About the ESD Trolleys and antistatic carts the standard CEI - IEC TECHNICAL SPECIFICATION 61340-5-2 say:.
A practice in some areas has been the use of conductive chains rather than conductive wheels (ESD).
When chains are used, sufficient pressure or weight should be ensured to avoid poor contact being made with the floor (ESD conductive). Some chains are often hooked to the surface rather than being allowed to trail on the floor.
A minimum of two wheels on trolleys or carts should provide a path to ground. A single wheel may have part of its surface covered by foreign matter, or may not be in contact with a conducting part of the floor, or it may rest on an uneven floor or, in the case of a poor quality cart, it may fail to be in contact with any part of the floor.
Resistance to ground measurements should be made to assure proper performance.