Magnetic Field Shielding
What is different about magnetic field shielding
This is a complex subject involving a different set of physics plus materials science. Taking the Faraday cage approach does not work as magnetism has no charge and will always try to reach the opposite pole by the easiest route. The only common feature is the need to construct a physical barrier.
Crucially, no material will stop a magnetic field; it can only be diverted and redirected around the protected area. In fact even the use of the word 'shielding' is a misnomer as the direction of the field is shifted resulting in the creation of a region of lesser field strength.
There are two basic scenarios for magnetic field shielding:
Shield the risk – deflect the field
Magnetic field flows through the object to be protected
Magnetic field diverted to flow through the shielding
Shield the threat – contain the field
Magnetic field intersects the object to be protected
Magnetic field contained to within acceptable limits
Current medical MRI scanners produce magnetic fields of between 0.5 and 3.0 Tesla* – a measure of magnetic strength, but more powerful machines do exist and the trend is upwards.
* For comparison, the Earth's magnetic field is infinitesimal varying from below 30µT (micro Tesla) to over 60μT (<1/30,000 Tesla to >1/60,000) depending on location: the higher level being nearer the magnetic poles.
Materials, Design and Installation
Irrespective of whether the magnetic field it is DC or AC driven i.e. derived from an MRI scanner or a power transformer, the shielding material A needs to be of high permeability B that makes it highly attractive to a magnetic field compared to adjacent materials. This technique is sometimes referred to as 'flux-shunting'.
A For the construction related work we are involved in this generally steel.
B Permeability is a measure of how effective a material becomes magnetised i.e. its ability to conduct magnetic lines of force, when exposed to a magnetic field (sometimes called magnetic flux or flux).This ranges from 1 for air to 1,000,000 using exotic materials.
Low frequency, AC generated, magnetic fields present a more difficult problem to resolve. Here, a current is induced to flow within the shielding material. In order to optimise the induced-current shielding effect, the material needs to possess both high electrical conductivity and high permeability is required.
Optimising the design requires addressing and evaluating factors such as the relative performance of grain orientated v. non-orientated steels, the number of shielding layers and relative spacing, fixing types and penetrations; all crucial to the overall performance of the shielding. EEP installations use up to three different types of special electrical sheet steel.
Careful installation is paramount if the shielding is to avoid the situation whereby one condition is resolved but transfers the problem to an adjacent area – often a consequence of paying insufficient attention to the critical wall–floor interface.