Hi Hans:
Regarding your several questions:
1) Printed wiring spark-gap.
This is a problem because the manufacturing variances of printed
wiring makes the spark gap extremely variable in breakdown voltage.
This is a further problem because when it operates, it creates a
carbon path across the surface of the PC board. This reduces the
breakdown voltage with every sparkover incident until finally the
creepage finally becomes a resistor.
2) Spacings less than those required in safety standards.
This simply cannot be done on a printed wiring board -- even if
the spacing is intended to be a spark gap. In fact, it cannot be
done even if it is a clearance gap rather than a creepage gap.
And, it cannot be done even though a clearance is a renewable
insulation and the construction is used to protect a safety
insulation from catastrophic failure. This is the way the
certification minds operate.
3) Discrete spark gaps.
Strangely, these are usually accepted by the various certification
houses even though an air gap is not accepted!. Usually, the
certification houses will require them to be certified.
4) MOVs.
No comment.
Best regards,
Rich
