Cable Knowledge
Shielded Cable or Unshielded Cable: How to Decide?
A shield controls electromagnetic coupling when it is selected and terminated as part of a complete system. Adding a metal layer without a grounding plan may provide little benefit and can create unwanted current paths.
Sources of Interference
Variable-frequency drives, motors, contactors, transformers and radio transmitters can couple noise into control and instrumentation circuits. Long parallel runs and large loop areas increase susceptibility.
Shield Options
Foil shields offer high coverage and compact construction, often with a drain wire. Copper braid is mechanically durable and suitable where flexibility is needed. Combined designs address a broader range of conditions but add diameter and cost.
Grounding Strategy
Single-end or both-end termination depends on frequency, circuit design, equipotential bonding and manufacturer instructions. There is no universal rule. A high-frequency shield generally benefits from low-impedance circumferential termination rather than a long pigtail.
Information for the Supplier
- Signal or protocol
- Operating voltage and frequency range
- Nearby power or VFD circuits
- Cable length and movement
- Required pairs, twists and individual or overall shields
- Grounding and connector arrangement
A Shielding Failure Seen in Practice
A high-coverage shield terminated through a long thin pigtail can perform poorly at high frequency because the termination adds impedance. Cable construction, gland, connector and bonding surface must be designed together.
- Control loop area and routing first.
- Define shield termination at both ends of the route.
- Separate power and sensitive signal cables.
Information to Confirm
- Signal type and susceptibility
- Noise sources and separation distance
- Foil, braid or combined shield
- Pair twisting and capacitance limits
- Termination method
- Flexing and environmental requirements
Engineering note: EMC performance depends on cable routing, connectors, bonding and equipment design as well as the cable itself.
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- PVC Insulated Power Cable
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- Welding Cable
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Continue reading
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