Re: Electromagnetic field question

[Michael Williamson <m.williamson@mailbox.uq.edu.au>]

Mitch and other Radsfers,

the magnetic field interference problem described is a common one at our
university.  Where you get magnetic flux densities exceeding about 10 mG,
there is potential for interference with computer screens.  It seems the
observed oscillations are the result of beating between the screen refresh
rate (anything between about 45 Hz and 120 Hz) and the mains frequency (60
Hz Nth America, 50 Hz elsewhere).

We usually find 2 causes: bad wiring (separation of active and neutral
conductors, multiple current return paths, etc) and proximity to the main
current feeders for the building (these lines can carry 400 - 800 Amps per
phase! - small load imbalances can produce quite large fields with currents
like this).

The solution is usually to either rewire or relocate the computer.

You might want to check to see if there are any relevant publications from
your electricity supply bodies such as the Edison Electric Power Research
Institute (in Australia the Electricity Supply Association of Australia has
brought out an excellent publication called Magnetic Field Mitigation to
reduce VDU interference - I'm sure there'll be something similar in the US).

Regards

Mike

Michael Williamson			Radiation Protection Adviser
(m.williamson@mailbox.uq.edu.au ) 	Occupational Health & Safety Unit
ph +61 7 3365 4504   			University of Queensland
fax +61 7 3365 1577			Brisbane 4072
					Australia

All opinions given in this message are those of the author and should not
be taken as representing the official policies of the University of
Queensland.




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