What is the difference between foil and braid shielding?

Foil shielding uses thin aluminum/polyester tape for 100% coverage and excellent high-frequency protection, but is fragile. Braid shielding uses woven copper strands for better mechanical durability and low-frequency protection, typically 70-95% coverage.

Should I ground the shield at one end or both ends?

It depends on the application. For low-frequency analog signals, ground at one end to prevent ground loops. For high-frequency digital signals (USB, Ethernet), ground at both ends for best EMI performance. When in doubt, ground both ends.

What is a drain wire?

A drain wire is an uninsulated conductor that runs alongside the foil shield, providing an easy termination point for grounding. It is necessary because you cannot solder directly to aluminum foil.

When do I need shielded cable?

Use shielded cable when: signals are sensitive to noise (analog sensors, audio), you are near EMI sources (motors, VFDs, switching power supplies), compliance requires it (FCC, CE, MIL-STD-461), or cable runs are long (greater than 10 meters for data).

How is shielding effectiveness measured?

Shielding effectiveness (SE) is measured in decibels (dB) and represents the ratio of EMI field strength without the shield versus with the shield. Testing is performed per MIL-STD-461 or IEEE 299 standards using spectrum analyzers and calibrated antennas. A shield with 40 dB SE reduces EMI by a factor of 100.

What is transfer impedance?

Transfer impedance (ZT) measures how much external interference voltage appears inside a shielded cable per unit length and per unit current on the shield. Lower transfer impedance means better shielding. It is measured in milliohms per meter and is particularly important at frequencies below 30 MHz where braid shields excel.

Do I need shielded cable for digital signals?

High-speed digital signals (USB, HDMI, Ethernet, PCIe) typically require shielded cable because their fast edge rates create broadband EMI. For slower digital signals like RS-232 or I2C at short distances, unshielded cable may suffice in benign EMI environments. When EMC compliance is required, always use shielded cable.

What is the difference between overall shield and individually shielded pairs?

An overall shield wraps around all conductors in a cable, protecting them collectively from external EMI. Individually shielded pairs (ISP) wrap each twisted pair separately, providing crosstalk isolation between pairs within the same cable. Instrumentation cables often use both for maximum protection.

Deep Dive15 min read

Cable Shielding: Complete Guide to EMI Protection

Understanding foil, braid, and combination shields for high-reliability wire harnesses.

Quick Definition

Cable shielding is a conductive barrier that surrounds wires to protect signals from electromagnetic interference (EMI) and prevent the cable from radiating noise. Common types include foil, braid, and spiral shields.

What is Shielded Cable?

Shielded cable is an electrical cable that includes one or more conductive layers surrounding its internal conductors. These conductive layers - made from metal foil, braided wire, or spiral-wound strands - form a barrier that blocks electromagnetic interference (EMI) and radio frequency interference (RFI) from corrupting the signals carried inside the cable.

The shield serves two critical purposes: it prevents external noise from coupling into the cable (immunity), and it prevents the cable from radiating its own signals into nearby circuits (emissions). This bidirectional protection is essential for maintaining signal integrity in environments with motors, switching power supplies, wireless transmitters, and other sources of electromagnetic noise.

Shielded cable construction is governed by several key standards. Military applications follow MIL-C-27500 for aerospace wire, while SAE AS50881 (formerly MIL-W-5088) defines wiring for aircraft and aerospace systems. Commercial data cables like Ethernet follow TIA/EIA-568 standards, which specify shielding requirements for different cable categories. Understanding these standards helps engineers select cables that meet both performance requirements and regulatory compliance.

Key Insight: The term "shielded cable" encompasses many constructions - from simple foil-wrapped data cables to complex aerospace harnesses with multiple shield layers. The right choice depends on your frequency range, flexibility requirements, and EMI environment.

EMI Shielding for Cables

EMI shielding for cables works by providing a low-impedance path that diverts interference currents away from the signal conductors. When electromagnetic fields strike the shield, they induce currents that flow through the shield material and are safely conducted to ground, rather than coupling into the wires inside.

How EMI Enters Cables

Radiated Coupling

Electromagnetic waves from nearby sources (antennas, motors, switching circuits) strike the cable and induce voltages in the conductors. This is the primary concern for cables running through open spaces or near RF equipment. Shield effectiveness against radiated EMI depends heavily on frequency.

Conducted Coupling

High-frequency noise travels along power lines, ground planes, or other conductors and couples into signal cables through shared grounds or parasitic capacitance. This is common in industrial settings with variable frequency drives (VFDs) and switching power supplies.

Shielding Effectiveness (SE)

Shielding effectiveness is measured in decibels (dB) and represents how much the shield attenuates electromagnetic fields. A shield with 20 dB SE reduces EMI by a factor of 10; 40 dB SE reduces it by a factor of 100; and 60 dB SE by a factor of 1,000. The required SE depends on your application:

20-40 dB: Adequate for general commercial applications
40-60 dB: Required for medical devices and industrial instrumentation
60-80 dB: Necessary for military and aerospace systems
80+ dB: Used in high-security applications (TEMPEST, sensitive RF equipment)

Frequency-Dependent Performance

Shield performance varies significantly with frequency. Braid shields excel at low frequencies (below 10 MHz) due to their low DC resistance, while foil shields provide better protection at high frequencies (above 30 MHz) due to their complete coverage. This is why combination shields (foil + braid) are used when broadband protection is required.

Shielded Cable Anatomy

Hover over the layers below to explore each component of a shielded cable.

Fig 1.1: Shielded Cable Cross-Section

Select a layer to inspect details

Types of Cable Shielding

Shield Type	Coverage	Best For	Flexibility
Foil (Tape) Aluminized Mylar	100%	High frequency (over 30 MHz)	Low (can crack)
Braid Tinned Copper	70-95%	Low frequency (under 10 MHz)	High
Spiral (Serve) Wrapped Wire	85-95%	Audio, flex applications	Very High
Combination Foil + Braid	~100%	Wideband, harsh EMI	Medium

Rule of Thumb: Use foil for high-frequency data protection. Use braid for mechanical strength and low-frequency noise. Use combination for noisy industrial environments.

Frequently Asked Questions

Related Glossary Terms

Drain Wire EMI Backshell Twisted Pair Crosstalk Impedance Grounding

Designing Shielded Assemblies?

WireIt helps you specify shields, select backshells, and generate complete BOMs for your shielded cable assemblies.

Start Free Preview Back to Glossary