Electromagnetic compatibility, or EMC for short, is an ongoing issue in industry. Wherever high currents flow, the risk of interference from electromagnetic fields is omnipresent - especially when more and more data lines are laid in factories as a result of digitalization. A system is electromagnetically compatible if its function is not disrupted by electromagnetic fields from other systems and does not interfere with them. In practice, however, there are enough gaps through which interference radiation can enter or leave a system. The most common causes are poorly shielded connections, especially in the cable gland or connector, but also unsuitable components or improper workmanship.

The most important barrier against electromagnetic fields in cables is the shielding, a braid of highly conductive wires that are braided around the cores inside like a plait, or metallized foil that is wrapped around the cores. Both act like a Faraday cage.

Unfortunately, some users forgo shielding in order to save a few euros. This can cost them dearly, for example if cables for high currents lie close to data cables, which can lead to interference and production downtimes. There are differences in the quality of shielding. The braiding is only dense enough to form an effective barrier to electromagnetic fields from a coverage level of around 80 percent.

Skintop MS-M Brush in cross-section. © Lapp

For moving cables in drag chains with many alternating bending cycles or in robotics, where the shielding has to withstand torsion as well as bending, the shielding must be designed in such a way that this degree of coverage is also achieved in the bent state, i.e. there are no gaps in the braid. This can be controlled via the braiding angle. For moving applications, the wire is braided around the cores at a more obtuse angle. The degree of coverage is therefore higher and the elasticity better. However, more copper is also required, which increases the costs.

The structure of the cores inside the cable also influences the EMC behavior. Individual cores in the cable can be additionally shielded with aluminum-laminated foil around the insulation. Ideally, the protective conductor should be divided into three conductors and distributed symmetrically to earth around the core bundle.

Zero resistance

Exemplary EMC behavior does not end with the cable. The connector and the cable gland are also an important, but unfortunately often the weakest link when it comes to protection against interference. From an EMC perspective, the perfect connection of cable, cable gland and connector has an electrical resistance close to zero between the cable shield and earth potential. This requires a large contact surface. This is not possible with a shielding braid twisted into a sausage and soldered to a connector lug. Instead, the cable shield should be in contact all around the transition from the cable gland to the plug without any gaps so that the Faraday cage continues from the cable to the plug and interference signals are blocked out. This shield contact must be made at both ends of the cable and connected to earth potential.

With the Skintop MS-M Brush, thousands of brush hairs arranged in a ring hold the shade in place. © Lapp

Large metallic surfaces and continuous electrical connections with good conductivity are favorable for shielding - as with the Epic Ultra rectangular connector from Lapp. This has a nickel-plated metal housing and the seal is on the inside so that the two metal housing parts touch over a large area. The Skintop MS-M Brush cable gland is available to match, as the transition from the connector to the cable must also be tight for a well-shielded overall system. Normally, the shield is fixed in place with a spring, but here this is done by ring-shaped brush hairs. The variable clamping range makes assembly, disassembly and assignment easier and faster. A single operation centers, fixes, relieves strain and hermetically seals the cable. Currents induced by interference signals flow away via the highly conductive surrounding brush shielding. Even when the cable is twisted and bent, the contact surface between the shielding braid and the brush insert remains intact. This is useful at the front of a robot arm, for example, where several power and data cables run in a very confined space.

Error in the switch cabinet

When the experts from Lapp open an electrical enclosure, they can usually see immediately if there are any problems. For example, if earthing straps are missing from the doors. Or if there is a tight space in the switch cabinet and the recommended bending radii of the cables are not met. Then the cables are kinked directly behind the cable outlet and the shielding braid does not cover a large area or slips out. Power cables with high currents can scatter strong electromagnetic pulses and cause interference in the system. EMC problems therefore do not always have a technical cause. Sometimes the fitter cuts too deeply when stripping and damages the shielding. He can then repair the damaged area with a conductive shielding tape.

Bernd Müller, freelance journalist from Bonn / am