MAGNETIC FIELD SHIELDING
Low Frequency Magnetic Fields
The effects of electro-magnetic fields on equipment and human health is a much-discussed topic. We are all continuously exposed to low frequency electro-magnetic fields (EMF), a common manifestation of these fields are flickering computer screens or VDU’s. With the rapid growth in the use of IT equipment in the office, it is no longer possible or acceptable to just move the computer to another part of the building that is unaffected.
The sources of magnetic fields in buildings can be grouped into two general categories.
- power supply and distribution equipment
- power consumption equipment
There are several mitigation strategies to reduce the affects of EMF, EEP can help advise at building design stage, or propose remedial actions for existing installation. This can include carrying out EMF surveys or pre-installation studies on EMF effects.
EMF shielding is only one strategy but should this be required EEP offer a full design and installation service.
EMF Screening
When the consideration of physical screening becomes the most practical of remedial actions this involves placing a physical barrier of shielding material between the source of interference and the affected area. The most commonly used materials are steel and high permeability materials such as Mu-metal.
EMF shielding works by diverting and deflecting the magnetic field away from the affected area. The design of a shield must take into account that a magnetic field will ‘flow’ around a partial shield, but as these fields decay rapidly with distance the extent of the shield needs to be large enough to take this into consideration. The most critical criteria when designing a shield is the level of attenuation required. Although different sizes and types of IT equipment and VDU’s are affected by different levels of interference, experience shows magnetic fields in excess of 0.5 μT (Micro Tesla) can cause VDU flickering and if the fields rise above 1 μT most screens will be affected. Test levels for IT equipment as detailed in the EU EMC Directive (EN 55024, Immunity of IT equipment) is 1.25 μT.
When the results of an EMF survey, or study are compared to an acceptable level the required attenuation can be determined.
Using the formula A = 20log(R/M) where A is the attenuation required in dB, R is the actual EMF level and M is the maximum EMF level acceptable.
For example a monitor susceptible to 0.8 μT located in an actual field of 10 μT will require 22 dB of attenuation.
Once this has been determined the type, thickness and extent of shielding material can be calculated. Particular consideration needs to be taken at floor / wall junctions as a poorly designed shield may solve a problem in one area but concentrate the magnetic field in an adjacent room causing a new problem in a different area.
Health effects from EMF and standards
The health effects from EMF are a hotly debated and contentious issue. There are many studies with many varying results and conclusions. The website PubMed lists over 700 summaries of reports into EMF and health effects. It should be emphasised that these studies are looking for a link with regard to LONG-TERM HEALTH (measured in years) as opposed to short term exposure. Therefore most discussions will centre on public dwellings, ie a bedroom where it could be expected that a person may
live for many years. THE PRECAUTIONARY PRINCIPLE FOR NATIONAL AUTHORITIES, that recommends dwelling should not be sited in EMF above, typically 0.4 μT. Is rapidly gaining acceptance.
EEP can advise and propose solutions when EMF and health cause concern.
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