L‑824 Primary AGL Cable

The L‑824 Primary AGL Cable is a specialised, high‑voltage (5kV) shielded power cable manufactured to the U.S. Federal Aviation Administration (FAA) specification L‑824. It is the standard cable used for primary series circuits in airfield ground lighting (AGL) systems, carrying the constant current (typically 6.6A or 20A) from the Constant Current Regulator (CCR) to every isolating transformer and light fixture in the loop. The cable is designed for direct burial, underground duct, or elevated installation and is engineered to withstand the mechanical, thermal, and environmental stresses of airfield operations.

Key characteristics of L‑824 primary cable include:

• 5kV Voltage Rating: A single‑conductor cable rated 5,000 V for the series circuit it carries, providing ample margin for the high transient voltages that occur in inductive series circuits.
• Shielded Construction: Features a metallic screen (copper tape or braid) that provides electromagnetic interference (EMI) protection, reduces radio frequency interference (RFI), and ensures safety by confining the electric field within the cable.
• XLPE Insulation: Cross‑linked polyethylene insulation offers excellent dielectric strength, high temperature rating (90°C continuous), and resistance to moisture, chemicals, and thermal ageing.
• PVC Outer Jacket: A tough, durable polyvinyl chloride jacket provides mechanical protection against abrasion, impact, and environmental exposure, with options for additional armouring in high‑risk areas.
• Conductor Options: Available with stranded (7‑wire Class B or 19‑wire Class C) annealed copper conductors, sized from 8 AWG to 4 AWG per FAA AC 150/5345‑7F Table 1, to suit circuit length and current rating requirements.

The L‑824 cable is the standard primary cable specified in FAA Advisory Circular AC 150/5345‑7 and is used in airports worldwide, providing reliable power transmission for runway, taxiway, approach, and obstruction lighting systems.

L-824 cable exists to carry the 5kV constant current safely across an entire airfield's primary lighting loop, connecting a CCR to potentially hundreds of isolating transformers and fixtures spread across kilometres of runway and taxiway. Because this cable typically runs buried underground, its construction has to anticipate decades of exposure to soil moisture, mechanical stress from surface loads, and thermal cycling, all while maintaining the insulation integrity that keeps 5kV safely contained rather than posing a shock or fire hazard.

The shielded construction serves a second, equally important purpose beyond insulation: containing the cable's electric field and reducing electromagnetic interference protects both the circuit's own signal integrity and any nearby sensitive equipment, which matters given how close primary cable runs often pass to navigation aids, sensors, and communication infrastructure across the airfield.

The L‑824 primary cable is used throughout the airfield for all primary series circuit applications, including:

• Runway Lighting Circuits: Powering runway edge lights, centreline lights, threshold lights, end lights, touchdown zone lights, and other runway fixtures on precision and non‑precision runways.
• Taxiway Lighting Circuits: Powering taxiway edge lights, centreline lights, stop bars, clearance bars, and guidance signs.
• Approach Lighting Systems: Powering ALSF‑1, ALSF‑2, MALSR, and other approach lighting configurations, where the cable is often routed in underground ducts from the CCR to the approach lighting structures.
• Obstruction Lighting: Powering low‑, medium‑, and high‑intensity obstruction lights on structures, where the cable may be installed in conduits or directly buried.
• CCR to Field Interface: Connecting the CCR output to the field distribution panel or directly to the series loop, often using L‑824 cable with L‑823 connectors at termination points.
• Runway and Taxiway Intersections: Cabling through underground duct banks and handholes, with the cable's robust construction providing protection against mechanical damage.

The cable is suitable for use in temperatures ranging from -40°C to +90°C and is resistant to oils, fuels, de‑icing chemicals, and UV exposure. It is typically installed in trenches at a depth of at least 0.5m, in concrete‑encased duct banks, or on elevated cable trays.

L-824 primary cable is installed in trenches at a depth of at least 0.5m, in concrete-encased duct banks beneath runway and taxiway intersections, or on elevated cable trays inside equipment shelters, following the route set out from the CCR to the field distribution panel and on to each transformer along the series loop. Direct-buried runs are laid on a bed of sand or fine fill to protect the jacket from sharp stones, with warning tape laid above the cable at a shallower depth so future excavation work is alerted to its presence before digging reaches it. Where the cable crosses paved surfaces or enters a structure, it's sleeved through duct or conduit, and slack loops are left at each transformer or junction box connection so a future splice or transformer swap doesn't require pulling in fresh cable from the nearest access point.

