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Understanding Shielding Materials

 

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There are two methods of shielding against low frequency electromagnetic fields produced by 50Hz power sources, active shielding and passive shielding.

Active shielding involves the use of electronic field sensors and generators to measure the incident field and produce an opposing EMF which actively cancels the incident magnetic flux. This is a complex and costly method of shielding, generally reserved for issues which cannot be solved by passive shielding.

Passive shielding involves the use of rigid electrical steels which have a high magnetic permeability.

There are two methods of action of high permeability shielding materials.

  • Flux-entrapment materials are ferromagnetic materials including mu-metal and nickel-iron alloys.
  • Lossy shield materials include highly conductive materials including iron, steel, aluminium and specialty electrical steels. The most common form of shielding installed by EMI Shielding is a lossy type shield.

A combination of highly permeable and highly conductive materials is used to achieve the required performance.

The shielding installation consists of multiple sheets of the specialty steels, the number and configuration of which is calculated to achieve the required shielding effectiveness.

People are often surprised by the thickness of the sheets – less than 1mm (shown below):

Low VOC (volatile organic compound) glue is used to secure the sheets to the slab. Subsequent layers are secured to prevent sideways movement of the sheets. Pin-type fixings are used to secure the final layer.

Rolling on the first layer of glue

Layers are installed with an overlap on all edges to provide conductivity and continuity of the shield. Joins in subsequent layers are offset to maintain shielding performance and to minimise surface undulation.

 

Laying Shielding – note staggering of edges

The extent of shielding is generally larger than that of the electrical installation and is required to be a continuous shield without large penetrations or patches and with extension up columns and walls to prevent fields from leaking at these areas.

Shielding turns up columns and allows small penetrations. An epoxy coating protects from moisture.

Shielding can be fixed to the soffit where protection is required for the floor above.

Shielding materials must not be exposed to moisture as they corrode easily and corrosion between the layers can lead to loss of performance.

Shielding materials can be painted using approved (non-water based) paints.

The finished floor shielding is taped at the joins to prevent ingress of dust between the layers.

The shielding is then covered over with carpet and plasterboard to the walls.

 


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