Electromagnetic compatibility is decided at the cables. The circuit board inside a metal enclosure can be internally pristine while its cable — an antenna, if built casually — radiates interference out and receives it back. This guide explains how cables behave electromagnetically and how to select, build and terminate them for EMC.
How Cables Radiate and Receive
Common-mode current is the villain: when a circuit's return currents find paths the designer didn't intend — through cable shields, ground wires and capacitance — the cable carries a net current whose fields radiate efficiently. Receivers work symmetrically: fields couple into cables, and common-mode currents ride into the electronics. The frequencies that matter are set by the fastest switching edges in the equipment — a 20ns edge radiates happily around 15MHz and up — which is why 'it's only a slow signal' reasoning fails: the edge, not the data rate, sets the spectrum.
Shield Selection by Frequency
Shield effectiveness is frequency-dependent and construction-dependent:
- Foil shields: excellent high-frequency coverage; fragile and hard to terminate (hence drain wires with their inductance penalty).
- Braided shields: low impedance, mechanically robust, terminate beautifully; coverage 60-95% by weave.
- Foil + braid: the standard for demanding EMC — braid handles low-frequency and mechanical duty, foil closes the high-frequency gaps.
- Spiral shields: flexible but inductive at high frequency — fine for audio, wrong for fast digital.
Our shielded cable analysis compares these constructions in depth.
Termination: Where EMC Is Won or Lost
The 360-degree principle rules: shield circumference clamped to a metal shell that continues the enclosure. Pigtails — even short ones — become the dominant impedance path at the frequencies that matter and convert a good shield into a mediocre antenna lead-in. Second only to pigtails in damage: shields that terminate to a floating or painted surface, and shield continuity that was assumed rather than tested.
Grounding and Routing Rules
- Shields ground per the system's scheme — single-end for low-level analog loop-breaking, both-end for emission containment — but never accidentally.
- Separate power and signal cables; cross at right angles where crossing is unavoidable; never run a drive output parallel to an encoder line in the same chain.
- Filter at the enclosure boundary: feedthrough filters and filtered connectors work because they sit where the cable crosses the shield plane.
Designing for EMC Compliance
Equipment that must pass emissions and immunity testing treats cables as design items: shield constructions specified per cable class, termination hardware chosen at connector-selection time, routing documented, and pre-compliance testing done early enough to fix what it finds. Harness suppliers serve this by building exactly the specified constructions — with shield-continuity testing on every assembly so the design's assumptions hold in production.
Kaweei EMC Cable Manufacturing
Kaweei manufactures EMC-oriented cable assemblies — foil/braid shielded builds, 360-degree backshell terminations, filtered-connector harnesses, shield-continuity-tested production — for industrial, automotive and medical equipment makers. Contact our engineering team with your EMC requirements; our shielded cable guide provides the construction detail.