Magnetic Fields of a Cylindrical Shell Excited by an Unbalanced Current Source

This paper presents an analysis of the low-frequency magnetic field generated from an unbalanced current source in the presence of a cylindrical shell. Analytical expressions of the magnetic field are given, as well as the shielding mechanism and shielding performance of the shell. The impulse magnetic field generated from a lightning discharge current in a coaxial cable is also addressed. It is not always correct that a conducting shell or enclosure is the panacea to shielding every kind of excitation. The shell may become a vain attempt at magnetic shielding if the source current is not balanced within the enclosure, such as the bend of a busbar trunking system. The presence of the enclosure may only result in a change in the distribution of the magnetic field. Effective shielding is achieved if the monopole field from the source is cancelled. This can be realised by placing the returning-current filament internally or injecting the current in the shell.

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