Are wrist straps necessary if all other ESD precautions are taken (i.e., two ground [foot] straps, jacket, dissipative flooring, grounded mats, etc.)? If so, why?
Wrist straps are not necessary if an operator is wearing two foot grounders on a conductive grounded floor and doesn’t lift both heels/toes at the same time, like some people do when sitting down. If an operator is also wearing a esd jacket, but is not electrically connecting the jacket to either their body’s skin or ground, then the jacket is providing only partial protection. Charges on the jacket may have no where to go or discharge to if the jacket is not grounded. A popular way to ground the jacket is with a coil cord either attached to a snap on the waist area of the jacket or via a wrist strap snapped to the inside cuff of an ESD jacket.
Source: Transforming Technologies