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AC EMI Explained



Strictly defined, AC (alternating current) electric and magnetic fields include ALL time-varying fields – those with frequencies other than 0 Hz – but for practical EMI purposes and because of ubiquity, the term AC typically refers to fields generated by the use and distribution of electric power. In some countries (the United States among them), that means fields at a frequency of 60 Hz and its harmonics (120 Hz, 180 Hz, 240 Hz, etc.); in the rest of the world, that means fields (electric and magnetic), oscillating at a frequency of 50 Hz and its harmonics (100 Hz, 150 Hz, 200 Hz etc.).

While we concentrate on power frequencies, it must be said that EMI is an issue at ALL AC frequencies. For example, the developed world makes broad use of variable frequency drives and motors (elevators, air-handling motors of all sizes, as well as sewing machines and countless other applications) and these produce a rich and complex set of emissions capable of interfering with other devices.


It is generally true that anything that is powered is, at some level, susceptible to interference from AC magnetic or electric fields but we tend to devote more time to magnetic fields since they are not materially affected (mitigated) by common building materials. The opposite is true of electric fields. But there are very important exceptions to that generalization, particularly in the area of biological diagnostic and research equipment.

microscopy suite 1A variety of research and diagnostic equipment can experience EMI problems when elevated levels of AC magnetic fields are present. Among the most susceptible, are the newest generations of SEM and TEM electron microscopes. These instruments are exquisitely sensitive and can exhibit interference from external AC fields as low as 0.1 mG or less. A variety of laboratory equipment including Gas Chromatograph and any device utilizing electron scanning technology will be sensitive to interference from external magnetic field sources.

High precision movement robotic systems can also exhibit problems if located in areas with elevated AC magnetic fields, as can research MRI & NMR imaging systems.

USoundBiomedical equipment found in hospitals, clinics and treatment centers can also be subject to interference from external AC magnetic fields at quite low levels. EKG & EEG equipment, Ultra Sound scanning systems, MRI imaging systems and patient worn medication delivery systems are examples of medical equipment which can have interference problems with elevated AC magnetic fields.

NFL - Saints Studio 2 - New OrleansElevated AC magnetic field levels can cause significant EMI problems in broadcasting and entertainment production facilities as they can induce “hum” or objectionable degradation of signal/noise ratios in a wide variety of equipment including microphones, musical instrument pickups, recording mixers, etc.

HP ServerUnfortunately, the potential of an EMI threat from elevated AC magnetic fields to computer and telecommunications equipment is not well defined. While there are numerous anecdotal reports of EMI problems with routers and distribution systems from external AC magnetic fields, very few manufacturers of such equipment or systems provide meaningful sensitivity or immunity specifications or guidelines. Moreover EMI guidelines for AC magnetic field immunity thresholds are internationally inconsistent.

For example, the governing specification for the EU, EN 55024, which speaks to a maximum AC field level of 1 A/m (12.6 mG) has no official standing in the United States and equipment is not required to meet either the immunity or emissions standards. Nevertheless, some IT data processing and communications manufacturers have begun to enforce EU site environmental requirements in order to extend performance guarantees.




Extremely low frequency (ELF) or 60 Hz (AC) magnetic fields are naturally emitted by current-carrying electrical conductors and devices. All other things being equal, the AC magnetic field strength emitted by electrical circuits is directly proportional to the magnitude of electrical current. But wiring configurations can be optimized for lower fields: 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 conductors from different 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.

Effects_of_distanceAC magnetic fields decrease naturally in intensity as a 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.

Transmission 1The most obvious sources of AC magnetic fields 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 light transformers, older 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 well-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 might be considered typical.

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