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EMF Mitigation & Shielding |
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Enertech Consultants
has pioneered research in EMF Mitigation and Shielding methods, and has worked on
development of key publications on the subject.
Mitigation is the process of reducing magnetic fields. The need for mitigation is often
raised when VDT interference or interference to other sensitive
equipment, such as an Electron Microscope occurs due to external magnetic fields.
Shielding is one of several options available in the mitigation process. Two types of
shielding are common: shielding with conductive or ferromagnetic materials, or
field reduction using passive or active cancellation wire loops. Shielding
with materials refers to using a material to provide an alternate path for the
magnetic flux (ferromagnetic materials) or creation of another magnetic field in the
opposite direction (conductive materials). Magnetic field cancellation with wire loops utilizes current carrying conductors to create a new magnetic field that partially cancels the existing field. Passive shielding uses the existing magnetic field to induce a current in the wire loop. Active shielding uses an external power supply with controlled magnitude and phase to force a current in the wire loop. Another method of reducing the magnetic field is to modify or change the source to produce less field. For power lines, including transmission and distribution lines, and local building wiring, mitigation actions such as rearranging the position of the conductors and creating balanced currents can reduce the magnetic field.
Have a look at these pictures
VDT image distortion due to strong (125 mG) external 60Hz magnetic field Investigation of Electron Microscope Interference EMF measurements near X-Ray Fluorescence Spectrometer DC Magnetic Field De-Gaussing of Magnetized Structural Steel in Building Wall
Interference MitigationExternal magnetic fields can cause interference in the form of image distortion to video display terminals (VDTs).Other sensitive equipment, such as electron microscopes, can also experience interference. This interference can be due to both static (DC) or 60 Hz alternating (AC) magnetic fields, which can deflect the electron beam path inside the VDT and cause distortion on the screen. The extent of interference will depend on magnetic field strength and direction, and the type of monitor. This distortion will appear as a jitter of lines or text on the VDT screen for AC magnetic fields and a fixed tilt and/or poor color purity for DC magnetic fields. Common sources of external AC magnetic fields that can cause VDT interference are: building wiring with unbalanced or net currents, transformer or electrical switchgear rooms in the building, and nearby transmission or distribution power lines. There is limited information available to define an acceptable level of external AC magnetic field with respect to the threshold for VDT image jitter. The threshold for AC (60 Hz) interference depends on many variables and has commonly been estimated at about 10 mG. A common source of DC magnetic field interference is building steel that has been magnetized by operation of an MRI machine or another DC field source. The threshold for DC (0 Hz) interference is not clearly defined, but Enertech has verified detectable image tilt and color purity problems for some VDTs in DC magnetic fields of about 1 gauss (and less). Enertech has extensive direct experience investigating VDT interference problems and recommending mitigation options, including: shielding, field cancellation with active and passive loops, correction of internal wiring problems, and/or power line reconfigurations or design changes. Shielding with MaterialsEnertech has extensively studied the use of ferromagnetic (high permeability), conductive (high conductivity), and layers of both materials to shield magnetic fields. Ferromagnetic materials include steel and mumetal. Conductive materials include aluminum and copper. Picture: Shielding of Electrical Equipment Room with Materials The mitigation process includes:
Passive Cancellation LoopsThe use of passive cancellation loops refers to the process whereby an existing magnetic field induces a current in a material or conductor. This inducted current is then routed in such a way that it partially cancels the existing field. Enertech has been involved in several passive shielding projects to reduce EMF due to power lines. Typical scenarios involve reducing the field from a transmission or distribution line, or from transformer buswork near an occupied area. Enertech determines the best way to implement cancellation loops by creating a computer model which simulates both the source and the cancellation loop to test various designs. Enertech engineers recently wrote a comprehensive handbook for EPRI on the design of cancellation loops for EMF Mitigation. Picture: Passive Magnetic Field Cancellation Loop on 69 kV Transmission Line Active Cancellation LoopsEnertech has performed several projects in conjunction with utilities, schools, and businesses to reduce EMF due to power lines using active cancellation loops. The use of active cancellation loops involves a system that senses the magnetic field in the region to be shielded and, via a feedback system, imposes a current on additional conductors such that it reduces the magnetic field in the region. If active loops are the best field reduction method for a situation, Enertech can design a system to meet the mitigation needs. Source ModificationEnertech has assisted clients in designing equipment modifications to reduce magnetic fields. Design changes for power line mitigation include opposite phasing (or revising conductor arrangements to reduce EMF), creating balanced currents, or other engineering design changes. For electrical equipment rooms, rearranging and moving electrical components is often the first step to consider since it is more cost effective than installing magnetic field shields. |
