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MERLIN LEGENDCommunications System Release 6.1
System Planning
555-661-112
Issue 1
August 1998
Before You Begin
Page 1-5Confirming the Location of the Control Unit
1
Electrical Noise/Radio-Frequency Interference 1
In most cases, electrical noise is introduced to the system through lines/trunks or
telephone cables. However, electromagnetic fields near the control unit may also
induce noise in the system. Therefore, the control unit and cable runs should not
be placed in areas where a high electromagnetic field strength exists. Radio
transmitters (AM and FM), television stations, induction heaters, motors (with
commutators) of 0.25 horsepower (200 watts) or greater, and similar equipment
are leading causes of interference. Small tools with universal motors do not
generally cause a problem when operated on separate power lines. Motors
without commutators generally do not cause interference.
Field strengths below 1.0 volt per meter are unlikely to cause interference.
Estimate the field strength produced by radio transmitters by dividing the square
root of the emitted power (in kilowatts) by the distance from the antenna in
kilometers.
This yields the approximate field strength in volts per meter and is relatively
accurate for distances greater than about half a wavelength (492 feet, or 150 m,
for a frequency of 1000 Hz).
To comply with FCC Part 15 requirements, each power supply on a newly
installed system must have a ferrite core installed around the AC power cord and
ground wire. Beginning with Release 2.1 this ferrite core is packaged with the
power supply module at the factory. Ferrite cores are also compatible with earlier
releases.