Model GAM-2A
AC Outlet Analyzerand Wrist Strap Tester

An easy-to-use, low cost, plug-in test to confirm the integrity of the
static discharge path from field service personnel to earth ground via
wrist strap grounding systems and the equipment grounding conductor.

Features:

• Verifies proper AC outlet wiring and ground path integrity
• Validly tests wrist straps and coil cords
      All types
      When being worn (system tests)
      When not being worn
• 27 VDC test voltage minimizes skin resistance effects
• Tests to practical resistance limits
• Tests ESD smock continuity
• Easy-to-use, fast and accurate

When the Model GAM-2A is plugged into an AC outlet, the green LED lights if the outlet's wiring is correct and the path to earth ground via the equipment grounding conductor (the green wire) is intact.  The red LED lights if either condition isn't met.

By touching the Test Button on the Model GAM-2A, a person can quickly and easily insure that their wrist strap grounding system contains adequate current limiting resistance and that there is continuity from one's skin to earth ground.  The GAM-2A detects poorly fitting, worn out, corroded or dirty wrist straps.  It can also be used to check the integrity of the conductive path from sleeve-to-sleeve of ESD protective garments.

Resistance Limits

Upper limits, most often, have been arbitrarily established, or were based on calculations that assume skin resistance to be constant.  Skin resistance is actually voltage dependent, dropping as voltage increases.  It can be tens of megohms when measured at under 10 volts and tens of kilohms, or less, at over 20 volts.  (Ref. 1)  Except when wrist straps are improperly worn, worn out, or broken; the upper limit is only exceeded when wrist straps are tested on persons whose skin resistance is excessively high at low voltage.  The resistance to ground of ESD workstation personnel wearing wrist straps is typically a bit over 1 megohm when measured at more than 20 volts.

The upper limit might better be based on the ESD susceptibility of the devices being worked with.  The voltage level to which a person can rise due to tribocharging current and changing capacitance during normal workstation activity is controlled by the person's resistance to ground.  Persons who are moving about and who are grounded through 100 megohms rarely rise to what are ordinarily deemed ESD hazardous voltage levels.

When persons contact a charged object, their voltage rises in nanoseconds.  It must be lowered before they touch an ESD-sensitive component.  The discharge time of an average person (100pF) from 5 KV to 100 volts who is grounded through 50 megohms is 15 milliseconds.  Persons at workstations ordinarily move in fractions of a second or more.

Upper limits of less than 50 megohms are unnecessarily stringent and can cause perfectly good wrist straps to fail test.  Lower limits are usually valid above 20 volts, or when wrist straps are tested as components, i.e., not being worn.

Ref. 1. Raymond Kallman, "Wrist Strap Monitoring Systems." 1994 ESD Symposium pp.34-41