Why shield type is a real design decision, not a checkbox
"Shielded cable" gets specified as if it's one thing, but the construction underneath that word determines what interference it actually blocks, how many bend cycles it survives, and how it has to be terminated. Picking the wrong shield type doesn't usually show up as a total failure — it shows up as intermittent noise that's hard to trace, or a cable that shields perfectly on the bench and then fractures three months into a drag-chain application. The three constructions in common use — foil, braid, and spiral (serve) — aren't interchangeable, and none of them is simply "better"; each is a different trade-off between frequency coverage, flexibility, and termination effort.
Foil shielding
Foil shielding is a thin layer of aluminum bonded to a polyester (Mylar) film, wrapped around the conductors with an integrated bare or tinned-copper drain wire for grounding, since you can't solder or crimp directly to aluminum foil.
Coverage: 100% continuous coverage — there are no gaps in the wrap the way there are in a woven or spiraled shield.
Frequency performance: Best at high frequencies (above roughly 10 MHz), where that continuous coverage matters most. Typical attenuation runs 25–45 dB.
Flex life: Poor. The aluminum layer fractures under repeated bending, so foil-only shielding is a bad fit for any cable that moves in service.
Termination: Requires the drain wire — there's no direct connection to the foil itself.
Typical applications: USB, HDMI, Cat6 and similar data cables, and other fixed or lightly-flexed high-frequency signal runs where 100% coverage against high-frequency EMI matters more than mechanical durability.
Braided shielding
Braided shielding is a woven mesh of (usually tinned) copper strands forming a flexible sleeve around the conductors — the shield you can see and feel as an actual braid if you strip the jacket back.
Coverage: 40–95% depending on weave (pick) density — denser braids cost more and add stiffness, so coverage is a real spec to check rather than assuming "braided" means fully covered.
Frequency performance: Strongest against low-frequency magnetic interference (below roughly 10 MHz), which foil handles poorly. Attenuation typically runs 30–50 dB.
Flex life: Moderate — mechanically robust and a good default for general handling, but individual strands eventually fatigue and break under sustained flexing.
Termination: Best terminated with a 360° shield termination at the backshell (the braid clamped or soldered around its full circumference) rather than a pigtail — a pigtail concentrates the whole shield's return current through one thin wire, which degrades high-frequency shielding effectiveness even though the connection is electrically continuous.
Typical applications: Motor and power cables, audio cabling, and general-purpose shielded harnessing where mechanical robustness and low-frequency rejection both matter.
Spiral (serve) shielding
Spiral, or serve, shielding wraps copper strands around the core in a single helical direction — a looser, more open construction than a braid, closer to a coil than a weave.
Coverage: 60–80%, with attenuation typically in the 15–30 dB range — the lowest shielding effectiveness of the three constructions, and at high frequencies a spiral shield can behave more like an inductor than a shield.
Frequency performance: Adequate at low-to-mid frequencies; not the right choice when high-frequency EMI rejection is the priority.
Flex life: Excellent — by far the best of the three, commonly cited at 5–10x a braid's flex life, because the helical strands can slide and reposition relative to each other as the cable bends rather than fatiguing at fixed crossover points.
Termination: Typically a pigtail connection.
Typical applications: Anywhere the cable itself is in constant motion — microphone and instrument cables that get coiled and uncoiled daily, robotic arms, and drag-chain applications where millions of bend cycles are a real spec, not a nice-to-have.
Combination (foil + braid) shielding
For applications where neither construction alone is enough, foil and braid are stacked: the foil layer (inner or outer, depending on manufacturer) handles high-frequency rejection with its continuous coverage, and the braid handles low-frequency magnetic interference while adding the mechanical durability foil lacks on its own. This hybrid construction commonly reaches 60–90 dB of attenuation across a 1 MHz–1 GHz band, and it's the standard choice for industrial Ethernet cabling run near variable-frequency drives, and for medical or aerospace assemblies that need to meet a defined EMC standard like MIL-STD-461 rather than just "shielded" as a loose spec.
Quick reference
Shield type Coverage Best frequency range Flex life Termination
Foil ~100% High (>10 MHz) Poor Drain wire only
Braid 40–95% Low (<10 MHz) Moderate 360° backshell termination preferred
Spiral/serve 60–80% Low-to-mid Excellent (5–10x braid) Pigtail
Foil + braid Combined Broadband (1 MHz–1 GHz+) Moderate (braid-limited) 360° termination, drain wire for foil layer
How to actually choose
Start from the failure mode you're protecting against, not the word "shielded." If the cable is fixed in place or only occasionally handled and the concern is high-frequency data-line noise, foil alone is usually sufficient and cheaper than braid. If the cable carries power or needs mechanical robustness in a general-purpose application, braid is the reasonable default. If the cable is going into continuous motion — a drag chain, a robotic joint, a coiled instrument cable — spiral shielding's flex-life advantage typically outweighs its weaker shielding effectiveness, since a foil or braid shield that's cracked from fatigue provides no shielding at all regardless of its spec-sheet attenuation number. And when the application has a real EMC compliance requirement rather than a general "keep the noise down" goal, foil+braid combination shielding is usually where the spec ends up regardless of cost, because it's the only construction of the three that reliably covers both frequency regimes.
Termination deserves the same attention as shield selection — a well-chosen braid shield terminated with a pigtail loses much of its high-frequency effectiveness, and a foil shield without a proper drain-wire connection isn't grounded at all. If a shielded cable is underperforming in the field, checking the termination is often a faster diagnosis than re-checking the shield construction itself.
Related reading
EMI/RFI Shielding Strategies for Multi-Board Enclosures — for shielding decisions at the board/enclosure level, once the signal is off the cable
What Is a Wire Harness? Complete Guide for Engineers
Signal Integrity Risk in Second-Sourced High-Speed Connectors
Have a specific shielding question for your application? Ask the Engineer — real questions get answered.