated and water-soaked: 15 kV DC on the primary and 5 kV DC on the secondary, with minimum resistance of 7,500 MΩ / 2,500 MΩ up to 150 W and 3,000 MΩ / 1,000 MΩ above 150 W (or the specified leakage-current limits). These are factory/qualification values, not field acceptance values.

Construction

Construction, Encapsulation & Watertight Design

Designed for demanding outdoor and underground service.

Windings and core

Electrically separate primary and secondary coils on a core, insulated from each other and the core. Capable of indefinite operation with the secondary short- or open-circuited at rated primary current.

Watertight case

Permanently sealed, no cracks, blisters or holes, case at least 0.25 in (6.35 mm) thick, moisture entering a lead cannot track along it into the windings.

Encapsulation

Core and coil assembly encapsulated with no internal air pockets; case material at least black cross-linked polychloroprene (TPE/TPV acceptable), durometer 75 ± 10.

Environment

Continuous outdoor service from −55 to +65 °C; withstands sunlight, oil, gasoline, water, runway deicing fluids and acid/alkaline soils over a 10-year service life.

Orientation

Operates in any orientation; fits a cylinder 8 in (20.3 cm) in diameter by 10 in (25.4 cm) high.

Mechanical robustness

Withstands shipping and installation; lead junction must not crack when carried by one lead; shock and lead-rigidity tests apply.

Water ingress is among the most important concerns in buried AGL systems. Causes include a damaged connector or case, improper sealing, lead damage, impact and long-term degradation. Effects: insulation deterioration, leakage current, corrosion, intermittent operation and winding failure. A transformer showing signs of water ingress should be investigated before it is returned to service.

Transformer Construction

MAGNETIC CORE MODEL · RATING · SERIAL PRIMARYCONNECTOR SECONDARYCONNECTOR PRIMARYSECONDARY OUTSIDE → IN: HOUSING · POTTING · INSULATION · WINDINGS · CORE CUTAWAY · ILLUSTRATIVE · CONSTRUCTION VARIES BY MANUFACTURER · NOT TO SCALE
Figure 6. Cutaway of a series isolation transformer showing how it is built.
AGL Visual Encyclopedia: Transformer Construction Transformer Housing Sealing system Insulation System
Leads & Connectors

Transformer Leads and Connectors

The connectors form part of the environmental and electrical protection system.

LeadRequirement (AC 150/5345-47C)
Primary H1Style 2 plug per AC 150/5345-26
Primary H2Style 9 receptacle per AC 150/5345-26
Primary cableNo. 8 AWG (min. 6 mm²), 19-strand minimum, insulated for not less than 5,000 V; extends 24 in ± 3 in (60 cm ± 7.5 cm) including connector
SecondaryTwo-conductor No. 12 or 14 AWG, 600 V cable, Style 7 or 8 receptacle; extends 48 in ± 3 in (120 cm ± 7.5 cm) including connector
ContactsLarge contact to X1, smaller contact to X2
CapsWater-resistant protective caps for the connector mating parts during shipping and handling

Poor connector installation can cause water ingress, increased contact resistance, intermittent operation, open circuit, local heating and insulation failure. A secondary fault can affect a single fixture while the primary series circuit continues to operate, so individual light failures should not automatically be blamed on the CCR or primary circuit.

Cables & ConnectorsTransformer Primary PlugTransformer Secondary PlugSecondary Connector

Plug-in Isolation Transformers

SAME TRANSFORMER, TWO WAYS TO CONNECT IT PLUG-IN · MOULDED CONNECTORS LOOP CABLE FROM CCR ON TO NEXT LIGHT PRIMARY PLUG PRIMARY PLUG SECONDARY PLUG TO FIXTURE SWAP = UNPLUG THREE, PLUG THREE FACTORY-MADE · KEYED · SEALED FIELD-JOINTED · JOINED IN THE PIT LOOP CABLE FROM CCR ON TO NEXT LIGHT FIELD JOINT FIELD JOINT FIELD JOINT TO FIXTURE SWAP = CUT, RE-JOINT AND SEAL THREE MADE ON SITE · QUALITY SET BY THE JOINT REPLACING A PLUG-IN UNIT · ON AN ISOLATED CIRCUIT ONLY ISOLATECCR OFF · LOCKED OFFPROVEN DEAD UNPLUGTHREE CONNECTORSLABEL THE LEADS SWAPSAME TYPE AND RATINGSEAT THE NEW UNIT PLUG INCLEAN · MATED · SEALEDMAKE EACH JOINT TESTELECTRICAL TESTSBEFORE ENERGISING PLUGS SPEED THE SWAP · BUT EVERY CONNECTOR STILL HAS TO BE CLEAN, FULLY MATED AND SEALED
Figure 7. Two series isolation transformers in a pit, one above the other.
AGL Visual Encyclopedia: Plug-in Isolation Transformers Primary Connector Secondary Connector Connector Sealing
Electrical Characteristics

Impedance, Efficiency & Power Factor

Two transformers of the same wattage are not necessarily interchangeable.

