Screen Connector

A Screen Connector (also known as a shield connector, screen termination, or cable screen clamp) is a specialised termination device used to connect the metallic screen (shield) of shielded AGL cables to earth, equipment chassis, or junction box earth terminals. The screen connector provides a low‑resistance, low‑inductance path to earth for induced currents and electromagnetic interference (EMI), ensuring the integrity of signal and power circuits in AGL systems.

Shielded cables are used in AGL systems where electromagnetic compatibility (EMC) is critical—such as communication links, control circuits, sensitive sensor wiring, and connections between CCRs and ALCMS systems. The screen connector:

• Terminates the cable screen: Provides a secure mechanical and electrical connection to the cable's metallic screen (braid, foil, or spiral wrap).
• Bonds to earth: Connects the screen to the earthing system, providing a low‑impedance path for interference currents.
• Reduces EMI: Prevents the cable screen from acting as an antenna, reducing radiated and conducted emissions.
• Prevents electrostatic discharge (ESD): Safely dissipates static charges that can build up on the cable screen.

Screen connectors are available in various configurations, including clamp‑on types, crimp‑on lugs, compression glands with integral screen termination, and solder‑type terminations. They are typically made of tinned copper, brass, or stainless steel for corrosion resistance and low contact resistance.

A shielded cable's screen only works if it is actually connected to earth at every termination point, and the screen connector is the component that makes that connection real rather than theoretical. Without it, the metallic screen is simply a floating conductor that can accumulate static charge or, worse, act as an unintended antenna that re-radiates the interference it was meant to block. By providing a secure, low-impedance bond from the screen to earth, the connector completes the circuit path that carries induced currents away safely, which is what allows the shielded cable elsewhere on the page to deliver its EMI protection in practice rather than only on paper.

Screen connectors are used throughout the airfield wherever shielded cables are employed and screen termination is required, including:

• AGL Control and Monitoring Cables: Terminating the screens of control cables between CCRs, ALCMS workstations, and field equipment to prevent EMI interference.
• Communication Links: Screen termination for fibre optic cable metallic members, telephone lines, and data communication cables.
• Sensor Cables: Terminating screens on weather sensors, RVR sensors, and other sensitive instrumentation cables.
• Junction Boxes and Distribution Panels: Providing a screen termination point within junction boxes and panels for incoming shielded cables.
• CCR Output Cables: For shielded primary cables used in environments with high EMI (e.g., near radar or high‑power transmitters).
• Retrofit and Upgrades: Adding screen termination to existing installations when upgrading to shielded cables for improved EMC performance.

The connector is selected based on the cable screen type (braid, foil, or combination), the screen material (copper, tinned copper, or aluminium), the cable diameter, and the required termination method (crimp, clamp, or solder). Proper screen termination is essential for ensuring the effectiveness of the cable shielding.

The connector is installed at each point along a shielded cable's run where the screen needs to be bonded to earth, typically at junction boxes, equipment enclosures, and cable termination points rather than at intermediate positions along a continuous run. The cable's outer jacket is stripped back to expose the screen (braid, foil, or combination) without damaging the underlying insulated conductors, and the connector is then clamped, crimped, or soldered onto the screen to create a secure mechanical and electrical bond. From there, a short bonding lead connects the screen connector to the local earth bar or chassis earth point, kept as short and direct as practicable, since a long or coiled bonding lead adds inductance that reduces its effectiveness at higher frequencies. Where a cable run enters multiple enclosures, the screen is typically terminated at each entry point rather than left floating partway along the route, so no section of the shield is left unbonded.

A screen connector provides a secure, low‑impedance electrical connection between the cable screen and the earthing system. Its design ensures that the screen is effectively bonded to earth, preventing the screen from becoming an antenna that radiates or picks up interference. The functional principles vary by connector type:

• Clamp‑on Screen Connectors: A split ring or clamp that wraps around the cable screen, with a tightening screw or bolt that compresses the clamp onto the screen. A tail or lug provides a connection point for the earth wire. This type is easy to install and allows for screen termination without cutting the screen.
• Crimp‑on Screen Lugs: A lug with a barrel that accepts the gathered screen braid. The braid is inserted into the barrel and crimped using a standard crimping tool. The lug is then bolted to the earth terminal. This provides a permanent, low‑resistance connection.
• Compression Glands with Screen Termination: Cable glands that incorporate a screen termination ring or spring. When the gland is tightened, the ring compresses against the cable screen, providing a 360° conductive path to the gland body, which is then earthed.
• Solder‑type Screen Terminations: The screen braid is tinned and soldered directly to a terminal or lug. This provides a very low‑resistance connection but requires skilled labour and is less common in field installations.

Key performance requirements for a screen connector include:

• Low Contact Resistance: The connection must have a resistance of a few milliohms to ensure effective EMI suppression.
• Low Inductance: A short, direct path to earth minimises the inductive impedance at high frequencies, which is critical for effective EMI suppression.
• Mechanical Integrity: The connection must withstand vibration, thermal cycling, and mechanical stress without loosening or breaking.
• Corrosion Resistance: The connector materials must resist corrosion to maintain long‑term low resistance.

The screen connector is typically connected to a dedicated earth terminal or to the equipment chassis, which is itself bonded to the main earthing system. The connection should be as short and direct as possible to minimise inductance.

The screen connector sits outside the constant current series circuit and its ALCMS/CCR control loop — it bonds a cable's screen or braid to earth for EMI suppression and carries no lighting current in normal operation. Because it plays no part in the current the CCR regulates, its condition produces no signal the loop-level fault detection can see: a loosened clamp or a degraded crimp does not register as a loop voltage anomaly or open circuit on the CCR. Its consequences are instead indirect and slow to surface — a compromised screen bond degrades EMI suppression on control and monitoring cabling, which can manifest as intermittent noise or unreliable signals on the ALCMS's own monitoring links rather than as a defined fault the system can localise. Verifying the connection therefore falls to periodic continuity and bonding-resistance checks carried out during scheduled maintenance, not to anything the lighting control system reports during normal operation.

Form follows the termination method chosen. A clamp‑on screen connector is a small split‑band or jaw of tinned copper, brass, or stainless steel that closes directly around the exposed screen (braid, foil, or spiral wrap) once the outer cable jacket has been stripped back, with a bolt or screw drawing the band tight to make solid metal‑to‑metal contact across the full circumference of the screen rather than a single point. A crimp‑on lug instead gathers the screen conductors — folded back over the jacket or twisted into a pigtail — into a barrel that is crimped in the same manner as a standard cable lug, with a tongue for bolting to the earth bar. Compression glands with an integral screen termination combine both jobs in one fitting: the gland body seals and strain‑relieves the cable entry into an enclosure while an internal cone or ring clamps the screen against the gland body itself, bonding it to the (earthed) enclosure automatically as the gland is tightened. Whichever method is used, a short, direct earth tail then runs from the connector to the local earth bar or bonding point, kept as short as practicable to preserve the low‑impedance path the screen depends on.