The EMC multiple washer with internal TRI contact springs allows several cables to be routed through one EMC cable gland to save space. The washer is securely contacted in the gland body via a circumferential iris spring.

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Another solution is the split EMC adapter: If this is screwed in between the housing wall and an EMC gland, this also results in double contacting with correspondingly higher attenuation values and higher current transmission with minimized technical and financial effort. If the user uses this adapter as a locknut, certain RF attenuation values can even be achieved cost-effectively and retrospectively for standard and plastic cable entries, which is usually sufficient for less critical applications.

With a split EMC connection block, thick, rigid cables can be fed into the control cabinet easily and in compliance with EMC regulations.

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Every cable gland requires a certain amount of installation space. However, if a user wants to insert various EMC cables into an enclosure, the trend towards ever more compact systems can sometimes lead to bottlenecks in the enclosure environment due to a lack of installation space. To solve this problem, Pflitsch relies on a sealing insert concept in which several cables - even with different diameters - can be fed through one cable gland.

Behind the sealing insert in the EMC solution is a metal disk that is manufactured precisely for the cable diameters used by the user. The shield of each cable is securely contacted in this disk by means of a TRI spring. Contact is made with the inserted washer in the cable gland via a circumferential iris spring ring. With this sealing concept - designed up to size M120 - several EMC cables with a diameter of 5 to 20 mm, including braided shield, can be integrated.

With rigid cables in the cabinet

Power cables and lines with larger cross-sections are usually very rigid and are therefore often difficult to install. In order to be able to insert such shielded cables into an enclosure and make EMC-safe contact, a divisible EMC connection block made of nickel-plated brass has been developed to simplify installation. This means that the stable lower part of the EMC block is first fitted to the control cabinet entry, the cable is positioned and the cable sheath is removed at the level of the contact point so that the cable's shielding braid is exposed. Once the prepared cable is positioned in the EMC connection block, the upper part of the block is pressed on and fixed in place with two screws. The non-magnetic TRI spring, which is also split, presses reliably around the braided shield of the cable. The EMC connection bracket is currently available in five sizes for cable diameters from 20 to 65 mm. Further sizes in M 25 and M 32 will follow shortly.

Contact multiple screens safely

In modern electronics, the cable shield often fulfills several tasks: On the one hand, it significantly determines the cable impedance and, on the other, it is intended to prevent signals from being coupled out and/or in. Unpleasantly, unwanted potential equalization can still take place via the braided shield. This is why there are special cables with multiple shields. The challenge now is to make EMC-compliant contact between these shielded cables in the shielded housing and the electronics. This is possible with an extended cable gland such as the aforementioned Blueglobe TRI with its two triangle springs positioned one behind the other.

The first spring is used to connect the outer shield of the cable directly to the outside of the housing, while the second spring connects the inner shield to the housing with low impedance. The possible third shield can then be routed to the ground test point in the switch cabinet and connected there. This solution achieves a shielding effect of better than -80 dB at frequencies up to over 1 GHz. Furthermore, the current-carrying capacity is doubled, which is important for frequency converters and in the field of electromobility, for example, where high-frequency shield currents of more than 25 A are nothing out of the ordinary.

'Channeling' EMC correctly

Ultimately, when it comes to EMC, the focus should not only be on the cable or its cable gland; particularly in demanding applications, the cable duct must also be designed for E and H fields specific to the application. Unlike cable glands, however, each EMC cable duct assembly is unique and must be specially adapted to the application. This first requires an analysis of the installation environment. The interference fields that occur are just as important for the design: Magnetic fields in the vicinity of electric motors or frequency converters can be attenuated by increasing the wall thickness of a trunking, for example, while in the case of electric fields, care must be taken to design the trunking with as few gaps as possible. In some cases, molded parts may even have to be made "from the solid".

Pflitsch, for example, uses 2 mm thick sheet metal to protect against magnetic fields. In order to keep out electric fields, the application-specific assemblies are provided with holes at a maximum distance of 50 mm along the trunking components so that the body and cover as well as the required fittings can be screwed tightly to the trunking body without gaps. Seals made of copper-beryllium, as used in safety doors, can also be inserted. Special connecting elements are used at the seams of the trunking parts.

Equipped in this way, only a few interference waves can escape from or penetrate into the trunking. In the test, this type of cable duct design achieved a magnetic shielding attenuation of -18 dB compared to an unprotected installation of cables and lines. For comparison: a standard cable duct system achieves around -10 dB attenuation.

Authors:
Günther Quednau is an EMC expert at Pflitsch in Hückeswagen;
Walter Lutz is a specialist journalist in Haiger.

EMC measurement without expensive measurement technology

The measuring principle of the KoKeT method and ...

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During the development of the EMC cable glands, Pflitsch realized that none of the established measurement methods for evaluating the shielding behavior of cable glands is really suitable for providing clear and reproducible measurement results on the shielding behavior of these components. The reason for this is that it is not the cable gland itself that is measured, but always the entire installation. In addition, the conventional methods require very expensive measurement technology.

... the open load cell.

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As a result, the Hückeswagen-based company further developed the internationally standardized measurement method for coaxial connectors - the triaxial method (in accordance with IEC 61 196-1, IEC 61 196-A, prEN 50 289-1-6 A +C, VG 95 214-12 and VG 95 214-13). The resulting KoKeT method (Coaxial Kelvin Tube) is able to measure the shielding effectiveness and the transfer impedance (absolute) of direct current up to over 1.5 GHz. KoKeT works according to the double-coaxial method. A partition wall is screwed into a well-shielded tube, which divides the chamber into two chambers one behind the other and at the same time contains a receptacle for shielded cable glands in sizes M8 to M85. Comparative measurements show that this method measures around 20 dB more accurately in the important frequency range of 25 to 130 MHz than the commonly used injectionline or absorber clamp methods.

KoKeT is now listed in the current IEC 62153-4-10 as a recognized measurement method.