As I recall, our facility (IBM at the time) first dealt with field ESD issues
long before there were the present FCC EMI rules for "digital devices".
This was back in the '60's.
At the time we made typewriters, some with internal magnetic cards for
storing typed documents. In low humidity regions, it was not uncommon
for two types of ESD problems. One was the effect on moving media
(paper or mylar based cards) within the product, which could build up
excessive ES charges, and stick to one another, viz. a "jam". Do you
recall when many early copiers (probably Xerox) had visible conductive
brushes at the output paper tray to bleed off some of the stored charge
so that the sheets did not stick together. Charges on the media could
also arc to grounded parts in the machine, causing functional problems.
The other major field problem was that of office personnel walking
across wool carpets to operate our equipment, sometimes drawing a
tidy arc when they first touched the equipment. This was distasteful to
the users and could often "lock up" the machine electronics. At times
all we could do was recommend a different carpet fabric.
So, how much ESD immunity should be designed into a product and
tested? This depends a lot on where and how the equipment will be used,
and the importance of its function. A glitch (wrong character) on a typed page
was not the end of the world for a typical typewriter user. However, a serious
ESD event on an air traffic controller's monitor in Tuscon (or other dry
climate)
could lead to mid-air collisions.
Although not an ESD problem, I recall an interesting EMS field incident that
Donald Bush (now retired) investigated. We had recently gone to electronic
keyboards to replace the older mechanical keyboards on our electronic
typewriters. A machine located in the Southwest (as I recall) was behaving
badly. Don went to the site, and found that there was a nearby airport with
a large radar dish within sight of the typewriter. The radar signal was being
picked up by the traces (acting as antennae) in the keyboard PCB, and
typing whatever the random noise caused the unit to type. The user claimed
it would even occassionally type something when it was turned off?
Adding a grounded plane under (or over?) the keyboard in the form of a
coated mylar sheet provided enough shielding to resolve the problem.
Ah, isn't it great to remember events from 30+ years ago and not be able to
remember what you had for lunch?
George Alspaugh
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