The L‑824 primary cable functions as the main power distribution conductor for the AGL series circuit, carrying constant current from the CCR to all isolating transformers. Its design ensures efficient, safe, and reliable power transmission over long distances:

• Conductor: High‑conductivity stranded annealed copper minimises I²R losses, ensuring that the voltage drop along the loop remains within acceptable limits. The conductor is sized to carry the full primary current (6.6A or 20A) with minimal temperature rise.
• Conductor Screen (Semiconductive Layer): A semiconductive layer over the conductor provides a smooth, stress‑relieving interface between the conductor and the insulation, preventing partial discharge and electrical treeing that can lead to premature failure.
• XLPE Insulation: The cross‑linked polyethylene insulation provides high dielectric strength (typically > 25 kV/mm) and excellent thermal stability. Its low dielectric constant and low loss tangent minimise capacitive currents and power losses in long cable runs.
• Insulation Screen (Semiconductive Layer): A semiconductive layer over the insulation ensures a uniform electric field distribution, preventing stress concentration and partial discharge at the insulation surface.
• Metallic Screen (Shield): A copper tape (overlapping) or copper braid screen provides a low‑impedance path for fault currents and shields the cable from external electromagnetic interference. The screen is designed to carry fault currents and is connected to earth at termination points.
• Outer Jacket: The PVC jacket provides mechanical protection against abrasion, moisture ingress, and chemical attack, ensuring the cable's long‑term reliability in buried and exposed installations.

The cable's shielded construction is particularly important in AGL applications, as it prevents the cable from radiating electromagnetic interference that could affect sensitive communication and navigation systems, and protects the series circuit from external EMI sources such as radar and radio transmitters.

The L-824 primary cable carries no control logic of its own — it is the physical medium that makes loop-level control possible in the first place. An ALCMS command sets the required brightness step, the CCR converts that into a fixed regulated current, and it is this cable that then delivers that exact current, unchanged in magnitude, around the entire series loop to every isolating transformer in turn: the cable's role is to keep the current identical at every point on the loop, not to route or vary it. Because the circuit is series-connected, the cable itself becomes part of the CCR's fault-monitoring loop — the same instrument that measures loop voltage against commanded current to detect an open-circuit lamp or failed transformer will just as readily flag a cable fault (an insulation breakdown, a screen discontinuity, or a full open circuit) as a voltage anomaly on that loop. It does not, however, distinguish a cable fault from a fixture fault: both present as the same class of loop-level electrical symptom to the ALCMS, and locating the actual break requires insulation-resistance testing and sectionalised loop testing in the field, rather than any diagnostic capability built into the cable itself.

L-824 primary cable is built as a single-conductor construction with a stranded, bare or tinned copper conductor sized per the current rating, surrounded by a cross-linked polyethylene (XLPE) or ethylene propylene rubber (EPR) insulation layer rated for the 5kV class. A semi-conducting insulation shield and a copper tape or braided shield are applied over the insulation to control the electric field and provide fault-current capacity, followed by an outer jacket of polyvinyl chloride (PVC), polyethylene, or a similar UV- and chemical-resistant compound. The finished cable is round and flexible enough for duct pulling or direct burial, and is marked at regular intervals with its type, voltage class, and manufacturer for identification during installation and maintenance. FAA L‑824 defines two insulation types at the 5,000 V rating: Type C (cross‑linked polyethylene, XLPE) and Type B (ethylene propylene rubber, EPR); Type C's XLPE insulation is the more common choice for direct burial and duct installation.

L-824 airport cable is cable made to the FAA L-824 specification for underground use in airport lighting circuits. The 5 kV type is commonly used for primary series circuits, and a lower-voltage type is used for secondary and other circuits. The cable is chosen for direct burial or duct use and for the voltage class of the circuit it serves.

Check the voltage rating, conductor size, shielding and insulation type against the project specification and the CCR loop design before ordering. For testing, see the AGL primary cable page, which covers cable insulation, continuity and fault checks.