Leakage reactance

Magnetic flux that does not link both windings. Transformer impedance = resistance + leakage reactance; it influences voltage drop, current under load, short-circuit current, power loss and circuit behaviour.

Efficiency

AC 150/5345-47C sets minimum efficiency by type: 70% (10/15 W and 20/25 W), 80% (30/45 W and 65 W), 85% (100 W and 150 W) and 90% (200 W and above).

Power factor

Minimum 0.95 for all listed types under the specified test conditions. Evaluate together with efficiency, loading, operating current and fixture type.

Winding design, impedance, efficiency, power factor, secondary voltage characteristics and short-circuit behaviour may differ and can influence the whole series circuit. Replace a transformer on the basis of the approved type and electrical characteristics, not physical size or wattage. The AC also gives secondary full-load current, short-circuit current, load ohms and maximum open-circuit voltage for each type.

Applications

Runway, Taxiway & Approach Lighting Transformers

Each is an AGL isolation transformer; type depends on fixture and circuit.

Runway lighting transformer

Supplies runway edge, threshold, end, centreline, touchdown zone or approach lights from the series circuit.

Taxiway lighting transformer

Taxiway edge and centreline lights, stop bars and other series-circuit visual aids.

Approach lighting transformer

Many fixtures along the approach; consider fixture wattage and number, series current, circuit design, environment and approved equipment.

Runway Lighting SystemsTaxiway Lighting SystemsApproach Lighting Systems

Approach-Light Transformer Arrangement

GROUND LEVEL SERIES LOOP from the CCR SERIES LOOP to the CCR APPROACH LIGHT FRANGIBLE STEM break-away coupling ONE TRANSFORMER PER LIGHT SECONDARY PRIMARY SHORT SECONDARY LEAD BASE CAN · BELOW GROUND ISOLATING TRANSFORMER LOOP PASSES THROUGH THE CAN SIDE VIEW OF THREE LIGHTS ON ONE SERIES CIRCUIT · SCHEMATIC · NOT TO SCALE
Figure 8. Side view of three approach lights on one series circuit.
AGL Visual Encyclopedia: Approach-Light Transformer Arrangement PAPI Transformer Arrangement Buried and Elevated Transformers
LED

LED AGL and Isolation Transformers

Replacing an incandescent fixture with LED does not automatically keep the old transformer valid.

LED and incandescent fixtures can differ substantially in power consumption, current characteristics, power factor, startup behaviour and electronic load. Review the complete system: CCR → series cable → isolation transformer → LED fixture, and confirm compatibility with the fixture's characteristics and the approved AGL design.

LED Isolating TransformerLED AGLLED Retrofit Projects

LED Power-Draw and CCR Re-sizing After Retrofit

SAME 60-FIXTURE CIRCUIT · LOAD BEFORE AND AFTER THE RETROFIT · ILLUSTRATIVE 0 2.5 kW 5 kW 7.5 kW 1 · HALOGEN (BEFORE) 7.5 kW CCR · ABOUT 556 V OUT 2,700 W 300 671 W 3,671 W 49% OF RATING CABLE LOSS · ABOUT 18% OF THE LOAD 2 · LED, CCR UNCHANGED 7.5 kW CCR · ABOUT 218 V OUT 600 W 169 671 W 1,440 W 19% OF RATING CABLE LOSS STAYS AT 671 W · NOW ABOUT 47% CHECK THE CCR'S MINIMUM LOADING AND REGULATION 3 · LED, CCR RE-SIZED 2.5 kW CCR · ABOUT 218 V OUT 600 W 169 671 W 1,440 W 58% OF RATING FIXTURES TRANSFORMER LOSS CABLE I²R LOSS · SAME CURRENT, SAME LOOP HOW TO RE-SIZE 1 · RE-ADD THE LOAD FIXTURES, TRANSFORMERS, CABLE 2 · COMPARE RATING AND MINIMUM LOADING 3 · CHOOSE KEEP, BALANCE, COMBINE, RE-SIZE 4 · PROVE IT CURRENT AND LIGHT AT EVERY STEP THE CABLE LOSS DOES NOT FALL WITH THE LAMPS · ADD UP THE NEW LOAD, DO NOT SCALE THE OLD ONE
Figure 9. Horizontal bars on one power scale showing the load of the same 60-fixture series circuit in three states, split into fixture power, transformer loss and cable loss.
AGL Visual Encyclopedia: LED Power-Draw and CCR Re-sizing After Retrofit Transformer Sizing LED Driver Interaction with Thyristor CCR
Substitution

