What is a shielded cable and its screen connection?
A metallic screen wrapped around the cable core — and the earth connection that makes it work, made at the cable end and checked end to end.
A shielded cable (also called a screened cable) has a layer of copper braid, tape or foil around its insulated conductors, under the outer jacket. The screen connection is how that layer is joined to earth at the cable end: the jacket is cut back at the gland, the screen is led out as a drain lead or clamped, and it is bolted to the earth bar.
A connected screen keeps the electric field inside the cable, carries leakage and induced current safely to earth, and stops outside interference reaching control and signal conductors. A screen that is floating, broken or shorted does none of this, can rise to a dangerous voltage, and can show up later as noise, nuisance alarms or an earth fault. Many primary series cables are unscreened, so the cable specification decides where a screen is used; the design drawing decides whether it is earthed at one end or both.
- Isolate. CCR or supply off, locked out and tagged; prove the cable dead.
- Strip. Cut back the jacket at the gland to the specified length without nicking the screen.
- Expose the screen. Fold back the braid or foil, or attach a drain lead, keeping it clear of the conductors.
- Connect. Fit a lug or clamp (or a 360° screen-bonding gland where the design calls for one) and bolt the screen to the earth bar at the specified torque.
- Insulate. Sleeve the screen lead so it cannot touch other metalwork or an unspecified earth.
- Test & record. Screen continuity end to end, and jacket insulation resistance with the screen temporarily lifted from earth; reconnect, re-check continuity and log the readings.
Frequently Asked Questions
Q1What is a shielded cable in airfield ground lighting?
A shielded (screened) cable has a metallic layer, such as a copper braid, tape or foil, wrapped around the insulated conductors under the outer jacket. The screen is connected to earth so it can carry leakage and induced currents safely, limit electrical noise, and keep the electric field inside the cable.
Q2Is every AGL cable shielded?
No. Many primary series cables are single-core and unscreened, and the specification or design decides. Screened cable is common for multicore control and signal cables, such as those used for ALCMS, monitoring and communication, and for some primary or medium-voltage cable where the design calls for a screen. Always follow the project specification.
Q3What is a screen connection?
It is the way the screen is terminated at the cable end: the jacket is cut back, the screen is exposed, and it is joined to an earth bar or gland through a lug, drain wire or clamp so that it makes a low-resistance path to the earthing system.
Q4Should the screen be earthed at one end or both ends?
It depends on the cable and its purpose, and the design states which. A signal or control screen is often earthed at one end to avoid an earth loop carrying noise current, while a screen that must also handle high-frequency interference or fault current is often earthed at both ends. Never decide this on site; follow the design drawing.
Q5What happens if a screen is left floating or broken?
An unearthed screen can rise to a dangerous voltage by induction or leakage, so it is a shock hazard, and it no longer protects against noise. A screen that is broken at a joint or termination breaks the protective path and can hide a fault that later shows as nuisance alarms, communication errors or an earth fault.
Q6How is a screen connection checked?
With the cable isolated, locked out and proved dead, by inspecting that the screen is terminated, secured and insulated from other metalwork where it should be, then testing screen continuity end to end and the insulation resistance of the jacket, which needs the screen temporarily lifted from earth, and comparing the readings with the commissioning baseline. The screen is then reconnected and its continuity re-checked before the cable returns to service.
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