Hi KC--
In the USA, the National Electric Code (NEC) establishes the equipment "grounding" requirements for both
equipment and personnel safety. There are specific requirements for these electrical "grounding"
connections.
In general, as soon as any electrical equipment is introduced onto a work bench or into a work station, the
"grounding" of electrical equipment is required (with very few exceptions). The "third" wire, the "U",
green-wire, green-yellow, or bare copper wire is the "grounding" conductor (in the parlance of the NEC.
The "grounding" conductor is different than the "grounded" conductor.
The key to personnel safety is that any and all conductive surfaces that "could become electrified" shall
be connected to the "grounding" conductor. As such, any ESD grounds must be connected to the grounding
conductor.
The best way to do this is not to run separate "grounding" conductors from the source of electrical power
(and the grounding conductor) for both electrical power "grounding" and ESD grounding. This is dangerous
because if some electrical equipment "faults" there will be a difference of electrical potential between
the two "grounding" systems, and this could result in an electrical shock to personnel (yes it's current
limited by resistances, but a shock is still a shock, whether it's felt or not).
The best way is when electrical power is introduced at a work station/bench, there should be a metal
(conductive) junction box or terminals. From this junction box, electrical power for plug-strips,
equipment, and illumination is distributed. Also from this junction box, there should be a separate wire
(low impedance) from the electrical power "grounding terminal" to the ESD "grounding" terminal, which
serves as the distribution point for ESD grounding connections.
In other words KC, you should not separate ESD and electrical power "grounding" in an EOS/ESD protected
area, it should be tied (bussed) together in/very near the protected area.
Additional comments:
1) The EOS/ESD Association has standards that reflect the above, the above guidelines requirements were
very successfully used to meet all NEC and ESD requirement for more than 20 years in my previous life of
employment.
2) Use "GFCI's" (ground fault circuit interrupters) for all electrical power (to the fullest extent
possible).
3) Use low impedance (large) wiring to minimize the effect of electrical transients in the grounding
system.
4) Be extremely careful if electrical testing of > ~50 Vdc or ac peak must be performed in an ESD
environment. If you must, let me know and I can supply you with some additional requirements and
suggestions to significantly improve personnel safety.
5) Be extremely careful if you must have "multiple" sources of electrical power in the same work area.
Hope this helps,
Ralph
KC Chan wrote:
> Hi all
>
> Regarding the ESD grounding, is there a requirement that separate
> groundings are needed for equipment, power supplier and human(eg,
> through wrist strap)? Any industrial standard has such requirement? I
> was questioned by a customer about that and was requested to separate
> them.
>
> Regards
> KC
>
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--
Ralph Hersey
Ralph Hersey & Associates
3885 Mills Way
Livermore, CA 94550-3319
PHN/FAX: 925.454.9805
e-mail: [log in to unmask]
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