Can Any Transformer Be Used With an Airport Light?

No.

An ordinary electrical transformer must not be substituted for an AGL isolation transformer. It must suit series-circuit operation, required current, fixture wattage, environment, insulation requirements, connector system, installation arrangement, applicable certification and project requirements. For FAA projects, AC 150/5345-47C is the transformer specification and AC 150/5345-53 provides the airport lighting equipment certification programme.

Isolation Transformer vs AutotransformerIn-Base vs Remote Transformer
Installation

Airport Transformer Installation

Follow the approved drawings and manufacturer instructions.

Before

Verify the transformer: part number, type, frequency, wattage, primary and secondary current, certification.
Inspect the housing: cracks, cuts, damage, deformation, connector and lead damage.
Prepare the installation point per the approved arrangement.
0 / 3 complete

Connect

Connect the primary circuit using the specified AGL connector system.
Connect the secondary lead to the light fixture.
Verify cable routing without excessive mechanical stress.
Check connector integrity: correct mating and sealing.
0 / 4 complete

Test & Verify

Check continuity and insulation with the applicable tests.
Energize the series circuit under controlled conditions.
Verify light operation, then perform photometry where required.
0 / 3 complete
SOP: Isolation TransformerInstallation & Commissioning

Transformer-Pit Installation

THE PIT · SECTION lid flush, cables in, firm drained base CABLE ENTRIES · PLAN primary in and out, secondary to the light PAVEMENT BACKFILL LID · FLUSH ISOLATING TRANSFORMER DRAINED BASE DRAIN PRIMARY IN PRIMARY OUT SECONDARY PAVEMENT LID AND PIT PRIMARY IN PRIMARY OUT SECONDARY LIGHT EARTH CONNECTION SIX INSTALLATION CHECKS · IN ORDER 1 POSITION set out from the design, clear of existing services 2 EXCAVATE size and depth to the design, firm free-draining base 3 SET THE PIT level, at the right height, lid flush with the surface 4 ENTRIES ducts at the design positions, sealed against water 5 DRAIN path or outlet working, not blocked 6 BACKFILL as specified, lid fits, debris cleared away
Figure 10. Transformer-pit installation in two views and six checks.
AGL Visual Encyclopedia: Transformer-Pit Installation Transformer Placement Transformer-Pit Safety Transformer Installation
Testing

Testing and Insulation Testing

Per the applicable specification, manufacturer instructions and project test procedure.

Visual inspection

Case, leads, connectors, physical damage, sealing.

Continuity

Appropriate windings and connections.

Insulation

Windings must be insulated from each other, the core and ground; use an approved procedure and suitably rated equipment.

Electrical characteristics

Primary and secondary current, voltage, power factor, efficiency and load characteristics as required.

Qualification

Certified equipment undergoes characteristics, shock, 20-cycle insulation, temperature-rise and production ratio and void tests.

AGL Testing & InspectionInspection: Testing & Calibration

Insulation-Resistance Testing

NEW SERIES LOOP · INSULATION-RESISTANCE TEST (DE-ENERGISED) BEFORE ENERGIZATION 1 · ISOLATE 2 · DISCHARGE 3 · APPLY DC 4 · RECORD CCR OFF · LOCKED OUT LOOP DISCONNECTED CABLE + TRANSFORMER INSULATION UNDER TEST MΩ DC TEST VOLTAGE EARTH LEAKAGETHROUGH THEINSULATION DC VOLTAGE TO EARTHread in megohms HIGH AND STEADY IS GOODlow or falling means a fault DISCHARGE AFTER TESTstored charge can shock SCHEMATIC · LOOP SHORTENED · NOT TO SCALE · VOLTAGE, TIME AND LIMITS COME FROM THE CABLE RATING AND SPECIFICATION
Figure 11. Insulation-resistance test on a new series lighting loop.
AGL Visual Encyclopedia: Insulation-Resistance Testing Transformer Testing Insulation Resistance Transformer Inspection
Failure Modes

Isolation Transformer Failure Modes

Tap a card for details.

