EMC Explained: A Practical Guide for Plant Teams to Stop “Mystery” Faults

A vision system on your case packer fails an inspection once an hour, stopping the line. The sensor tests fine. The PLC code is correct. After two days of chasing this “ghost,” a senior tech finally opens the control cabinet. The vision system’s cable shield is a “pigtail”—a single twisted wire held down by a screw.

He replaces it with a proper 360° EMC gland. The fault disappears.

That “ghost error” was not a ghost. It was electrical noise, or Electromagnetic Interference (EMI). In any modern plant, Variable Frequency Drives (VFDs), servo motors, and power supplies are all “noisy.” They act like small radio transmitters, flooding the air with electrical noise that can corrupt “quiet” data signals on Ethernet, sensor, and encoder cables.

Intermittent faults, drive trips, and sensor jitter are rarely a sign of a bad device. They are often a sign of ineffective EMC practices. Understanding the following three principles is key to fixing the root cause and eliminating these faults for good.

Principle 1: Understanding 360° Shielding (The Pigtail Problem)

A shielded cable is designed to “catch” electrical noise and drain it safely to ground, protecting the data wires inside. For this to work, the “drain” must be wide open.

The Problem: The “Pigtail” The most common EMC mistake is the “pigtail.” This is when a technician painstakingly unbraids the shield, twists it into a single wire, and puts it under a terminal screw. A pigtail is like a 10-lane highway draining into a tiny garden hose. It chokes the flow of noise. Even worse, at high frequencies, the pigtail itself acts as a tiny antenna, broadcasting the very noise it was supposed to stop.

The Solution: 360° Contact The solution involves connecting the entire surface of the shield to ground. The easiest, most reliable way to do this is at the point of entry. This is why SKINTOP® EMC cable glands are so critical. Instead of a single screw, they use a 360-degree spring or brush that clamps down on the entire circumference of the shield, creating a low-impedance, “wide-open” path for noise to drain away.

Principle 2: The Importance of Cable Separation

Electrical noise can “jump” from a high-power cable to a sensitive data cable, just like heat from a hot pipe. This is called coupling. Physical separation is one of the most effective strategies to prevent it.

Cable Routing Guidelines:

• “Noisy” Cables: VFD motor leads (ÖLFLEX® VFD), servo power, 230/400V AC lines.
• “Quiet” Cables: Industrial Ethernet, sensor/encoder signals, 4-20mA loops.

Best Practices:

1. Separation: A best practice is to keep “Noisy” and “Quiet” cables in separate trays or conduits. The more distance, the better.
2. Crossing Cables: If a “Quiet” data cable must cross a “Noisy” power cable, cross them at a right angle (90°), like a road intersection. Never run them side-by-side or diagonally.
   • The Logic: Think of the power cable as a noisy highway. If you run a data cable parallel to it, it is like walking along the highway—you are exposed to the noise for a long time. If you cross it at 90 degrees, you step over it instantly, minimizing the exposure.

Common Pitfall: Zip-tying “Noisy” and “Quiet” cables together in the same bundle is a primary cause of noise coupling.

A robustly shielded data cable like LAPP ETHERLINE® provides a strong first line of defence. Pairing it with a high-quality shielded power cable, like ÖLFLEX® SERVO FD, helps contain the noise at its source.

Principle 3: Matching Glands to the Application

A technician in a hurry might grab any gland that fits the hole. This can be a critical error.

Using a standard plastic or non-EMC metal gland on a shielded cable is like building a firewall and leaving the front door wide open. It completely breaks the shield’s 360° connection to ground at the most critical point: the cabinet entry.

A proper industrial gland must do two jobs:

1. Be EMC-Clean: It must provide the 360° grounding path (Principle 1).
2. Be IP-Rated: It must seal the enclosure against dust, moisture, and oil (e.g., IP68).

A cheap gland does neither. This same logic applies to connectors. Using robust, shielded EPIC® industrial connectors ensures shield integrity is maintained all the way to the device.

A 5-Point Troubleshooting Guide for “Ghost” Faults

Next time an intermittent “ghost” fault appears, it may not be the PLC. The root cause is often found in one of these five areas:

1. Gland & Cabinet Entry: A common culprit is a standard plastic/metal gland being used on a shielded cable where an EMC gland was needed.
2. Shield Termination: Inside the cabinet, the “pigtail” is the most frequent source of EMC failure.
3. Cable Routing: The “Quiet” cable (sensor, Ethernet) is often found bundled with or running parallel to a “Noisy” VFD or servo cable.
4. Panel Bonding: The metal cabinet panel itself may not be properly bonded to the main earth (e.g., bolted to a painted surface), preventing noise from draining.
5. Component Mismatch: An unshielded patch cord may be used in an otherwise shielded system, creating a “hole” for noise to get in.

The Bottom Line: EMC Is a Process, Not a Mystery

EMC is not “black magic.” It is a set of simple, repeatable processes. By understanding these three principles—360° shielding, clean routing, and using the correct EMC glands—teams can move from chasing random faults to building robust, reliable lines. Standardising components and practices turns an expert’s “knack” into a standard operating procedure everyone on the team can follow.

Frequently Asked Questions (FAQ)

Do I really need an EMC gland for every shielded cable? If a shielded cable is entering a grounded metal cabinet, a proper EMC gland is the only way to correctly terminate the shield and maintain the cabinet’s own shielding. It’s a critical part of the system.

What’s the difference between grounding and shielding? It’s simple: Shielding blocks noise, and Grounding drains noise away. A shield is a barrier. A ground is the “drain pipe.” For either to work, they must be connected, and that connection should be 360°.

My pigtail seems to work fine. Why change it? A pigtail is a hidden risk. It might work today, but as soon as a new, “noisier” drive is installed nearby, or an old motor starts to wear, the noise floor will rise. That “fine” pigtail will then become the weak link that shuts your line down.Still fighting “ghost” faults in your plant?Speak to an engineer today for a site audit or advice on the best EMC components for your facility.