Abstract Power frequency magnetic fields are difficult and expensive to shield, particularly when the fields originate from sources with complicated field pattern. A method is discussed and test data presented which suggests that carefully combining active and passive shielding techniques can produce results which are importantly superior to either technique when used alone. The results are generally consistent with an analysis which considers sources as a superposition of elementary source fields with different order terms. Key Words - Magnetic Fields, Active Field Cancellation, Passive Shielding Introduction Questions about possible adverse health effects of power frequency magnetic fields, coupled with the capacity of such fields to interfere with sensitive instruments and computer monitors, have spurred efforts to find effective and economical means to reduce fields in a variety of settings. Attention has focused on power lines but other sources are both as prevalent and as difficult to manage.
Examples are transformers, cables, bus bars and service entrance panels.
Field management techniques proposed for power lines have typically employed either compaction, reconfiguration or distance. These techniques are well documented, though not universally employed, and essentially involve cancellation of one field by another of opposite phase. Accordingly, fields from transformers, cable and bus bars and service panels are not effectively managed by such active cancellation devices due to their geometry and the complexity of the fields produced. These sources and the fields produced by them are typically managed, more or less effectively, by shielding with either permeable or conductive materials, or a combination of both. This is done on such a way as to take advantage of their desirable characteristics but uses them in combination to minimize their respective undesirable characteristics.