Using a current limiting resistor in the ground cord is the user’s choice; however, the resistor is not for ESD control purposes. The ESD Association standard for grounding is ANSI/ESD S6.1. ANSI/ESD S6.1 recommends a hard ground (no resistor), but allows the use of a current limiting resistor in the mat’s ground cord.
“The grounding conductors (wires) from wrist straps, worksurfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments and other ESD technical elements may or may not contain added resistance. Where added resistance is not present, a direct connection from the ESD technical element to the common point ground or common connection point is acceptable and recommended.
Note: Manufacturers may add resistance to the grounding conductors for purposes other than ESD (e.g. current limiting). Added resistance is acceptable for the purposes of controlling ESD provided electrostatic accumulation does not exceed specific EPA requirements. The typical added resistance in grounding conductors is 1 megohm, although other values may be specified.” [ANSI/ESD S6.1 section 5.3.3 ESD Technical Element Conductors]
When an in-line resistor is used in Desco’s common point grounds it is isolated from the banana jacks