The decision of whether or not to use shielded Cat cable (typically Cat5e, Cat6, or higher) in PLC and HMI systems depends heavily on the environment and the specific application.Here's a breakdown of factors to consider and why shielded cable is sometimes (but not always) necessary

When to use Shielded Cat Cable (STP - Shielded Twisted Pair):
High Electrical Noise Environment

This is the primary reason. If your PLC/HMI system is located near: Large motors Variable Frequency Drives (VFDs) High-power electrical equipment Welding equipment Radio transmitters/antennas Arcing equipment Switching power supplies Any source of significant electromagnetic interference (EMI) or radio frequency interference (RFI) Lighting strikes or high static electricity

Long Cable Runs

Longer cable runs are more susceptible to noise pickup. Generally, if you're exceeding the recommended length for Ethernet (100 meters or 328 feet), shielding becomes increasingly important, especially in a noisy environment. Even shorter runs can benefit from shielding if the environment is very noisy.

Safety-Critical Applications

In applications where a communication failure could have serious consequences (e.g., safety interlocks, emergency shutdown systems), the extra protection offered by shielding can be justified.

Compliance with Standards

Some industrial standards or regulations may mandate the use of shielded cable in certain situations. Check the relevant standards for your specific application.

Outdoor Installations

Although not directly related to electrical noise, shielded cable with proper grounding can offer some protection against lightning strikes. However, remember to use outdoor-rated cable.

Areas with High ESD (Electrostatic Discharge) Potential

Some industries (like textiles, or handling certain powders) have high ESD risks. Shielded cable can help protect the equipment from ESD damage.

Manufacturer Recommendation

If the PLC, HMI, or other equipment manufacturer specifically recommends shielded cable, follow their guidance.

Future-Proofing

If you suspect the environment might become noisier in the future, it's often a good idea to use shielded cable preemptively.

High Data Rates

While typically not as crucial as noise, higher data rates over long distances benefit from the reduced signal degradation that shielding provides.

When Unshielded Cat Cable (UTP - Unshielded Twisted Pair) Might Be OK



Clean Electrical Environment

If your PLC/HMI system is located in an environment with minimal electrical noise (e.g., a clean office environment, a small lab, or a very well-controlled industrial setting), unshielded cable is often sufficient.

Short Cable Runs

For short cable runs (e.g., connecting a PLC directly to a nearby HMI), the risk of noise pickup is lower.

Cost Considerations

Unshielded cable is typically less expensive and easier to terminate than shielded cable.

Simpler Installation

Unshielded cable is generally easier to work with.

No Significant Electrical Equipment Nearby

If there are no large motors, VFDs, or other significant sources of electrical noise in the vicinity.

Environmentally Controlled Areas

In environments such as climate-controlled server rooms, where equipment is typically well-isolated.

Important Considerations When Using Shielded Cable



Proper Grounding is Essential

The shield must be properly grounded to be effective. If the shield isn't grounded, it can actually increase noise by acting as an antenna. Usually, you ground the shield at one end only (typically at the control panel or cabinet). Grounding at both ends can create a ground loop, which can introduce even more noise. Consult the equipment manufacturer's documentation for their recommended grounding practices.

Shielded Connectors

Use shielded RJ45 connectors (or other appropriate connectors) to maintain the continuity of the shield. The connector should make good electrical contact with the shield of the cable.

Cable Quality

Buy good quality cable from a reputable manufacturer. Poor quality shielded cable may have a poorly constructed shield that doesn't provide adequate protection.

Cable Management

Avoid running shielded cable in close proximity to power cables or other sources of noise. Keep Ethernet cables separated from power cables in cable trays or conduits.

In summary



When in doubt, use shielded cable. The added cost is usually worth the peace of mind, especially in industrial environments. Assess your environment. If you're unsure about the level of electrical noise, it's best to err on the side of caution and use shielded cable. Follow the manufacturer's recommendations. Proper grounding is absolutely crucial for shielded cable to be effective.Your Specific Situation (Non-Industrial):

Since you mention that your systems were never in an industrial setting, the need for shielded cable is likely less critical. If you're in a relatively clean electrical environment (e.g., a home office, a small lab away from heavy equipment), unshielded cable might be sufficient. However, if you are experiencing any communication problems or suspect electrical noise, switching to shielded cable is a good first step in troubleshooting.Start with UTP cable and only switch to STP if you experience problems. Flag for review