ELF or AC Magnetic Fields



Unshielded bus channels
Extremely low frequency (ELF) or 60 Hz (AC) magnetic fields are naturally emitted by current-carrying electrical conductors and devices. The AC magnetic field strength emitted by electrical circuits is directly proportional to the magnitude of electrical current. However, multiple adjacent conductors, carrying balanced currents have a low net field emission, a consequence of the natural cancellation of magnetic fields created by currents traveling in opposite directions (single phase) or with different phase angles (three-phase). Rigid metallic conduit generally provides good magnetic field reduction, provided that the feed and return currents are equal, in single-phase circuits, and if all of the currents (both feed and return) are present, in three-phase circuits. If electrical current from a circuit returns via an alternate path, then magnetic field levels emitted from such a circuit can increase significantly. This condition usually occurs if neutral circuits are "cross connected" or illicit connections are made between a neutral and ground in a building's electrical distribution system. This is often referred to as "stray", "ground", "zero-sequence", or "net-current" conditions, usually a result of a wiring error.

AC magnetic fields decrease naturally in intensity as function of distance (d) from the source. The rate of decrease however, can vary dramatically depending on the source. For example, magnetic fields from motors, transformers, etc. decrease very quickly (1/d3) while circuits in a typical multi-conductor circuit decay slower (1/d2). Magnetic fields from "stray" current on water pipes, building steel, etc. tend to decay much slower (1/d). Simply increasing the distance from the source(s) of an area with elevated magnetic field strengths can often reduce magnetic fields to an acceptable level.


Electrical transmission lines
AC magnetic field sources include heavy current-carrying devices such as Transmission and Distribution Power Lines, Transformers, Electric Service Panels, and Conduit or Bus Bars. Less obvious sources include fluorescent lights, computer and television monitors, and other electronic equipment. Even the wires in the wall are potential sources. For example, if a distribution circuit inside a wall is incorrectly wired ("wiring errors"), the resulting magnetic field can extend across a substantial portion of the room or building.

Unlike electric fields that are relatively easily shielded by common materials used in commercial construction, magnetic fields are capable of penetrating all but ferromagnetic and a very few, specially manufactured and installed materials. AC magnetic fields will pass undiminished through earth, concrete and most metals, including lead. The actual AC magnetic field strengths encountered within a given commercial building typically range from under 0.2 mG in open areas to several hundred near electrical equipment, but for practical purposes, an ambient range of from 0.2 to 4 mG can be considered typical.

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