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Title: Extremely low frequency magnetic field shielding in large office building
Authors: Kong, Siu-kuen Sammy
Degree: M.Phil.
Issue Date: 2000
Abstract: Recently the extremely low frequency (ELF) magnetic field environment inside buildings has been the subject of greater attention. This is because of the concerns over actual or potential ELF magnetic field interference and possible associated health hazard. In response to these concerns, this project has examined the mitigation of magnetic fields in office buildings, with the focus on the ELF magnetic shielding. Until now, there is no standards or other known methods given in literature for designing a shield in buildings. The objectives of the thesis are to characterize two major types of commercial shields, metallic trucking and planar sheets, and to develop semi-empirical formulas for design purposes through experiment measurements and numerical analysis. This thesis includes the discussions on these issues as well as on the electromagnetic environment in buildings, the associated EMI, problems, ELF magnetic field shielding theory, and consideration of their application in buildings. The study began with an investigation into the magnetic environment in office buildings. Various magnetic field sources and possible mitigation methods have been identified. It is found that shielding is considering as one of the most efficient methods to mitigate any unwanted fields. In office buildings, two different types of shields can be employed: planar shield and rectangular shield (trucking). The planar shield refers to the use of flat metallic sheets. It can be used for shielding large sources, such as a transformer room, or for mitigation in an affected room. Metallic trunking can be used for shielding line sources, such as, heavy current-carrying cables or busbars (sometimes referred to the busduct). A large amount of experimental works and computer simulations have been carried out to study shielding characteristics of both rectangular shields and planar shields. For the trunking type shield, a set of semi-empirical formulas for shielding effectiveness against shield size, thickness, and material were derived for different trunking types that are commonly used in buildings in Hong Kong. These formulas were obtained by using numerical simulation software and with verification using experimental data. They describe the shielding phenomenon in trunking and provide an easier way to estimate shielding effectiveness of the trunking. The characteristics of the planar shields were also investigated in great detail through both numerical analysis and experimental measurements Practical issues, such as the leakage at the seam position were also investigated. In the past, the shielding performance of the applied shield can only be estimated roughly. However, with the semi-empirical formulas provided in this thesis, the predication of the shielding effectiveness for trunking shield design is possible. It can overcome the burden in shielding design. On the other hand, the shielding characteristics of the finite planar shield that are related to the real practice were addressed in the thesis. With these findings, the electrical engineers can design the shield more efficiently. The design procedure is simplified and the cost-effective shield design is feasible. The ELF magnetic fields inside a building can be effectively mitigated, and the compatible electromagnetic environment in the building can be ensured.
Subjects: ELF electromagnetic fields -- Health aspects
Office buildings -- Health aspects
Hong Kong Polytechnic University -- Dissertations
Pages: xvi, 137 p., [24] leaves : ill. ; 30 cm
Appears in Collections:Thesis

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