Electromagnetic Interference (EMI)

Electrical device selection and location and the routing of electrical cables and wiring should result in the ElectroMagnetic Interference (EMI) levels of which the sum does not exceed the levels listed in the table below in the listening area.  If EMI levels exceed these limits in listening areas, the resultant noise (hum and buzz) may make hearing device telecoil listening impossible.  Design for low EMI with quality lighting, dimming devices and good wiring practice.  Create venues that provide opportunity to use the hearing aid for more ....

Background Noise
from EMI Sources
 Magnetic Field Intensity

H mA/m
 Magnetic Flux Density
 μTesla   or    mGauss
Data provided by
J.M. Woodgate
and D.G. Edworthy

 -47  1.8  2.3                      0.23
Preferred maximum for extended listening
 -32  10.1 12.6                     1.26
Maximum for extended listening
 -22  31.8 39.6                     3.96
Maximum for transient locations, e.g. service and ticket counters
 0  400  503                     50.3
400 mA/m is the reference signal strength
 8  1000 1257                  125.7
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 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 more slowly (1/d2). Magnetic fields from "stray" current on water pipes, building steel, etc. tend to decay even more slowly (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 (paragraphs above and EMIDecay.gif attachment below sourced from www.fms-corp.com).

Another excellent resource on the subject of EMI prevention and correction is a book written by Karl Riley, "Tracing EMFs in Building Wiring and Grounding", ELF Magnetic Surveys, 104 Old Farm Rd., Hampstead NC 28443, 1995, 2005, 2nd printing 2007 - highly recommended.

A number of Mr. Riley's articles on the same subject are posted at Mike Holt Enterprises

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  • BuildingDesign.pdf - on Dec 27, 2010 4:17 PM by Bill Droogendyk (version 1) Remove
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  • EMIDecay.gif - on Dec 27, 2010 4:37 PM by Bill Droogendyk (version 1) Remove
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