From: Douglas McKean ([email protected])
Date: Thu Oct 28 1999 - 11:24:20 PDT
At 05:15 PM 10/27/99 -0700, D. C. Sessions wrote:
>
>In many ESD situations multipoint grounding is a Good Thing
>because it minimizes the net local change in potential for
>strikes far from the groundpoint. OTOH it can cause problems
>in extreme cases if the chassis is relatively high impedance.
>
>For EMI the same tradeoff applies. A multipoint connection
>to the chassis will be good if the chassis itself is low
>impedance and the contacts are relatively high impedance.
>A single-point ground *can* be a problem if the enclosure
>impedance is high enough that internal fields can induce
>potential differences across it; in that case shunting the
>induced currents through internal conductors may be better
>than leaving the skin to flap around.
In absolute agreement with you.
I'd start with multi-point grounding with say metal
alloy cases with good conductive coatings. OTOH, I'd
start with single point grounding with say conductive
coating on plastic or anodized coatings on metal being
the primary shielding. I have on occasion used single
point grounding in metal enclosures with good conductivity
for things such as plugin modules that only have only
to insert into a backplane connector and have only
leds to the outside world. No other connections.
With the same type of module assembly having several
external interfaces to the outside world, I had to
use multi-point.
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