When selecting cables for industrial applications, many engineers encounter the same question:
Why do some cables have shielding while others do not? Is a shielded cable always better? Can unshielded cables be used safely?
The answer is not simply about price or cable quality.
Choosing the wrong cable can lead to signal fluctuations, communication failures, false alarms, unexpected equipment behavior, and endless troubleshooting during commissioning.
In industrial automation systems involving PLCs, sensors, transmitters, variable frequency drives (VFDs), servo systems, and communication networks, cable selection and wiring practices have a direct impact on system reliability and performance.
Let’s take a practical look at the differences between shielded and unshielded cables and understand when each should be used.
1. What Is the Difference Between Shielded and Unshielded Cables?
A shielded cable contains an additional conductive layer surrounding the signal conductors.
Common shielding types include:
- Aluminum foil shielding
- Copper braid shielding
- Foil plus braid combined shielding
The purpose of the shield is to reduce electromagnetic interference (EMI) from external sources and protect the integrity of the transmitted signal.
An unshielded cable does not contain this shielding layer. Its structure is simpler, installation is easier, and cost is lower, making it suitable for applications with low interference levels and short cable runs.
As a general rule:
If the environment contains electrical noise, weak signals, or long transmission distances, use shielded cable.
If the environment is electrically clean and the application is simple, unshielded cable is usually sufficient.
2. Where Are Shielded Cables Necessary?
Shielded cables should be considered whenever the installation includes:
- Variable Frequency Drives (VFDs)
- Large motors
- Servo drives
- Switching power supplies
- Frequent contactor operations
- High-current equipment
These devices generate electromagnetic fields that can couple into nearby signal cables and cause interference.
Typical symptoms include:
- Unstable analog signals
- Fluctuating transmitter readings
- PLC input abnormalities
- Intermittent communication failures
- Encoder errors
- Servo alarms
Many of these problems are mistakenly diagnosed as equipment failures when the real cause is improper cable selection or poor wiring practices.
The following cable types should generally be shielded:
- Pressure transmitter signal cables
- Temperature sensor cables
- Flowmeter signal cables
- Level transmitter cables
- Load cell cables
- Encoder cables
- Servo feedback cables
- Pulse control cables
- RS-485 communication cables
- CAN bus cables
- Profibus communication cables
- Analog reference and feedback signals for VFDs
- Long-distance low-level signal transmission
Because these signals are relatively weak and sensitive to interference, shielding greatly improves reliability.
3. Unshielded Cables Are Not Inferior
The absence of shielding does not mean a cable is low quality.
For many applications, unshielded cables are entirely appropriate, including:
- Power distribution
- Lighting circuits
- Standard receptacles
- Short-distance digital signals
- Simple control circuits
- General industrial wiring
Unshielded cables offer several advantages:
Easier Installation
Stripping, routing, bending, and terminating are generally simpler.
Lower Cost
For identical conductor size and core count, unshielded cables are significantly cheaper.
Reduced Installation Errors
Since there is no shield termination requirement, there are fewer opportunities for wiring mistakes.
For ordinary power and control applications in low-noise environments, unshielded cables provide excellent long-term performance.
4. Proper Shield Grounding Is Critical
Many facilities install shielded cables but still experience unstable signals.
In most cases, the issue is not the cable itself but improper shield termination.
Shielding effectiveness depends heavily on correct grounding practices.
The most common method is:
- Single-end grounding
- Grounding according to equipment manufacturer recommendations
Common mistakes include:
- Connecting shield wires randomly to cabinet screws
- Twisting shield wires together without proper grounding
- Leaving shield conductors exposed near terminals
- Allowing shield strands to contact other wiring
Improper grounding can actually create additional interference problems, especially when different cabinets use different grounding potentials.
Best practices include:
- Secure shield termination
- Clearly defined grounding points
- Reliable grounding bus connections
- Proper insulation of shield ends
- Preventing loose shield strands
Remember:
Installing shielded cable alone does not solve interference problems.
Cable routing, grounding quality, and installation practices are equally important.
5. Cable Routing Is Just as Important as Cable Selection
Many electrical noise issues are caused by poor cable routing rather than poor cable quality.
Signal cables should never run parallel to power cables for long distances.
This is especially important for:
- Motor cables
- VFD output cables
- Servo power cables
- High-current supply lines
If crossing is unavoidable, cables should cross at approximately 90 degrees.
Avoid bundling power and signal cables together using cable ties.
Inside control cabinets:
- Power wiring should use dedicated cable ducts.
- Signal wiring should use separate cable ducts.
- High-voltage and low-voltage terminals should be physically separated.
Sensor cables, communication cables, and encoder cables should be kept away from:
- Contactors
- VFDs
- Switching power supplies
- Brake resistors
- Large transformers
Simply increasing physical separation often eliminates many interference problems immediately.
6. How to Choose Between Shielded and Unshielded Cables
The following guidelines can help:
Use Unshielded Cable For:
- Lighting circuits
- Power distribution
- Standard receptacles
- Short digital I/O signals
- Simple control wiring
Use Shielded Cable For:
- Instrumentation signals
- Analog inputs and outputs
- Sensor wiring
- Encoder feedback
- Communication networks
- Long-distance signal transmission
- Installations near VFDs or motors
Most importantly:
If the equipment manufacturer specifies shielded cable, always follow the manufacturer’s recommendation.
7. Practical Experience from Industrial Sites
Two common mistakes are frequently seen in the field.
Mistake #1: Using Ordinary Cables Everywhere
This may reduce initial material costs, but later causes:
- Signal instability
- Communication failures
- Long commissioning delays
- Increased labor costs
The cost of troubleshooting often exceeds the savings from cheaper cables.
Mistake #2: Using Shielded Cables Everywhere
Installing shielded cable without proper grounding or routing practices provides little benefit.
Many installations use shielded cables but:
- Leave shields floating
- Ground shields incorrectly
- Mix power and signal cables in the same tray
This increases cost without improving performance.
The correct approach is straightforward:
- Determine the signal type.
- Identify nearby interference sources.
- Evaluate transmission distance.
- Follow equipment manufacturer requirements.
8. Conclusion
Shielded cables are not a universal solution.
Unshielded cables are not low-grade products.
The difference lies in the application.
Shielded cables are ideal for:
- High-noise environments
- Weak signals
- Long cable runs
- Precision measurements
- Industrial communication systems
Unshielded cables are perfectly suitable for:
- Power distribution
- Lighting systems
- Basic control circuits
- Electrically clean environments
A reliable industrial system is not achieved by using the most expensive materials.
It is achieved through:
- Correct cable selection
- Proper grounding
- Good installation practices
- Reasonable cable routing
In many cases, a single cable, one grounding point, or an improperly routed wire can become the root cause of an entire system failure.
Cables may appear simple, but selecting the right one can save countless hours of troubleshooting and maintenance.