Hi Jeff:
> My company is proposing to use conductive paint on our enclosures, and I
> would like your input as to the acceptability of this vis-a-vis protective
> earth bonding of enclosure panels. I am interested in the perspective of
> both European and North American requirements.
The implication of your question is whether conductive paint can
be used to carry fault current in the event of an insulation fault
to the conductive paint.
In my experience, effective bonding of conductive paint requires
large-area contacts to the paint. This is because the paint has
a very small cross-sectional area, so the current from the paint
to the grounding circuit must be distributed with large-area
contacts, i.e., emc finger stock.
Some certifiers may question whether such contact mechanisms
constitute bonding as defined in the various safety standards.
But, this only addresses the grounding of the conductive paint.
A fault to the paint would start with a point contact and may
grow to a large-area contact depending on the nature of the
fault. Regardless of whether or not the fault connection is
large-area, at the instant of contact, it will be a small-area
contact. The initial current density at the point of contact
to the conductive paint will be extremely high. This high
current will cause the conductive paint to vaporize, and the
connection to ground will immediately open.
(You can perform this experiment by touching a live, 120-volt
or 230-volt 18 AWG or 0.75 mm wire to the grounded painted
surface.)
For this reason, conductive paint will not be effective as a
part of a protective earth circuit.
However, the paint can be grounded -- it just cannot be
designated as a part of the protective earth circuit, and it
need not be grounded through bonding techniques.
The implication is that your construction employs a plastic
enclosure with grounded conductive paint on the inner surface.
You can designate the plastic material as supplementary
insulation, and thereby achieve compliance with safety
requirements throughout the world. If you choose to designate
the plastic as supplementary insulation, you will need to test
the insulation for electric strength. This will be easy. You
wrap the outer side of the plastic in conductive foil and apply
the test voltage between the inner conductive paint and the
foil. You need to keep the paint away from accessible parts of
the enclosure to prevent breakdown around edges and through
holes.
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: ri...@sdd.hp.com
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