Practical Aspects Of Shielding


Also if the use of shielded cable is not mandatory to comply with national and local regulations, it is still often recommended to use a shielded cable as supply or signal cable. But using a shielded cable alone is not enough. Shielding is only effective when the right type of shielded cable is used and when the cable is connected properly.

Ground connection

The ground connection of a shield is crucial for its effectiveness. For this reason, serrated washers or split washers must be fitted under enclosure assembly screws and painted surfaces must be scraped clean in order to obtain a low impedance contact. Anodised aluminium enclosures, for example, provide inadequate ground bonding if plain washers are used under the fastening screws. Earth and ground leads should be made from wire with a large cross-section, or better yet from multi-cored grounding wire. If wire cross-sections less than 10 mm² are used with low-power motors, a separate PE line with a cross-section of at least 10 mm² must be run from the converter to the motor.

Motor supply cable

In order to comply with radio frequency interference limits, cables between frequency converters and motors must be shielded cables with the shield connected to the equipment at both ends.

Signal cable

The distance between the motor cable and the signal cable should be more than 20 cm, and the mains cable and motor cable should be routed separately as much as possible. Interference effects decrease significantly with increasing distance. Additional measures (such as divider strips) are essential with smaller separations. Otherwise interference may be coupled in or transferred.
Control cable shields must be connected at both ends in the same way as motor cable shields. In practice, single-ended grounding may be considered in exceptional cases. However, it is not recommended.

Types of shields

Frequency converter manufacturers recommend using shielded cable to shield the wiring between the frequency converter and the motor. Two factors are important for selection: the shield coverage and the type of shielding.

The shield coverage, which means the amount of cable surface covered by the shield, should be at least 80%. With regard to the shield type, a single-layer braided copper shield has proven to be extremely effective in practice. Here it is important that the shield is braided. By contrast, a wound copper wire shield (such as type NYCWY) leaves long slit-shaped areas uncovered, and HF components can easily escape through these gaps. The surface area for leakage currents is also significantly smaller.

Shielding is available in bulk for retrofitting. It can be pulled over the cable to provide the desired shielding effect. For short connections, metal hoses or pipes can be used as an alternative. Cable ducts can replace shielding only under certain conditions (a radiation-proof duct with good cover contact and a good connection between the duct components and ground).

Cables with double shielding further improve the attenuation of emitted and radiated interference. The inner shield is connected at one end, while the outer shield is connected at both ends. Twisted conductors reduce magnetic fields.

Shielded cables with twisted conductors can be used for signal lines. The attenuation of the magnetic field increases from around 30 dB with a single shield to 60 dB with a double shield and to approximately 75 dB if the conductors are also twisted.



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