Study on Shielding of Power-Frequency Magnetic Field in Workplace
Date Issued
2004
Date
2004
Author(s)
Chen, Hsin-Hung
DOI
zh-TW
Abstract
In Taiwan, it was common to introduce 60 Hz alternating current in most places such as households, offices and electrical equipments; this study aimed to investigate on the sources of power frequency magnetic fields and the efficiency of controlling power frequency magnetic fields, in order to avoid detrimental effects of power frequency magnetic fields.
Wherever electricity is generated, transmitted or used, power frequency magnetic fields are created. Magnetic flux density is influenced by multiple factors such as distance, electric current and the configuration of power transmission lines, etc. There are various methods to control power frequency magnetic fields, including the extension of distance and space, direction arrangement and shielding, etc.
This study attempted to probe for the control parameters of power frequency magnetic fields while manipulating the shielding methods. In addition, we sampled in workplace and evaluated the shielding efficiency of various materials in high magnetic field sources.
Results:
(1) When testing plane materials in magnetic fields generated by long electric line, the order of intensity of shielding effectiveness is the oriented silicon steel > the non-oriented silicon steel > the copper material > the aluminum material.
(2) The shielding effectiveness is the highest when the oriented silicon steel is placed perpendicularly to the axis of magnetic flux density.
(3) While using long electric line and loop as the magnetic field source with a magnetic flux density around 90 mG, the thicker the aluminum plate (not exceeding 7 mm), the higher the shielding effectiveness.
(4) It could be considered to avoid the magnetic leakage using superimposition of shielding materials rather using welding method.
(5) When testing different shapes of permeability ferromagnetic materials in magnetic fields generated by long electric line, the order of intensity of shielding effectiveness is cylindrical > square> double U shaped > U shaped > plate.
(6) While surveying in workplace, we found that:
1. The large equipments such as electric welding machines, transformers, motors, power pane land power conditioners often are with higher magnetic flux density (up to 100 - 1000 mG).
2. It is often with less magnetic flux density in business buildings. If there’s a transformer located below a building, the magnetic flux density might be up to 10 - 100 mG.
3. While designing and installing the shield in workplace, we suggest that it is more efficient and economic to use plate shape of shielding material. The shielding effectiveness was more than 14 dB in the two cases of this study.
Subjects
60 Hz
磁場屏蔽
電力頻磁場
遮蔽效果
power frequency magnetic fields
shielding
Type
thesis