In a message dated 97-03-29 21:52:12 EST, you write:
<< 1. Do I have a problem or is the field involved negligible? Note that
all
of the electrical wiring in the trailer accesses this unit.
The question has been hotly contested for many years, with no definitive
conclusions. Most Electrical Engineers would not consider your setup to be a
problem. Keep in mind that in your house and office, you are surrounded by
many feet of wire carrying 120 or 220 volts at the same frequency. There are
wires in the ceiling, as well as the walls. People have been sleeping under
electric blankets for years, with no known cases of harm. If you have an
electric range, she has been standing in close proximaty to the heat coils,
which are unshielded 220 volts. Every time you use a power tool, you are
holding 120 volts, unshielded, in your hand. However, since your wife is
uncomfortable with it, I would recommend a shield, just for her piece of
mind. No matter how well you rationalize it, the unknown can be frightening.
<< 2. Would a metal barrier of some sort, attached to the underside of the
bed support plywood, act as a suitable protector?
It would help.
<< 3. Would this barrier have to be grounded to the chassis of the trailer
in
some way?
Yes, and the chassis would have to be grounded at the power supply. For
safety purposes, you should have a chassis ground anyway. Electrical codes
call for trailer outlets to be grounded.
4. Is there a better suggestion? >>
Build a complete enclosure around the unit, and ground it well. For heat
purposes, use a perforated material. Your wife may have a problem with the
perforations, so to put her mind at ease, point out a few satelite TV
receptors, and note that they are also perforated. As long as the openings
are very small compared to the wavelength of the field, they are opaque to
the field - in other words, the shield looks like a solid piece to the
fields. The wavelength of 60 cycles is on the order of 100s of miles (I don't
remember the exact value, and I am to lazy to calculate it tonight), whereas
the satellite wavelengths are fractions of an inch.
It has been many years since I was involved in EMI work (electro-magnetic
interference), so I am a bit rusty. I will pass this question on to some of
my former co-workers who are still active in this area. As soon as I hear
from them, I will pass their answer on to you.
I retired two years ago from the Tennessee Valley Authority after 28 years
designing instrumentation and control systems for nuclear power plants. My
involvement with EMI was about 15 years ago.
Hope this helps.
Dan Masters,
Alcoa, TN
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