Hi Chris:
> To me, it's sort of funny in that it just says that the Creepage and
> Clearance distances do not apply on inner layers of void free PCBs.
> That's nice; but I can't find where a distance is specified. I mean, I
> would think that there should be some minimum distance between an AC line
> and a 5V SELV line on an inner layer of the board!!!!
To answer this comment, we need to look at what
a creepage is and its role in the scheme of the
product.
Almost all product constructions employ solid
and air insulations, both in parallel and in
series, between conductors. We call solid
insulation "solid insulation." We call air
insulation "clearance."
We call the interface between solid insulation
and air insulation "creepage."
Note that solid insulation and air insulation
are truly electrical insulations.
Creepage is NOT an insulation.
Creepage is not a material. It is simply a
surface at which solid and air insulations
meet.
The surface of solid insulation is subject
to deposition of airborne "pollution."
Typical products provide little or no control
of airborne materials to prevent deposition
of the "polluting" material onto the surface
of a solid insulating material.
Polluting material is a solid, uncontrolled
(i.e., not a known insulating) material in
parallel with the solid (and air) insulations.
The polluting material bridges the solid
insulation, and therefore could jeopardize
the safety function provided by the solid
insulation.
When sufficient polluting material accumulates
on the surface of the solid insulation, the
voltage across the insulator and the pollution
causes micro-arcs in the pollution. These
micro-arcs are high temperature, and cause
thermal decomposition of the surface of the
solid insulation. When organic materials
decompose, they free up the carbon atoms,
leaving a tiny carbon resistor on the surface
of the solid insulation.
Each tiny carbon resistor is in parallel with
the adjacent solid insulation beneath the
surface of the solid insulation. So the tiny
carbon resistor is shorting out a small part
of the surface of the solid insulation.
This phenomenon is known as "treeing" due to
the tracking pattern of the carbon path on the
solid insulation surface.
To account for the effect of pollutants on
the surface of organic solid insulations, we
require the creepage distance to be larger
than the air distance (clearance). Because
some materials are more resistant to tracking
across the surface, the creepage distance is
a function of the "relative tracking index"
characteristic of the insulation.
Where a solid insulation is not subject to
pollution, there is no requirement for a
creepage distance. Many standards specify
that a hermetically sealed assembly is not
subject to creepage distance requirements.
Likewise, the inner layer of a multi-layer
printed wiring board is not subject to
pollution and therefore is not subject to
creepage distance requirements.
Best regards,
Rich
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Ron Pickard: emc-p...@hypercom.com
Dave Heald: davehe...@attbi.com
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
Jim Bacher: j.bac...@ieee.org
All emc-pstc postings are archived and searchable on the web at:
http://ieeepstc.mindcruiser.com/
Click on "browse" and then "emc-pstc mailing list"