Electromagnetic Interference Despite Screening: What Could Be the Cause?

EMC problems in machines and systems are typically difficult to diagnose. They appear as sporadic sensor glitches, inexplicable field bus errors, or irregularities in servo drives. The culprit is often the level of electromagnetic interference in modern systems. Frequency converters and motors operate with fast switching edges, and vision systems, IO-Link components, and industrial Ethernet communication are concurrently in play. This requires that screening, earthing, and cable routing are all implemented consistently and correctly.

Electromagnetic compatibility is a question of contact surfaces, transitions, and, above all, impedance. This is understood to be the frequency-independent total resistance of an electrical conductor or component against alternating current, which is a combination of ohmic resistance, inductance, and capacitance.

A typical question that users have is, "should the screen be bonded at both ends or only one?" The answer depends on what type of interference is needing to be managed. In industrial applications, high-frequency interference currents dominate the scene. Typically, these are most effectively reduced when the screen is bonded at both ends. A screen that is bonded only on one side cannot reliably dissipate these currents, thus increasing the danger of cables emitting interference or coupling with adjacent cables.

For very sensitive, low-frequency, analogue signals, a one-sided bond can make sense in individual cases to avoid earth loops. In modern systems, however, another method is preferred: a properly engineered equipotential bonding system and a defined screen connection. This is a central component of stable EMC, especially for modular machines.

A screen, however, is only as effective as its connection. A common installation mistake is the so-called pigtail solution. This is when the screen is separated, twisted into its own wire, and then connected to the earth. This is a problem for high frequencies because this wire introduces inductance into the circuit, thereby counteracting the function of the screen itself. The better alternative is a full-coverage, fully enclosed, 360° connection via EMC cable glands, screen clamps, or screen brackets on a screen connection bar, and preferably directly on a conductive mounting surface. The rule of thumb is "wide, not long". Additionally, contact surfaces must be electrically reliable. Varnish, layers of oxidation, or grime significantly reduce effectiveness.

In control cabinet engineering, motor cables are one of the strongest sources of interference. Best practice, therefore, is to use rigorous screening with a defined connection at both ends, positioned as close as possible to the inverter output and to the motor, or, where applicable, to the connector. This also requires a properly connected protective earth (PE) and equipotential bonding. If the door of the control cabinet, mounting surface, PE rail, and machine body are not connected properly, the effectiveness of even a well-connected screen is limited.

Installation also plays a large role—power, signal, and data cables should always be run separately. Running these in parallel within the same channel significantly increases the risk of interference coupling. If crossings are required, experience has shown that 90° crossings are typically more robust. Following these guidelines during installation can prevent many EMC issues and noticeably increase system availability.