Shielding (electrical engineering)

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Shielding plates inside a cell phone

The shielding of electrotechnical devices, facilities and rooms is used to keep electrical and / or magnetic fields occurring at higher frequencies away from them or, conversely, to protect the environment from the fields emanating from the facility.

Electromagnetic waves , which have both a magnetic and an electrical component, also often have to be shielded in order to prevent or reduce their radiation or radiation .

Shields serve to improve electromagnetic compatibility and also to ensure interference-free signal acquisition, transmission and processing. In the case of cables in particular , shields also protect against undesired mutual interference ( crosstalk ) between actually independent signal channels.

The effectiveness of a shield is quantified by the shielding attenuation . In the case of cable shields, the measurand of the shielding effect is the transfer impedance .

Shielding measures

Influence of the course of the magnetic flux density lines by ferromagnetic material. Shielding results in a virtually field-free space within the ring profile.
Magnetic shielding of the mains transformer from a tube oscilloscope: three orthogonally nested sheet metal coils made of Mu-metal form a box that is closed on all sides without air gaps.
Copper ribbon loop around the core of a switched-mode power supply transformer , which prevents the vertical field components from spreading.

Static and low frequency electric fields

The electrostatic shielding works on the principle of influence (see also Faraday cage ). Shielding is achieved with electrically conductive shielding materials. Metal sheets, conductive foils or layers connected to earth or reference potential are used. These include B. also metallized plastic foils, aluminum foil-laminated paper, graphite and conductive lacquer layers. In plastic housings ( ABS ) autocatalytically deposited layers ( chemical nickel ) and subsequently electrolytically reinforced with copper are used.

The connection to the reference potential is made by contact springs or fastening screws. Electrical shielding is necessary for microphones and audio amplifiers , among other things, or whenever high-impedance signals and / or low levels have to be transmitted or processed.

Static and low frequency magnetic fields

Soft magnetic materials, d. H. Ferromagnetic materials of high permeability and low remanence also counteract the passage of magnetic fields of low frequency or constant fields. Magnetic shielding also has an electrically shielding effect if it is sufficiently conductive.

Magnetic shields are used e.g. B. used in CRT monitors and oscilloscopes with cathode ray tubes , since magnetic interference sources can cause picture interference. Permanent magnets of loudspeakers in television sets with picture tubes are often magnetically shielded. Other applications are the shielding of power transformers and motors in tape recorders and turntables with magnetic scanning systems .

A material suitable for this purpose is the highly permeable, so-called mu - metal , which, however, is sensitive to deformation and therefore often has to be annealed under protective gas after processing . There are also other materials for flexible cable shielding that are largely insensitive to deformation and that can be used without the customer's heat treatment.

The effect of the shielding can be explained by the breaking of the field lines when B-fields enter matter. In the case of substances with a permeability of the order of magnitude of 10,000 and above, every incoming field line is practically broken in the tangential direction and every falling out in the direction of the perpendicular. The field lines are thus conducted along the shield and do not penetrate through. It follows that magnetic shields must be self-contained to be effective. In the adjacent illustration of a transformer shield, this is achieved by three sheet metal coils lying one inside the other in such a way that the winding planes are orthogonal to one another. So no magnetic field, however directed, can penetrate to the outside.

High frequency magnetic fields

Magnetic fields of higher frequencies can be shielded with electrically conductive sheets that do not have to be ferromagnetic. The cause is the induced eddy currents that counteract the generating magnetic field. The sheet thickness must be greater than the skin depth to avoid currents on the shielded side.

If you know the direction of the fields, you can change the shielding u. U. reduce to a kind of short-circuit winding. This is used, for example, to shield the transformers of switched-mode power supplies , which are wrapped for this purpose with a copper tape that is soldered to form a ring. Such rings must enclose the field lines to be shielded.

electromagnetic fields

High-frequency electromagnetic fields ( electromagnetic waves ) can only be completely shielded with electrically conductive covers that are closed on all sides: due to the skin effect , an alternating magnetic field in electrically conductive material decreases exponentially. Up to the skin depth, the magnetic field drops to the 1 / eth part (≈ 37%) of the value at the outer edge. The skin effect facilitates the shielding of electromagnetic fields at high frequencies, as even very thin sheet metal is effective.

Gaps or openings reduce the shielding attenuation or even destroy it if the largest dimension of the openings or gaps reaches or exceeds the order of magnitude of half the wavelength to be shielded. As a rule of thumb, openings already reduce the shielding significantly when they are about a tenth of the wavelength. The deterioration occurs because the current generated by the field to be screened on the screen surface flows around the openings (apertures) and acts as a transmitting antenna. These surface currents cause the field to penetrate through the shielding and produce a field which corresponds to that of an electromagnetic dipole or multipole at the point of the opening. If components or cables protrude into this field, a shaft can become detached.

For this reason, doors and housing parts of a shield, switch cabinet or housing are sealed with conductive lamellas or metal braids, which result in an electrical contact that is as continuously closed as possible.

The shielding effect of metallic housings can be significantly impaired by cables and wires that penetrate the housing wall. Such cable entries , connectors and terminal points therefore require careful mechanical design to shield high-frequency interference signals:

  • Cable shields are applied (connected) on both sides to shield against the magnetic components of electromagnetic fields so that a compensating current can flow that counteracts the incident field.
  • Cable shields should not penetrate the shielding housing in an isolated manner, but should be connected to the housing wall directly at the point of entry over their entire circumference.
  • Unshielded cables should have filters ( feedthrough capacitors , line filter ) are performed.

Dissipation of interference on the cable shield

The standards VDE 0113-1 and DIN EN 60204-1 prescribe equipotential bonding for the electrical equipment of a machine. This can only partially prevent undesirable consequences from electrostatic , electromagnetic and grid-related interference. On the contrary, protective earthing often creates earth loops , as the cable shields create additional connections. In contrast, helps a neutral earthing of the signal covers a balanced signal transmission , a ground breaker or electrical isolation .

Ground connections should be as short, large and thick as possible in order to reduce the inductance and the effective resistance and thus to keep the potential differences between the ground connections of different parts of the system low.

In order to effectively divert potentially occurring disturbances on the cable shield, it is necessary to contact the cable shield as frequently and over a large area as possible.

The values ​​of the transfer impedance of the shielded cable should be as low as possible. Particularly in the case of high-frequency line interference, a small transfer impedance ensures that the interference voltages induced inside by the sheath current are kept low.

Shielding attenuation

The shielding effect is usually measured using the size of the dimension number shielding attenuation . For the magnetic field component, the shielding attenuation is the ratio of the undamped external field H a at a given location to the remaining residual field H i at the same location after inserting a shield. The shielding of coaxial lines quantifies the transfer impedance ; Colloquially, the transfer impedance is also called coupling resistance.


  • H. Kaden: Eddy currents and shielding in communications engineering . 2nd, completely revised edition. Springer Verlag, 2006, ISBN 3-540-32569-7 (first edition: 1959).
  • H. Wolfsperger: Electromagnetic shielding - theory and practical examples . Springer Verlag, 2008, ISBN 978-3-540-76912-5 .

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