Water ingress

Possible causes

  • Damaged connector
  • Damaged transformer case
  • Improper sealing
  • Lead damage
  • Mechanical impact
  • Incorrect installation
  • Long-term degradation

Open circuit

Possible causes

  • Broken primary lead
  • Failed winding
  • Loose connector
  • Damaged cable
  • Poor termination
  • Note: an open primary can affect the series circuit; an open secondary usually affects only its fixture

Short circuit / abnormal impedance

Possible causes

  • Abnormal CCR operation
  • Increased current demand
  • Incorrect circuit voltage
  • Protection operation
  • Fixture failure
  • Transformer heating

Other mechanisms

Possible causes

  • Insulation breakdown
  • Overheating and excessive loading
  • Mechanical or installation damage
  • Environmental degradation
  • Internal winding failure
Common Isolating Transformer IssuesIsolation Transformer FailuresSeries Circuit Electrical Faults

Open-Secondary Behaviour and Core Saturation

SAME SERIES CURRENT · THREE SECONDARY CONDITIONS 1 · LOADED FLUX VOLTS KNEE LAMP LIT SECONDARY OPPOSES PRIMARY SMALL FLUX · LOW SINE VOLTAGE 2 · SECONDARY OPEN FLUX VOLTS LAMP FAILED NOTHING OPPOSES THE PRIMARY FLUX DRIVEN TO THE KNEE 3 · CORE SATURATED FLUX VOLTS FLUX FLAT-TOPS VOLTAGE TURNS PEAKY CORE HEATS · TREAT AS LIVE CCR HOLDS THE LOOP CURRENT CONSTANT CURRENT PRIMARY STAYS IN THE LOOP FULL CURRENT OPEN SECONDARY LAMP OR LEAD OPEN NOTHING OPPOSES CORE SATURATES FLUX FLAT-TOPS TERMINALS PEAKY VOLTAGE TREAT AS LIVE AN OPEN SECONDARY IS NOT AN OPEN CIRCUIT · THE LOOP STAYS CLOSED, BUT THE CORE IS DRIVEN HARD AND THE TERMINALS ARE LIVE
Figure 12. Three panels with the same series current.
AGL Visual Encyclopedia: Open-Secondary Behaviour and Core Saturation Transformer Failure Modes Secondary Shorting and Overvoltage Events Film Cutout Operation
Troubleshooting

Is the Transformer the Problem?

A dim or dead light does not automatically mean the transformer failed.

CCR
Primary Circuit
Transformer
Secondary Circuit
Fixture
Photometry

Light not working

  • CCR operating condition
  • Series-circuit current
  • Primary cable and connector
  • Transformer
  • Secondary connector
  • Fixture

Light is dim

  • CCR current step and circuit current
  • Transformer type and loading
  • Secondary connection
  • Fixture condition
  • Photometric performance

Repeated transformer failures

  • Incorrect rating or excessive load
  • Incorrect series current
  • Cable / connector issues, water ingress
  • Installation practice, environment
  • Fixture incompatibility

Repeated failures should be treated as a system problem rather than solved by repeatedly replacing the transformer. A transformer can contribute to low intensity, abnormal fixture operation or uneven performance, but low output can equally come from fixture optics, alignment or contamination.

Troubleshooting EngineAGL PhotometryPhotometric Testing Guide

Transformer Troubleshooting

TRANSFORMER TROUBLESHOOTING · SYMPTOM, CAUSE, CHECKPOTTED HOUSINGP1P2SERIESLOOPS1S2FIXTUREPRIMARYSECONDARY123451 PRIMARY CONTINUITY · 2 SECONDARY CONTINUITY · 3 PRIMARY TO EARTH IR4 PRIMARY TO SECONDARY IR · 5 VISUAL: HOUSING, POTTING, WATERALL LIGHTS OUTPrimary winding open · check 1CCR TRIPS, EARTH FAULTInsulation breakdown · checks 3, 4, 5ONE LIGHT OUT, LOOP COMPLETESecondary side · check 2DIM OR FLICKERING LIGHTSecondary connection · checks 2, 5HOT, DISCOLOURED, SMELLOverload or internal fault · check 5ALWAYSTELL ATC · ISOLATE AND LOCK OFF THE CCR · TEST THE TRANSFORMER DISCONNECTED · AUTHORISED ONLYTest methods and limits come from the transformer manufacturer and the airport’s safe-working rules.
Figure 13. Transformer troubleshooting map.
AGL Visual Encyclopedia: Transformer Troubleshooting Transformer Failure Transformer Continuity Connector Troubleshooting
Maintenance & Replacement

