I think you are expecting too much from the cart.  The ANSI/ESD S20.20
recommend that the resistance to ground from the surface of the moble
equipment be less than 1 billion ohms.
Are you working with some extrordinary components? To transform the cart
into a grounded workstation you would need to connect it to ground with
a wire/resistor of appropriate value.
Remember dirt and dust prevent good contact with the floor, which is
probably greater than 1
Meg Ohm.

> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Joseph
> Sent: Tuesday, December 03, 2002 2:29 PM
> To: [log in to unmask]
> Subject: [TN] ESD-Protective Carts
>
>
> Our stockroom is looking for some carts that are
> ESD-protective and ground through the conductive floor
> underneath.  In other words, we want the cart to be grounded
> while moving, so we can transport sensitive parts from the
> stock location to the point of use.
>
> Basic practice says that ESD-sensitive material must be kept
> in protective packaging or at a static-protective
> workstation.  These carts must be considered portable
> workstations, so we'd want the surface of the cart to meet
> the same grounding criteria as a static mat sitting on a
> table.  Of course, we'd lay out specific routes to minimize
> exposure to fields.
>
> We've bought some carts that claimed to ground through the
> wheels, but they failed test.  We tried some metal carts, and
> hung chains underneath to drag on the floor, still no good
> (not enough contact
> pressure?)  And even if either of these concepts works now,
> what happens after the cart rolls around for a while and the
> wheel, or the chain link, or the grounding strap inevitably
> picks up a layer of insulating dirt and dust?
>
> If anyone is using grounded carts successfully, could you
> please let me know make and model, how you test and how
> often, and how you handle the buildup of insulating dust?
>
> Joe Kane
> BAE SYSTEMS Controls
> Johnson City, NY
>
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