AC ELECTROMAGNETIC INTERFERENCE (EMI)
AC typically refers to fields generated electric power systems. In Europe that means fields (electric and magnetic), oscillating at a frequency of 50 Hz and its harmonics (primarily odd harmonics, 150Hz etc).
AC EMI is caused by variable speed drives (VSD) and motors (lifts, chiller units and air-handling motors being the most predominant. These produce a rich and complex set of emissions capable of interfering with other devices. The cables from the VSD to the motor are the primary source of interference which is highly dependant on installation practice.
The victims of AC EMI includes transducers such as temperature sensors and data processing equipment. Telecommunication systems are also vulnerable to this type of interferences the harmonics are in the audio band up to 4 kHz. Magnetic fields are not materially affected (mitigated) by common building materials. The opposite is true of electric fields. But there are very important exceptions to that generalisation, particularly in the area of biological diagnostic and research equipment.
A 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 extremely sensitive and the specifications for these devices have detailed immunity criteria.
And can exhibit interference from external AC fields as low as 10 nT or less. A variety of laboratory equipment including Gas Chromatograph and any device utilising 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.
Biomedical 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.
Elevated 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.
In Europe the standard, EN 55035 for information Technology Equipment (ITE), allows a maximum AC field level of 1 A/m (1.26 µT)
THE BASICS OF ELF OR AC MAGNETIC FIELDS
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. 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.
AC 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.
The 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. 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 0.02 µT in open areas to several hundred near electrical equipment, but for practical purposes, an ambient range of from 0.02 to 0.4 µT is typical.
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