Maintenance and Replacement Procedure

Physical condition

  • Case, leads, connectors
  • Cracks, abrasion, damage
  • Contamination

Electrical condition

  • Continuity and insulation
  • Circuit performance
  • Abnormal current / voltage

Installation

  • Correct transformer type and connections
  • Cable condition
  • Connector sealing

Operational performance

  • Fixture operation and light output
  • CCR operation
  • Photometry where applicable
Replacement stepAction
1. Identify existing unitRecord manufacturer, part number, L-830/L-831 class, wattage, primary and secondary current, frequency
2. Identify the fixtureType, manufacturer, rated load, lamp/LED technology
3. Verify the circuitCCR current, series configuration, cable arrangement, connector type
4. Select the replacementMeets or exceeds the approved specification and matches the application
5. Inspect connectionsPrimary and secondary connectors
6. TestApplicable electrical and operational tests
7. Verify lightingPhotometry where required
Workflow

Selection Engineering Workflow

Better than choosing from a catalogue photograph or nominal wattage.

Identify AGL circuit
Primary current
Frequency
Fixture and wattage
Secondary requirement
Select type
Verify characteristics
Verify connectors
Environment
Certification
Install
Test and commission
Standards

Standards and Applicability

FAA AC 150/5345-47C — Specification for Series to Series Isolation Transformers for Airport Lighting Systems (22 July 2011; FAA status Active). It covers L-830 (60 Hz) and L-831 (50 Hz). Related documents are AC 150/5345-26 (L-823 plug and receptacle connectors) and AC 150/5345-53 (Airport Lighting Equipment Certification Program).

Use of the AC is generally recommended; it is mandatory for projects funded through the AIP and PFC programs. L-830 and L-831 are FAA classifications — the applicable transformer standard at a given airport depends on the governing aviation authority, project specification and equipment-approval requirements. For non-FAA projects review ICAO requirements, the national authority's requirements, the airport specification and project documentation.

FAA AC 150/5340 SeriesICAO Annex 14 RequirementsStandards Companion
Pitfalls

8 Common Mistakes

1. Selecting only by wattage

Wattage alone does not identify the transformer.

2. Ignoring primary current

6.6 A and 20 A units can share a wattage but suit different circuits.

3. Ignoring frequency

A 50 Hz unit is not automatically equivalent to a 60 Hz unit.

4. Using an ordinary transformer

AGL transformers are purpose-designed for airport series circuits.

5. Ignoring connector compatibility

A similar-looking connector does not make it electrically or mechanically suitable.

6. Ignoring environment

Buried units need water, chemical and mechanical resistance.

7. Replacing without checking the fixture

Select as part of the complete fixture circuit.

8. Testing only the transformer

Consider CCR, cable, transformer, connector and fixture together.

Quick Reference

Airport Isolating Transformer — Quick Reference

ParameterDescription
PurposeElectrical isolation and power transfer
ApplicationAGL series lighting
Primary / secondaryPrimary in the series circuit; secondary to the light fixture
Common series current6.6 A (20 A and project-specific also used)
FAA designationsL-830 (60 Hz), L-831 (50 Hz)
Winding ratings (AC 47C)Primary 5,000 V; secondary 600 V
ConstructionEncapsulated, sealed, watertight; outdoor and buried service
SelectionCurrent + wattage + frequency + fixture + specification
TestingElectrical + insulation + operational as applicable
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FAQ

Frequently Asked Questions

Q1What is an airport isolating transformer?

A purpose-designed transformer used in an AGL series circuit to electrically isolate the primary series circuit from an individual light fixture while transferring electrical power to the fixture.

Q2What is an AGL isolation transformer?

An isolation transformer specifically designed for Airfield Ground Lighting series circuits.

Q3What is a runway lighting transformer?

An AGL isolation transformer used to supply a runway lighting fixture from the airport series circuit.

Q4What is an airport lighting transformer?

A transformer designed for the electrical requirements of airport visual-aid lighting systems, particularly series-circuit AGL installations.

Q5What is an L-830 transformer?

