Shielded Cable or Unshielded Cable: How to Decide?

When does a shield actually help?

Variable-frequency drives, motors, contactors, transformers and radio transmitters can all couple noise into control and instrumentation circuits. Coupling becomes more likely when sensitive conductors run in long parallel with a noisy cable, or when the loop formed by the signal wires encloses a large area. Long parallel runs and large loop areas both increase susceptibility.

A shield does not replace good routing. Keeping loop areas small and routing sensitive pairs away from power circuits is usually the first and cheapest step. A shield is then added where the remaining risk justifies the extra cost and termination work.

A shield is most useful when the signal is low level, when it carries high-frequency content, or when the cable must run beside equipment that switches quickly. A slow, high-level signal over a short run inside a quiet panel may need little or no shielding.

What is the difference between foil, braid and combined shields?

Each shield construction has a typical use. The main options are listed below.

• Foil shield: high coverage and compact construction, often supplied with a drain wire for termination.
• Copper braid: mechanically durable, and suitable where the cable must stay flexible.
• Combined foil and braid: addresses a broader range of conditions, but adds diameter and cost.

Foil and braid behave differently at high frequency. Braid generally gives lower impedance at many frequencies, while foil gives high coverage in a thin layer. Neither is better in every case. The right choice depends on the frequency range, the mechanical duty of the cable and the termination that can actually be achieved in the installation.

Should the shield be grounded at one end or both ends?

Single-end or both-end termination depends on the frequency, the circuit design, the equipotential bonding and the manufacturer's instructions. There is no universal rule. The method should be confirmed with the equipment designer or the cable manufacturer before the cable is installed.

At high frequency, a shield generally works best with a low-impedance circumferential termination, meaning the shield is bonded all the way around the cable at the gland or connector. A long, thin pigtail, which is a wire lead from the shield to earth, adds impedance at high frequency and can make an otherwise good shield perform poorly.

The bonding arrangement also has to fit the plant earthing system. Two earth points that sit at different potentials can drive current through a shield that is bonded at both ends, so the bonding design should be checked against the site layout rather than chosen by habit.

Which applications usually need a shield?

The table below is a starting point for discussion. It does not replace the equipment manufacturer's requirements or a site assessment of the electrical noise environment.

Requirements vary by equipment and site. Confirm the final choice against the datasheet and the installer's specification. For shielded signal cables, see the computer shielded cable range.

What about cables feeding variable-frequency drives?

Drives switch quickly and can produce common-mode current, which can cause EMC problems on nearby control and communication circuits. Symmetrical cable construction, with three phase conductors and three symmetrically placed earth conductors, is used together with an overall shield bonded at both ends to reduce common-mode current and EMC problems.

A symmetrical shielded cable does not solve every drive problem by itself. Reflected-wave overvoltage at the motor is a separate issue. It is addressed by cable length limits, dv/dt or sine filters, and motors with insulation suited to inverter duty. For a fuller discussion, see the VFD motor cable guide.

Frequently asked questions