Shielding is neither a “standard feature“ nor an “optional extra.“ It is a functional component with clearly defined application conditions.
At its core, shielding is a Faraday cage — a conductive enclosure that wraps around signal wires, keeping external electromagnetic interference out and internal signals in.
Control cables need shielding; power cables do not. The reason is simple: control cables carry milliamp-level signals or digital signals — even a small amount of interference can distort the signal. Power cables carry hundreds of volts — interference superimposed on power is negligible.
This article answers three questions: When is shielding mandatory? When can you skip it? How do you decide?
| Shielding Type | Principle | Advantages | Disadvantages | Key Data |
| Braid Shield | Tinned copper wire braided into a mesh | Good flexibility, high mechanical strength | Gaps in braid allow high-frequency “leakage“ | Braid coverage typically 65-85% |
| Foil Shield | Aluminium foil wrapped or laminated | 100% coverage, effective at high frequencies | Prone to cracking when bent | 100% coverage |
| Combination Shield | Foil + braid double layer | Covers both high and low frequencies | Highest cost, largest diameter | Braid coverage up to 100% |
Braid shield is suitable for applications requiring frequent bending (such as cable carriers), as the braided structure offers good flexibility and resistance to breakage. Foil shield is suitable for high-frequency signal transmission (above 100MHz) in fixed installations, as foil provides better high-frequency shielding than braid. Combination shield is suitable for demanding environments requiring both high-frequency protection and dynamic flexing, combining the advantages of both but at the highest cost.
External Links:
Anixter Low Voltage Cable Shielding Guide (PDF):
L-com Understanding Shielding in Flexible Control Cables (Foil, Braid, Combination):
VFDs typically operate at switching frequencies of 2-16kHz, generating significant harmonics and high-frequency noise. Without shielding, this noise will completely overwhelm the control signal.
Consequence of getting it wrong: The control system receives incorrect feedback signals, potentially causing sudden motor acceleration or deceleration, leading to equipment damage or personal injury.
A 4-20mA signal has only a 16mA range; thermocouple and RTD signals are even smaller (millivolt level). Even minimal interference can completely distort the reading.
Consequence of getting it wrong: The PLC reads temperature or pressure values that are several times off from actual values, potentially causing production safety incidents.
Higher communication speeds demand higher signal integrity. Profibus DP typically runs at 12Mbps; Ethernet/IP can reach 100Mbps or higher — unshielded cables are almost unusable at these speeds.
Consequence of getting it wrong: Frequent communication interruptions, data packet loss, and complete control system failure.
The 50Hz power-frequency electromagnetic field generated by power cables induces voltage onto adjacent control cables. The smaller the spacing and the longer the parallel run, the higher the induced voltage.
Consequence of getting it wrong: Control signals are completely overwhelmed by power-frequency interference and cannot function.
The transient electromagnetic field intensity during switching operations in substations is extremely high. IEEE research indicates this interference can damage electronic equipment.
Consequence of getting it wrong: Not only are signals unstable — the control modules themselves may be damaged by induced voltage.
The longer the distance, the greater the signal attenuation and the more interference coupling. 30 meters is an industry rule of thumb — beyond this distance, shielding is recommended even in relatively clean environments.
Consequence of getting it wrong: Signal attenuation combined with interference means the receiving end cannot decode valid signals.
Digital signals have only two states (0 and 1) with a wide voltage range (typically 24V DC ±20%). Small interference is unlikely to change the logic state. However: If the switching signal cable passes directly below a VFD output, the interference can be strong enough to cause false triggering — in this case, unshielded cable cannot be used.
The distance is extremely short, interference coupling is minimal, and the cabinet's grounded metal enclosure provides natural shielding. However: If there is a VFD inside the cabinet and the signal cable is less than 10cm from the VFD output terminals, interference can still couple in — in this case, shielding is recommended.
If the project is not in an industrial area, has no high-power equipment, and no VFDs — shielding can be skipped. But verify: Are there temporary interference sources such as portable welding equipment or handheld two-way radios on site? These may not appear on design drawings but can be present during construction.
Shielded cables cost 20-45% more than unshielded cables. If the budget is tight and the signal being transmitted is not critical (such as equipment status indicator lights), skipping shielding may be an option. However, consider this trade-off: the labour cost of troubleshooting interference issues during commissioning can far exceed the cable cost savings. A few days of site debugging delay can easily wipe out any savings from choosing unshielded cables.
Step 1: Check the RFQ
If the RFQ specifies “control cable, shielded“ — purchase shielded cable. If it says “control cable“ without specifying shielding — do not default to unshielded. Ask the design engineer.
Step 2: Ask the Design Engineer — Confirm Signal Type and Routing Path
Ask the design engineer or technical lead three questions:
1.What type of signal is being transmitted?
2.Where does the routing path go?
3.How long is the transmission distance?
Under 30 meters → Depends on signal type and environment
Step 3: Verify Site Conditions
If possible, visit the site (or ask the supplier to provide site survey recommendations):
Note: The routing path on design drawings and the actual site installation often differ — cable tray routing may change, cables may be rerouted closer to power cables, and other unexpected conditions are common. If possible, reserve the option for a shielding upgrade, or include a clause in the procurement contract stating that “the supplier may provide a shielding upgrade option if site conditions require it.“
If you are still uncertain which shielding solution fits your project after reading this article, feel free to reach out.
We can provide one-on-one selection recommendations based on your specific signal type, routing path, and site environment.
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