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For foot grounders, the ANSI/ESD-S20.20-1999 recommends that the system resistance of the floor and the wearer be less than 35 megohms (35x106 ohms) and again does not specify a lower range.
In both cases the lower range is really dictated by the application and safety concerns. Regarding safety, UL will only list products that include a lower resistance boundary of 1 megohm using currently available technology. Current technology readily has available carbon film resistors with a tolerance of +/- 10%. This would mean the minimum resistance you could run into would be 900 kilohms (- 10% of 1 megohm). Add to this the tolerance of the measuring unit/system and you could have a range as wide as 15 to 20 %.