Shielded cable is specified everywhere noise is suspected — and then silently defeated by a construction detail at the ends. The physics of shielding is well understood; the failure of shielded systems is overwhelmingly a termination and grounding problem. This analysis covers the shield types, the termination methods that preserve performance, and the mistakes that turn an expensive shielded cable into an expensive unshielded one.
What a Shield Does — Two Mechanisms
A shield works in two ways. Against external fields, it reflects and absorbs coupled energy, keeping it out of the conductors (and, symmetrically, keeps internal emissions from radiating out — often the legally binding half of the job). Against capacitive coupling between circuits inside the same harness, individual shields give each sensitive pair its own Faraday enclosure. Magnetic-field coupling is barely affected by shields at low frequency — twisted pairs and geometry do that work — which is why 'shielded' is not a universal fix.
Shield Constructions Compared
- Foil shield (aluminum/polyester): 100% coverage, excellent at high frequency, but fragile at flex points and hard to terminate — hence the drain wire, which concentrates current and adds inductance.
- Braided shield: mechanically robust, low-resistance, terminates beautifully; coverage is 60-95% depending on weave, so at GHz frequencies foil-plus-braid combos are standard.
- Spiral shield: very flexible, good for audio and flexing service, but behaves like a coil at high frequency — wrong choice for fast digital signals.
Choosing among them is a frequency-and-flex decision, not a cost decision alone: a flexing robot harness with spiral shields 'to save money' will radiate and receive noise at frequencies a braid would have contained.
Termination: The 360-Degree Principle
Shield performance is set at its ends. The gold standard is 360-degree termination — the shield circumference clamped directly to a metal backshell that mates to a metal connector shell, continuing the enclosure around the conductors. The common alternative, the pigtail (drain wire twisted and screwed to a lug), forces all shield current through one inductive path; above a few MHz the pigtail's own impedance becomes the dominant path for noise, and shielding effectiveness drops by orders of magnitude. Pigtail terminations are the single most common reason shielded systems underperform. For flexing harnesses where full backshells are impractical, short, wide, soldered shield clamps approach 360-degree performance.
Grounding Schemes
Classic low-frequency practice grounds the shield at one end to break ground loops; high-frequency and emission-control practice grounds both ends, accepting loop currents in exchange for containment, with the loop managed by bonding design. Real systems mix: signal shields grounded per the system's noise model, motor cable shields grounded both ends for emissions. The rule that never breaks: shield currents must have a designed path — never let them find one through your signal returns.
Common Mistakes That Disable Shielding
- Pigtails of more than a few centimeters on fast signals
- Shields cut back to fit a connector cavity 'temporarily' — in production
- Shield continuity assumed but never tested
- Foil shields abraided through by clamp edges, unnoticed inside sleeving
Kaweei manufactures shielded harnesses with braided, foil and hybrid constructions, 360-degree backshell terminations and shield-continuity testing on every assembly. Contact our engineering team with your noise environment and interface list — shield design is a conversation, not a checkbox.