The difficulty to shield magnetic fields

shielding magnetic fields

red is very strong , blue is weak

Transformers are often integrated in large office complexes or factories in the building. The current flow generates low-frequency alternating magnetic fields that can reach into the work area of employees. The workplace should be installed on the grounds of preventive occupational health and safety as far away as possible from the transformer station. As a guideline, the specification of the TCO standard is used with a value of 0.2 micro Tesla. In confined work spaces, the question may arise whether a shielding of the alternating magnetic fields is possible.

How does the shielding theoretical?

Small-scale magnetic field shields are frequently encountered in industrial applications. As a common example screen of the tube is called. A magnetic shield works as a sort of sponge that passes around the magnetic field around the shield, rather than be passed through the sensitive device. The magnetic field lines are strongly attracted by the shielding material, and therefore it requires a high permeability. The magnetic permeability μ determines the permeability of matter for magnetic fields. The higher μ, the better the shielding effect. Mu-metal as a nickel-iron combination achieved a permeability 50000-140000 μ, pure iron, however, only 300-10000 μ.

Encapsulating is necessary for shielding of room surfaces

With the shield of alternating magnetic fields, it is unfortunately not enough to cover only one side of the room with shielding material. The magnetic field would even penetrate reinforced around the edges in the room (see photo above left). The shielding effect of open shield is so deceptive. Near the plate, a good shielding effect observed. At a greater distance from the plate, the field strength, however, is almost identical to an unshielded source. It remains only a complete covering of the space left with a material of high permeability. Upon completion, it is very difficult to cover the transformer room from all sides. Therefore, a magnetic shield should be planned before installation.

Leaks in the area of penetration

Even with optimum design of the shield is to be expected in the transformer with a penetration of the walls. Finally, the lines must supply the entire building with electricity. The shielding is therefore as close as possible to the lines to move around. Every little slit leads to a weakening of the screening effect.

Not suitable for laymen

When planning shields the magnetic field on the intensity and frequency shall be determined. In addition, the basis weight and the distance from the field source are required. In specialized laboratories the appropriate choice of materials and the necessary permeability can be determined. When mounting caution is again commanded: in the mechanical processing or at a welding treatment, the shielding effect will be lost. Building biologists should not tend to offer the customer a quick fix. As stated above, the shot can backfire. Moreover, the material is relatively expensive.
A good introduction to the topic of the magnetic shielding provides the company Sekels Ober-Moerlen (Germany) on their website: www.sekels.de

http://www.sekels.de/fileadmin/user_upload/english/33_Brochure_Magnetic_Shielding.pdf

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