Hi Jeff:
> I need to clarify my earlier question on conductive paint. We bond our
> enclosure panels together with screws. We use external-tooth starwashers
> between the screws and sheet metal to achieve a reliable, protective-earth
> bond. On our painted panels, we mask the paint so that the starwasher makes
> good contact with the metal. We are now considering using conductive paint
> without masking, but we would still use starwashers. I don't see how this
> could be a problem, but it's not something I've seen done before and so I'd
> like your input.
The main point to keep in mind is that conductive paint is resistive,
and not near zero ohms.
I presume that the need to ground the panels is for safety, not necessarily
for EMC. If for EMC, then disregard these comments.
There are two scenarios to consider:
1) Star washer tooth cuts through the conductive paint and contacts
the panel; electrical connection is direct from the panel to the
star washer.
2) Star washer tooth does not cut through the conductive paint;
electrical connection between the panel and the star washer is
through the conductive paint to the points and the flat surface
of the star washer.
1) This scenario is nearly the same as the masked paint scenario. The
current path is two fold: a) from the panel directly to the teeth
of the star washer, and b) from the panel to the conductive paint to
the flat part of the star washer. Since this latter path is likely
to be higher resistance, most of the current will pass through the
points of the star washer.
2) This scenario may suffer due to the electrical resistance of the paint
and the contact area of the star washer with the paint. Presume that
the teeth of the star penetrate the conductive paint, but not all the
way through to the metal panel. The teeth of the star washer will
establish an equipotential environment within the conductive paint.
Therefore, all of the current from the paint to the star washer will
be through the teeth of the washer, not through the flat portion of
the star washer. When subjected to the 25- or 30-ampere fault current.
the paint may vaporize at each tooth of the star washer, and the ground
connection will be lost.
This can be easily tested with a simple experiment using a high-current
dc power supply.
My personal opinion is that reliance on conductive paint for safety is not
a good idea. The reason behind this opinion is the fragility of the paint
to both current density and physical scratches. (This reasoning follows
the same reasoning why fragile enamel and varnish wire insulation is not
relied upon for safety insulation.)
Best regards,
Rich
-------------------------------------------------------------
Richard Nute Product Safety Engineer
Hewlett-Packard Company Product Regulations Group
AiO Division Tel : +1 619 655 3329
Effective 6/12/99: +1 858 655 3329
16399 West Bernardo Drive FAX : +1 619 655 4979
Effective 6/12/99: +1 858 655 4979
San Diego, California 92127 e-mail: [email protected]
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