L-830 is the FAA designation for a 60 Hz series-to-series isolation transformer used in airport lighting systems.

Q6What is an L-831 transformer?

L-831 is the FAA designation for a 50 Hz series-to-series isolation transformer used in airport lighting systems.

Q7What is a 6.6/6.6 A isolation transformer?

A transformer with a 6.6 A primary current arrangement and a 6.6 A secondary current arrangement under the applicable transformer specification.

Q8What is a 20/6.6 A airport transformer?

A transformer configured for a 20 A primary and 6.6 A secondary current arrangement.

Q9Why are 6.6 A transformers used in airport lighting?

6.6 A is a common series-circuit current in AGL systems. Specific transformer ratings must still match the fixture and circuit requirements.

Q10Can a 20 A transformer be used on a 6.6 A circuit?

Not simply because the physical transformer looks similar. It must be specifically suitable for the intended circuit and application.

Q11What wattage airport isolation transformer do I need?

It depends on the light fixture, lamp or LED characteristics, secondary requirements, losses and the applicable specification. Select it from approved equipment data rather than by wattage alone.

Q12What does 5 kV mean on an airport isolation transformer?

In FAA AC 150/5345-47C the primary winding voltage rating is 5,000 V and the secondary winding rating is 600 V.

Q13Does a 5 kV transformer require a 5 kV insulation-resistance test?

Not necessarily. The voltage rating and the field insulation-resistance test voltage are different parameters; the approved test procedure must specify the test voltage.

Q14Can airport isolation transformers be buried?

Yes. AC 150/5345-47C requires the transformer to operate when buried and submerged, with runway deicing agents present.

Q15Why are airport isolation transformers watertight?

They may be installed underground and exposed to water, soil, deicing fluids and other environmental conditions.

Q16What is leakage reactance in an AGL transformer?

Part of transformer impedance, contributing to voltage drop and current behaviour and so to the transformer's electrical performance in the AGL circuit.

Q17What causes an airport isolation transformer to fail?

Insulation degradation, water ingress, connector failure, excessive loading, physical damage, incorrect selection, overheating and internal winding faults.

Q18Can a transformer cause a runway light to become dim?

Yes, but the CCR, cable, connectors, fixture and optical system should also be checked.

Q19How do you test an airport isolation transformer?

Depending on the procedure: visual inspection, continuity, insulation testing and electrical characteristic testing, followed by verification of fixture operation.

Q20Should an airport transformer be tested after replacement?

Yes. Apply the applicable commissioning and electrical tests, and verify lighting performance and photometry where required.

Q21What transformer is used with LED airport lights?

It depends on the specific LED fixture, series-circuit current and manufacturer requirements. Verify compatibility rather than assuming the incandescent-era transformer is suitable.

Related

Related AGL Engineering Pages

CCR

CCR Working Principle

Open →
PHOTOMETRY

AGL Photometry

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CABLES

AGL Cables & Connectors

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TESTING

AGL Testing & Inspection

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OVERVIEW

AGL Isolating Transformers

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ARTICLE

Isolating Transformers: The Backbone

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ARTICLE

Series Circuits & Isolating Transformers

Open →
DIVISION

Isolation Transformer Division

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CALCULATOR

Isolation Transformer Calculator

Open →
RUNWAY

Runway Lighting Systems

Open →
TAXIWAY

Taxiway Lighting Systems

Open →
PAPI

PAPI Engineering Guide

Open →

Select correctly. Connect properly. Test thoroughly.

Protect the complete AGL circuit: match current, frequency, wattage, fixture, circuit, environment and specification.

AGL Isolating Transformers AGL Electrical Services
Field Insight

Engineering & Expert Notes

RS
By Rahul Sharma
Airfield Ground Lighting Specialist
Check Connectors Before Replacing the Unit
Connectors

Open circuits and intermittent faults often trace to moulded connectors or leads, not the transformer core.

Choose the Rating With Margin
Selection

Select wattage for the actual load and consider LED retrofits when the load changes.

Insulation Class and Primary Rating Matter
Insulation

Verify the transformer is suitable for the circuit's insulation requirements and the application.

Moisture Is the Main Long-Term Threat
Water

Inspect housing, leads and seals, and fix drainage problems that expose transformers to standing water.

Measure Before Condemning
Testing

Insulation and continuity measurements identify the actual fault location before a part is replaced.

Record Type and Location
Records

Keep a register of transformer ratings and locations to speed up future maintenance.

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