Why are drag chains needed when other ESD measures are being used?
A cart with no ground chain or conductive castors on an ESD floor is a "floating conductor"; its insulative wheels are insulating it from ground giving it the ability to hold a charge. As anti static measures have presumably been taken in the setting you mention, it can be assumed that you are in a facility where static is a threat, and therefore all measures to eliminate static need be taken. This is why ESD experts recommend taking every precaution, including using ground chains.
What is a floating conductor?
A floating conductor is any conductive object that is electrically separated from ground. In the example above, the floor is considered ground. Another example of a floating conductor is people wearing regular footwear and no heel straps walking on an ESD floor.
How do I use a drag chain?
The proper amount of drag area or surface area that is for a drag chain on a cart is 12”-18”: that is chain to ground contact. Flooring type can play a role in the effectiveness of drag chains. For instance, ESD carpet is going to be the “best” flooring option for drag chains because of the tufts in the carpet weave. The chain will actually have more and better contact with carpet over any other floor type.
The connection of the chain plays a part in maintaining safety. On a cart, it is best to connect one side of the chain on one end of the cart to the opposite side of the chain on the other end of the cart to create a diagonal connection. This will ensure the chain does not pose any tripping or whipping hazards because it will remain under the cart.
Drag chains are essential for all carts, tables, etc. in settings where other anti static measures have been taken. They provide an electrical bond between the material handling device (cart, wheeled platform, etc.) and the grounded floor. Without the chain, these items are isolated floating conductors, and are a potential threat.
As anti static measures have presumably been taken in the setting you mention, it can be assumed that you are in a facility where static is a threat, and therefore all measures to eliminate static need be taken. This is why ESD experts recommend taking every precaution, including using ground chains.