How does the electrostatic shielding work

electromagnetic shielding

electromagnetic shielding, Shielding, keeping away or confining electrostatic, magnetostatic, or electromagnetic fields from or in a confined area. This means, especially in the case of magnetic shielding, the ratio of the field strengths inside and outside the shielded area Shield factor S., its logarithm Shielding attenuation.

1) electrostatic shielding, electrical shielding: The room to be screened is surrounded on all sides with a conductive wall. No field can penetrate into the cavity created in this way, since the interior of a conductor, including enclosed cavities, is always field-free. The electric field lines end in the surface charges that form on the outside of the wall as a result of the electric influence. Practical designs consist either of solid metal walls with high conductivity (Faraday cage), or, if the requirements for the shielding effect are lower, of perforated metal surfaces, e.g. wire mesh. However, these show a certain penetration of the electric field in the vicinity of the grid. This shielding is also effective with low-frequency alternating fields. In measurement technology, electrostatic shielding is primarily used to measure large resistances (R.

10 MΩ) or small currents or charges are essential.

2) Shielding against leakage currents: Shielding against leakage currents is required wherever the currents flowing through the insulation are comparable with the currents to be measured or the currents flowing through the test object. This is done either with Support stresses, which reduce the potential difference at the insulation, or with additional protective electrodes (Guard technique).

3) Magnetic shield: In this case, the room to be screened is covered with a magnetically soft material with a high permeability number (μr ≈ 103-105) surround. The shielding effect takes with it μr and the wall thickness too. In the magnetostatic case, the shielding is based solely on the shunt effect of the magnetically highly conductive envelope (magnetic circuit); in the case of alternating fields, the effect can be increased considerably by eddy current effects.

4) Shielding of alternating electromagnetic fields, Shielding in high frequency technology: The shielding is similar to the electrostatic one, but there are additional eddy current losses (Joule heat) and a current displacement towards the surface (skin effect). The ideal case of high-frequency shielding is a completely closed metallic hollow sphere. While the electric field component is completely shielded in this way, the magnetic shielding is frequency-dependent.

In addition to measuring technology, shields are necessary for low-frequency and high-frequency cables, telephone lines, as a protective device for work rooms against strong fields (e.g. in the vicinity of radio transmitters or high-voltage discharges), especially for people with pacemakers and similar devices (electrosmog).