This response is a little late, but here's my two cents.  The exposed metal
on line-powered equipment _must_ be grounded (or earthed, as our British
friends prefer to say) because this is an integral part of the safety system
that protects the operator.  All the safety standards require this because
in the event of a short circuit from an internal part at power line
potential to the exposed metal surface, the ground wire has to conduct
enough current out of the exposed metal to blow the circuit breaker quickly.
If there is any significant resistance in the ground circuit, the circuit
breaker will not blow and the operator will be exposed to a hazardous
voltage.  In short, do _not_ isolate exposed metallic surfaces on powered
equipment from ground as you will create a safety hazard.

An alternative is to use double-insulated systems with no third wire (no
earth ground) and allow the metal surfaces to float.  While this is popular
with hand-held power tools and kitchen appliances, I would not recommend
this approach for high-power industrial equipment.  This is permitted in the
safety standards based on the premise that two separate insulation systems
failing simultaneously is so unlikely as to be of negligible probability.
At the energy levels available in large industrial equipment, this
assumption becomes questionable.

If the static mat is very conductive, as in Jim's installation, you may need
to isolate the metal enclosure from the static mat to prevent the equipment
ground path from defeating the high ground resistance required of the static
mats.  Just don't isolate the equipment itself from ground.

I know it seems paradoxical that the equipment exterior is required to have
a low impedance path to ground while the static mats are required to have a
very high impedance path to ground.  This is a lesser of two evils safety
choice.  The low ground impedance of an exposed metal chassis is to protect
the user against internal shorts in the equipment, which we regard as
foreseeable single faults.  The high ground impedance of static mats is to
protect the user in case they come in contact with an external source of
hazardous voltage.  It is true that if the user comes in contact with an
external low-impedance source of hazardous voltage and also touches a metal
cabinet with a low-impedance ground, they will receive a potentially lethal
shock.  However, this is regarded as less likely than internal shorts to the
metal enclosures of equipment, so that is the rationale behind the practice.
If there is any likelihood that a user _can_ come in contact with an
external source of hazardous voltage, ground fault circuit interrupters
should certainly be used.  Note that these do not actually guarantee that
you will not be harmed by a fault condition, as the GFCI trip current can
still be lethal under some conditions, but they make it very unlikely.

As for ground voltage spikes getting into a system through the low-impedance
metal enclosure ground, I do not agree that this has any significant chance
of doing damage.  Since the static mats and the exposed metal surfaces at a
workstation should be connected to the building ground system at the same
ground point (you always do this, don't you? I mean always, always, always!)
everything is at the same potential, regardless of its absolute potential
compared to earth ground.  While it is true that the one megOhm resistor and
the distributed capacitance of the static mat plus user have a short time
constant during which a voltage difference can theoretically exist, this
would require an _enormous_ local ground disturbance to produce enough
voltage to cause problems.  IMHO, this will not happen in actual practice,
even in buildings with poor grounding systems.  If it ever does occur, ESD
will be the least of your problems.

Regards,

Seth Goodman
Goodman Associates, LLC
tel 608.833.9933
fax 608.833.9966




> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Jim Jenkins
> Sent: Thursday, May 30, 2002 1:26 PM
> To: [log in to unmask]
> Subject: [TN] ESD and equipment isolation
>
>
> Hi everybody,
>
> One of our internal specifications states that processing
> equipment needs to
> be "electrically isolated" from the static dissipative mat.  We are
> interpreting this to mean that if the equipment has a metal case it is not
> allowed to be in contact with the mat.  There has been some
> controversy over
> this, because we have whole areas and labs that have test equipment and
> other items that do not have the little feet on the bottom and
> make contact
> with the mat.
>
> We have people arguing on both sides here.  Some say that the contact
> provides a direct path to ground taking out the resistance needed to slow
> down the discharge.  I say that the dissipative (rather than conductive)
> nature of the mat provides the necessary resistance.  That is why
> resistors
> are no longer required in ESD specifications.
>
> Others say that the equipment can inject electrical energy through the mat
> into the ESDS devices.
>
> What do you all think?
>
> Thanks,
>
> Jim Jenkins
>
> [log in to unmask]
>
> ------------------------------------------------------------------
> ---------------
> Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF Technet
> To temporarily halt delivery of Technet send e-mail to
> [log in to unmask]: SET Technet NOMAIL
> To receive ONE mailing per day of all the posts: send e-mail to
> [log in to unmask]: SET Technet Digest
> Search the archives of previous posts at: http://listserv.ipc.org/archives
> Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
> information, or contact Keach Sasamori at [log in to unmask] or
> 847-509-9700 ext.5315
> ------------------------------------------------------------------
> ---------------
>

---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------