Carts
and Shelves don't really have to be grounded...do they?
It depends:
They do NOT
need to be grounded if:
-
The
product placed on the shelves is NOT susceptible to damage
from ESD
-
The
product is ESD Sensitive but will ALWAYS be stored in a
sealed ESD Bag, ESD Tote, or some other acceptable method to
protect it from from risks from ESD damage, and as long as
the ESD protective container will NOT be opened unless it is
moved to an ESD-safe area before removing it from the
protective packaging
Explanation:
ESD
Floor, does it really matter if a few areas fail?
The ESD Floor (or
conductive floor) is typically the cornerstone of an ESD program.
A good conductive floor provides a dependable ESD ground, allowing
people, carts, and product to move around an area, increasing
mobility and productivity with low risk of damage from Electro
Static Discharge.
If your customers
had a choice of having their product built on a part of the ESD
Floor that passed or failed, which would they choose? How
would you choose if your job or perhaps your life depended on the
quality of the product?
How should I ground my ESD carts? Should I use a grounding chain,
grounding cable, or conductive wheel?
The nice thing
about grounding chains is that they are cheap, but the down side is
that they have a higher failure rate. Grounding chains and
grounding cables are typically lightweight and tend to lose
connection with the floor when they run over the smallest "dust
bunny".
Another problem
that I see with both the chains and cables is that they work well
when they are clean and are used on type of floor such as ESD wax or
other homogonous material, but when they are used on conductive
floors with discrete conductive elements such as conductive vinyl
floors that use vertical carbon
strands. The floor is conductive wherever these vertical
conductive elements come to the surface, but the rest of the surface
is typically insulative. If a link of a chain (or ball of a
cable) happens to stop on a conductive element, the chain or ball
may make contact if the link is clean and heavy enough to make
contact.
In many cases a cart seems to be grounded when it is moving but fails about 50%
or more of the times when it stops. If you are relying on the
cart's ground to protect your product, then you typically have less
than a 50% possibility of the ground working.
Conductive wheels
are consistently conductive while moving or stopped (assuming the
floor is conductive and the wheel is relatively clean)
and provide a much more consistent ground than cables or chains.
More to come
......
