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July 1997

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Subject:
From:
Steve Mikell <[log in to unmask]>
Date:
Sat, 26 Jul 1997 11:04:29 -0500 (CDT)
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At 12:55 PM 7/26/97 +0800, you wrote:
>        Thanks Denis, Drake, Stuart, Stevem and Doug for your fast response.
>
>        To make things clearer, we refer electrical ground as the AC ground
>and for ESD ground we need to plunge a electrode 6 -7 feet into the ground.
>As a safety precautions for the operators, all the wrist straps has got a 1
>Meg Ohm resistor installed, whether you are grounded to the ESD or
>Electrical ground. We are using a 240 V power supply, so with 1 Meg Ohm
>resistor, even if there is a short, the current that flows through the
>operator will be less than 0.5 mili-amps. This is alright at the safety
>stand point.
>
>        My concern is that will there be a potential difference between the
>ESD and electrical ground? Say for example, my board(machine) is connected
>to the AC ground and the operator is connected (by wrist straps) to the ESD
>ground, if a potential difference of 5 volts exsist, there will be a flow of
>charge whenever the operator touches the board.
>
>        The reason my management is thinking of switching to the ESD ground
>is that sometimes there is a short of the live to the AC ground in our
>machine. That way we can isolate the operators grounding from the machines'.
>
>        As for the lightning scenario, I am not sure but can the ELCB helps
>in this case?
>


In 1983 I was involved in moving a factory into a brand new facility in
southern California.  The building electrician required the use of the AC
ground for ESD grounds instead of any alternative earth grounds, including
water pipes or ground rods.  The building was equipped with Ground Fault
Interrupters that could be defeated if the electrical equipment could short
to an alternative ground.  I was told that GFI was now a nationwide
requirement (in 1983), so any building less than 15 years old should be GFI
equipped (in the US).  Please refer this question to a licensed electrician
familiar with the code requirements in your location.  No one on the technet
can properly answer this question for you.

Steve Mikell
SCI Systems

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