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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Static worse for computer on the floor???
Message-ID: <17660047@hpfcdj.HP.COM>
Date: 28 Feb 90 19:30:22 GMT
References: <3618@sage.cc.purdue.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 37

>I'd like to propose an alternate question:  Is it possible/probable to damage
>a shielded, grounded (enclosed) computer with common static electricity
>buildup?  Anything over 16MHz is usually heavily shielded against creating
>RFI.  Wouldn't this shielding prevent external static damage?

Not necessarily; even though a particular system may pass the FCC or VDE
requirements for RFI, there is still the potential for damage via ESD.
(For that matter, there's still SOME RFI leaking out - it's REALLY
difficult to make a system that produces NO or even very little radiated
or conducted emissions; just ask the folks who do Tempest* work for a living!)

Damage from ESD can occur in at least two ways.  The first and most obvious
is the destruction of a component via a discharge which sends its current
through that device; say, zapping an input on a TTL device.  The other, and
sometimes overlooked, factor is that even if the discharge goes directly to
"ground" damage can still occur.  Electrostatic discharges can have pretty
remarkable amounts of energy (even if only for a very short time), and the
path through your system's "ground" to the ESD's Final Resting Place (call it
"earth ground", or the Local Source of the Most Holy Truly Zero Potential)
may contain appreciable impedance.  If this is the case, the high currents of
an ESD zap can induce momentary high-voltage spikes *on the ground of the 
system*, and thereby damage components.

RFI protection and ESD protection are related, but doing a good job for one
doesn't necessarily mean you're safe from the other.


* - "Tempest": referring to the requirements of certain military/government
     contracts, etc., which adhere to a set of very stringent emission
     specs known as "Tempest."  The concern here is that the other side (the
     Bad Guys) can very effectively listen in on what your computer is
     doing by monitoring the RFI it generates.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.