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May 2002

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Subject:
From:
"Kane, Joseph" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 30 May 2002 16:17:53 -0400
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Jim:

DOD-HDBK-263 Appendix E para. 50.2 says in part "Tools and test
equipment on grounded work work benches with metal or other
conductive coverings can shunt the protective resistance in the
work bench ground cable if allowed to contact the work surface."

It's not likely that equipment will inject electrical energy
into the work through the mat.  One-megohm resistance from mat
to ground protects the operator so that if they touch something
live, the current doesn't run through them directly to ground
(like standing in a puddle).  That's why wrist straps and
static mats have resistors in the snaps.  Some mats might be
resistive enough that a resistor is not needed in the ground
connection, but I know that the 3M mats that we use are quite
conductive, and the resistor is absolutely needed.

The handbook suggests the possibility of using ground fault
circuit interrupters, but Appendix G para. 30.1.f also says
"Electrically powered tools, test equipment, and fixtures used
in ESD protected areas should be properly grounded.  Grounding
of electrical test equipment should be via a grounded plug,
not through the surface of the ESD protective work station."

We require 3-pronged plugs or UL listing on anything electrical
at an ESD protective work station.

Joe Kane
BAE SYSTEMS Controls


-----Original Message-----
From: Jim Jenkins [mailto:[log in to unmask]]
Sent: Thursday, May 30, 2002 2:26 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]

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