Peter,
I looked through some of my books last night and this morning, and found
suggested spacings of:
* Greater than 20mm, 8.4mm, 5mm, or 10mm between circuits and:
- Points that a person could touch.
- Ungrounded metal parts that a person could touch.
* Greater than 2.2mm, 0.5mm, or 1mm between circuits (including traces on
printed circuit boards) and chassis
ground.
The first set of spacings is to prevent direct electrostatic discharge (ESD) to
the circuit. The second set of spacings is to prevent secondary arcing due to L
* dI/dt voltage rise on the bonding wires/straps when carrying the current from
an ESD hit.
Page 28 or 218 (I can't read my own notes) of Boxleitner, Warren, Electrostatic
Discharge and Electronic Equipment-- A Practical Guide for Designing to Prevent
ESD Problems (IEEE Press, New York, 1989) suggested the 20mm and 2.2mm spacings.
Pages 337 and 344 of Ott, Henry, Noise Reduction Techniques in Electronic
Systems, 2nd ed. (John Wiley & Sons, New York, 1988) suggested the 8.4mm and
0.5mm spacings.
Page 369 of Hartal, Oren, Electromagnetic Compatibility By Design 4th ed. (R&B
Enterprises, West Conshohocken, PA, 1996) suggested the 5mm spacing.
Page 677 of Paul, Clayton R., Introduction to Electromagnetic Compatibility
(John Wiley & Sons, New York, 1992) suggested the 10mm and 1mm spacings, based
on air having a breakdown voltage of about 3kV/mm.
John Barnes Advisor Engineer
Lexmark International
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