The key word in EMC is "compatibility". This implies that electrical and
electronic
equipment are (ideally) designed so that each can operate normally in the
presence
of another. This requires limiting both the emissions and sensitivity of such
devices.
Historically, only a limited number of product types have been subject to EMC
limits.
Most EMC requirements are based on the assumption that the emission of specific
frequencies is more likely to interfere with other equipment than "white" or
broad
spectrum emissions. For example, the FCC rules apply to devices using clocked
frequencies of 10K and above, but place no limits on vacuum cleaners, blenders,
arc welders, etc. unless they contain clocked electronics.
The exclusion of so many products from emission/susceptibility requirements is
often
the cause of EMC related "accidents". Some years ago, in one of the U.S.
Southwestern
states, the local public safety (police/fire/etc) communications were often
disrupted by an unknown source. The source was eventually traced to a pin ball
machine in a roadside tavern. The owner was told he must get rid of the
machine. A few weeks later, the noise re-appeared. It turned out that the same
pinball machine was placed in service at another pub in the county.
In some cases, the interaction of two devices is not exactly foreseeable. We
once
received reports of one of our typewriters typing occasionally without human
assistance. It turned out that the typewriter was in use fairly close to an
airport radar beacon. When the radar beam swept the area of the typewriter
installation, it could cause the capacitor coupled keyboard to create false
keystrokes. We added a large grounded template to cover most of the interior
keypad area, to increase its immunity.
There can be, and have been, safety related consequences of EM incompatibility.
In the 1980's (as I recall) a U.S. aircraft carrier suffered a major EMC
disaster. The powerful on-board electronics, particularly the radar units,
triggered the launch of a missle from one of the on-deck planes. The missle
struck the bridge tower, resulting in a fire costing millions of dollars in
repairs and the loss of some lives. I cannot find my copy of this event,
reported some years ago in one of the electronics magazines.
In general, Navies are far more sensitive to EMI due to the concentration of on-
board electronics. As a result, the U.S. Navy version of the Blackhawk
helicopter
had few EMI problems, while the Army version had several early crashes due to
interference from nearby radio stations.
The moral, if there is one, is that emissions and susceptibility of unregulated
devices is more often the problem than the emissions or susceptibility of a
regulated device.
George Alspaugh
Lexmark International
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