The following basic information gives our experience in the attenuation of mains frequency magnetic field reduction.
The following is based on full continuity fully welded and gapless joints to all shielded surfaces, augmentation of joints with an overlapping strap may be required.
The behaviour of mains frequency magnetic field is very different to RF and microwave energies and the treatment of reducing the fields is also very different.
Here are the basic facts based on experiences from laboratory tests and site installations:
The shield medium has to cause the magnetic field to flow easier in the shield rather than in free air, this is known as magnetic reluctance
There are varying opinions on the best shielding material, during Global EMC experimental testing three materials were exposed to 50Hz magnetic field in a 1m3 cube with a radiating coil external and the sensor internal supported on a plastic stand:
Aluminium – This medium is common in US Electron Microscopy chambers but not common in Europe, in the Global EMC 50Hz magnetic field cube tests this medium was the lowest in terms of field attenuation.
Low Carbon steel – This medium has the lowest commercial cost and is commonly used for 50Hz shielding due to budget constraints, in the Global EMC 50Hz magnetic field cube tests this medium was slightly better than Aluminium in terms of field attenuation.
Pure Iron – This medium has the highest permeability compared to the other two shield mediums. In the Global EMC 50Hz cube tests this medium was easily the best at shielding 50Hz magnetic fields.
Note – There are other exotic ultra-high permeability materials such as Mu-metal, these were not considered as they are not commercially viable.
When a shielding medium causes ‘magnetic reluctance’ the energies flow in the medium. Taking the example of a flat plate the energies cannot flow to anywhere, so, the magnetic field will want to re-radiate at the shielding medium edge, thus drastically reducing the shield effect. It is always better to get as close to total encapsulation as possible, thus allowing the energies to flow around the Faraday cage.
The difference experienced in ‘line-of-sight’ shielding can be as low as 20% that of encapsulation shielding even considering a 1m deep return flange (we have comparative data.) Typically ‘line-of-sight’ can be as low as 1.5dB/mm of shield thickness (we have experienced this!)
The effect of shielding versus the field strength is not a constant, if the field is low strength then the shield has less effect. When the field is higher in intensity the shield works better. When the field strength becomes very high the shielding can go into saturation and becomes less effective. These effects can be seen on the Hysteresis curve.
The following attenuation was measured in the Global EMC 50Hz cube tests on the same shield medium:
The good thing about 50Hz magnetic field is that the field rapidly decays from source. Taking the example of a sub-station transformer, if enough distance can be afforded then shielding may not be required, although, this is rarely the case!
Shielding of cables which emit high magnetic fields can be treated so as to reduce the adverse effects. Encapsulation will be required with a choice of permanent shield or with inspection covers. The shielded should run past a sensitive area by at least 3m but could be as much as 6m so as to negate the re-radiation effect.
For more information about Global EMC's 50Hz Electromagnetic Shielding service, please contact us