One question often comes up during the specification phase of a custom cable assembly: do I need a shielded cable or not? The usual answer is \u201cit depends\u201d. But it depends on very specific factors, and understanding them helps avoid two equally costly mistakes: using shielding where it is not needed, or failing to use shielding where it is. <\/p>\n\n
The first mistake unnecessarily increases the cost of the harness and adds stiffness and weight without any real benefit. The second can lead to interference, corrupted signals, intermittent faults that are difficult to diagnose and, in the worst case, equipment that fails electromagnetic compatibility testing. <\/p>\n\n
In this article, we explain what shielding actually does, what types of shielding exist, when it is necessary and when it is not, and which design decisions affect its real-world effectiveness.<\/p>\n\n
Shielding is a conductive layer that surrounds one or more cable conductors in order to reduce the influence of external electromagnetic fields on the signal being carried, and to reduce the electromagnetic emissions generated by the cable itself into the surrounding environment.<\/p>\n\n
Put simply: shielding protects the signal from what is outside the cable, and protects the environment from what is inside the cable.<\/p>\n\n
What shielding does not do is increase the cable\u2019s current-carrying capacity, reduce the electrical resistance of the conductor, or replace a correct installation. Poorly terminated shielding can be worse than no shielding at all, because it can act as an antenna instead of a shield. <\/p>\n\n
Engineering note<\/strong>: Shielding effectiveness depends to a very large extent on how the shield is terminated, not only on the shield itself. At JM Cableados, we treat shield termination as a critical process step, including electrical verification. A well-manufactured shield that is poorly terminated at the connector provides real protection close to zero. <\/p>\n\n Not all shields are the same. The three most common shielding types in industrial cabling are: <\/p>\n\n Braided shield<\/strong>: a mesh of copper or tinned copper wires braided around the cable. It provides good mechanical coverage, excellent performance at low frequencies and allows easier termination with a shield clamp or drain pigtail. Its typical coverage is between 85% and 98%. <\/p>\n\n Foil shield<\/strong>: an aluminium or copper foil bonded to a polyester backing, with a drain wire that allows connection to ground. It provides 100% coverage at high frequencies, with lower weight and smaller diameter than braid. However, it is mechanically more fragile and more difficult to terminate correctly. <\/p>\n\n Combined shield<\/strong>: this combines the advantages of both systems. It is the reference solution for applications with demanding requirements across the full frequency spectrum: precision analogue signals, high-speed industrial communications or environments with intense interference. <\/p>\n\n Shielding is necessary when any of the following conditions apply:<\/p>\n\n Low-amplitude analogue signals<\/strong>: temperature sensors, thermocouples, RTDs, load cells, 4\u201320 mA signals, encoder outputs. Any low-amplitude signal in an environment with interference sources \u2014 variable frequency drives, motors, switched-mode power supplies \u2014 needs protection against inductive and capacitive noise that can corrupt or offset the signal. <\/p>\n\n High-speed serial communications<\/strong>: protocols such as RS-485, CAN bus, EtherCAT, Profibus or DeviceNet have signal integrity requirements that, in noisy industrial environments, can only be guaranteed with shielded cable and correct termination.<\/p>\n\n Electromagnetic compatibility requirements<\/strong>: if the final equipment must pass EMC tests \u2014 CE marking, EMC Directive 2014\/30\/EU \u2014 radiated and conducted emissions from the cabling are a determining factor. Correct shielding can make the difference between passing and failing the test. <\/p>\n\n Environments with intense interference sources<\/strong>: proximity to variable frequency drives, servomotors, switched-mode power supplies, arc welding or radiofrequency equipment. In these environments, even digital signals can be affected if the cabling is not protected. <\/p>\n\n Specific sector regulations<\/strong>: sectors such as medical, railway or defence may impose shielding requirements as part of the equipment design, regardless of the interference analysis.<\/p>\n\n Shielding is not necessary and adds cost, weight and rigidity without benefit in the following situations:<\/p>\n\n Robust digital signals in clean environments<\/strong>: 24 V DC power lines, PLC digital signals with high voltage levels in environments without significant sources of interference. The noise immunity of these signals is high enough to operate without shielding. <\/p>\n\n Low-voltage power cables without EMC requirements<\/strong>: power supply lines for actuators, solenoid valves or heating resistors where the key requirement is current-carrying capacity, not signal integrity.<\/p>\n\n Very short cable lengths<\/strong>: below a few metres, susceptibility to interference is significantly reduced. In internal runs inside electrical cabinets with good circuit segregation, shielding rarely adds value. <\/p>\n\n Environments with low interference density<\/strong>: machinery without variable frequency drives, without high-power motors nearby and without high-frequency switched-mode power supplies.<\/p>\n\n Shielding is only effective if it is correctly connected to ground. And the way the shield is terminated at the connector or equipment largely determines its real effectiveness. <\/p>\n\n The two main ways to terminate a shield are:<\/p>\n\n Braid pigtail termination<\/strong>: a bundle of wires is extracted from the braid and soldered or crimped to a terminal connected to ground. This is the most common and most economical method, but it introduces parasitic inductance that reduces shielding effectiveness at high frequencies. For low-frequency signals, it is perfectly valid. <\/p>\n\n 360\u00b0 shield clamp termination<\/strong>: the shield is connected to ground through full circumferential contact at the connector. This is the correct method for high-frequency applications and for equipment with demanding EMC requirements. It requires specific connectors and accessories, but ensures shield continuity without introducing parasitic inductance. <\/p>\n\n Technical note: A shield termination with a 50 mm pigtail introduces parasitic inductance that can reduce shielding attenuation by more than 20 dB at frequencies above 10 MHz. For high-frequency signal applications, 360\u00b0 termination is not optional: it is the only method that maintains shielding effectiveness across the full spectrum. <\/p>\n\n The shield must be connected to ground. But at which end, and whether at one end or both, depends on the type of interference you need to combat. <\/p>\n\n Shield grounded at one end only<\/strong>: this protects against electric field interference, or capacitive coupling. It is the correct configuration for low-frequency analogue signals where ground currents that could introduce common-mode noise need to be avoided. <\/p>\n\n Shield grounded at both ends<\/strong>: this protects against magnetic field interference, or inductive coupling, and is necessary for high-frequency signals and for meeting EMC requirements. It requires the potential difference between the two ground points to be negligible; otherwise, ground currents are created and introduce noise into the signal. <\/p>\n\n Before writing \u201cshielded cable\u201d in a BOM or on a drawing, it is worth answering the following questions:<\/p>\n\nTypes of Shielding: Braid, Foil and Combined Shielding<\/h2>\n\n
Shield type<\/strong><\/td> Coverage<\/strong><\/td> Effective frequency range<\/strong><\/td> Mechanical resistance<\/strong><\/td> Ease of termination<\/strong><\/td><\/tr><\/thead> Braid<\/td> 85\u201398%<\/td> Low and medium frequency<\/td> High<\/td> Good<\/td><\/tr> Foil<\/td> 100%<\/td> High frequency<\/td> Low<\/td> Requires care<\/td><\/tr> Foil + braid<\/td> 100%<\/td> Full spectrum<\/td> High<\/td> More complex<\/td><\/tr><\/tbody><\/table><\/figure>\n\n When Shielded Cable Is Necessary<\/h2>\n\n
When Shielded Cable Is Not Necessary<\/h2>\n\n
Shield Termination: Where Everything Is Won or Lost<\/h2>\n\n
Shielding and Grounding: The Most Common Mistake<\/h2>\n\n
What to Define Before Specifying Shielding<\/h2>\n\n