Electromagnetic shielding

The shielding of electro-technical equipment, facilities and spaces is used to keep electrical and / or magnetic fields of these, or vice versa to protect the environment from the risks associated with the device fields.

Electromagnetic waves having both a magnetic and an electrical component, must also frequently be shielded to prevent the radiation or light or reduced. These tasks are part of the electromagnetic compatibility ( EMC).

The effect of shielding is quantified on the screen attenuation. In line shields the measure of the shielding effect is the transfer impedance.

Measures for screening

Static and low-frequency electric fields

The shielding of electric fields is achieved with electrically conductive shield materials. As a rule, you use natural gas or reference potential connected or related metal sheets or foils. The electrostatic shielding works on the principle of induction and found inter alia in microphones and amplifiers NF application. See also the Faraday cage.

Static and low-frequency magnetic

Soft magnetic materials, i.e., ferromagnetic materials with high permeability and low remanence, also counteract the passage of low frequency magnetic fields and constant fields. A magnetic shield effect at the same time also electrically shielding, if it is sufficiently conductive.

Magnetic shields are used, for example in CRT monitors and oscilloscopes with CRT, as it may cause picture interference due to magnetic interference. Permanent magnets of speakers in televisions CRT are often magnetically shielded. Other uses include shields of power transformers and motors in tape recorders and turntables with magnetic scanning system.

A suitable material for this purpose is the highly permeable, so-called mu-metal, but the opposite deformation is sensitive and needs to be frequently annealed after machining under protective gas. For flexible cable shields there are also other materials that are largely insensitive to deformation and can be used without client -side heat treatment.

The effect of shielding can be explained by the refraction of the field lines at the entrance of B- fields in matter. For substances with a permeability on the order of 10,000 and above any incident field line is practically broken in the tangential direction, and any failure in the direction of the solder. The field lines are thus led to the shield along and do not penetrate through. It follows that magnetic shields should be as closed as possible in itself, to be effective.

Electromagnetic fields

High-frequency electromagnetic fields ( electromagnetic waves) can be completely shielded only with electrically conductive, fully enclosed cases sufficient thickness: due to the skin effect penetrates alternating electromagnetic field only up to the skin depth in an electrically conductive material. The skin effect facilitates the shielding of electromagnetic fields at high frequencies, since very thin sheet is already effective. Gaps or openings reduce the shielding effectiveness and make them naught when the largest dimension of the openings or gaps, the order of half the wavelength reaches or exceeds shielded. As a rule of thumb, that openings already significantly reduce the shielding if their expansion reaches about one-tenth of the wavelength. The deterioration results from the fact that the current generated by the shielded area on the panel surface to flow around the openings ( apertures) and acts as a transmitting antenna. Said surface currents have a penetration of the field through the screen, causing a result field which corresponds to an electromagnetic dipole or multipole at the site of opening. The dipole or multipole superimposed on the unprotected side of the screen of the incoming electromagnetic wave. Therefore, the doors and the housing parts of a shield of a switchgear cabinet or housing can be sealed with metal or conductive fins meshes which provide a closed electrical contact as continuously as possible. The shielding effect of metallic housings can be greatly affected by wires and cables that penetrate the enclosure boundary. Such cable glands, connectors and terminal points therefore need for protection against high frequency interference signals a careful mechanical design:

  • Cable shields are possible over its entire circumference closed ( 360 ) connected to the shield wall.
  • Cable shields are connected at both ends for shielding against magnetic component of electromagnetic fields, so that a compensating current may flow, which counteracts the incident field. Cable shield should therefore be contacted at both ends to the respective screen housing.
  • Cable shield should a conductive housing is not isolated to penetrate, but can be contacted directly at the entry point.
  • Unshielded cables should filter ( feedthrough capacitors, mains filter ) are performed.

Shielding effectiveness

The shielding effect is recognized over the dimensionless shielding attenuation usually by measurement. The shielding effectiveness is the ratio of the undamped external field Ha at a given location to the remaining field Hi in the same place after inserting a shield for the magnetic field component. The shield coaxial cables quantifies the transfer impedance; colloquially called the transfer impedance and coupling resistor.

25338
de