• Start with a metal shelving unit with metal legs
• Make sure the shelves and legs are conductive (they should be chrome or stainless)
•    Plastic or powder coated metals are insulators and are difficult to make ESD safe
• Pick one or more posts to be a "grounding post" for your ESD shelving
• Connect the metal shelf to the metal grounding post(s)
• Connect the metal post(s) to ground using a conductive floor (ESD floor)
• If you don't have an ESD floor, connect the shelf to a common ground (typically tied to an or earth ground or electrical ground)

For a Mobile ESD Cart

• Add one or more Conductive wheels to the cart to connect the cart to ground through the conductive floor (see diagram below)
• Optional:  to assure that a cart is grounded while at rest, connect a ground wire between the cart and an ESD ground when the cart is stationary

Minimum requirements

• One "grounding post"
   

• One connection per shelf to the "ground post" using an ESD ClipTM , ESD Key TM , or metal split sleeve
   

• Provides one connection to the ESD floor by the single "grounding post" contacting the conductive floor

OR, for a mobile cart

• Use an ESD Wheel (conductive wheel) for grounding a mobile ESD safe cart

• One post connection per shelf using ESD ClipTM , ESD Key TM , or metal split sleeve
   
• Add one extra connection to a 2nd "grounding post"
   
• TWO connections to the ESD floor are provided by the Two "grounding posts" contacting the conductive floor

OR, for a mobile cart

• Use Two ESD Wheels (conductive wheels) for grounding a mobile ESD safe cart

• TWO post connections per shelf using ESD ClipTM , ESD Key TM , or metal split sleeve provides redundant connections to each shelf
   
• FOUR connections to the ESD floor are provided by the FOUR "grounding posts" contacting the conductive floor

OR, for a mobile cart

• Use Four ESD Wheels (conductive wheels) for grounding a mobile ESD safe cart

Summary

If you are considering conductive wheels, you have probably concluded that your "drag chains" are not working well for you.  Conductive wheels are typically much more reliable and are less prone to corrosion and contamination than drag chains or cables.

While even 4 conductive wheels or posts will not provide a good connection to a dirty or non-functioning ESD floor, more connections means a higher probability of making a successful connection.    With only one leg / wheel, you only have one possibility for connection.  All it takes to disrupt the ESD ground connection is to move the cart onto a small scrap of paper or plastic or perhaps move the leg onto a place on the floor that is not conductive.  In this case, more is better.