The wrist strap is connected to ground through a coiled retractable cable and 1 megaohm resistor, which allows high-voltage charges to leak through but prevents a shock hazard when working with low-voltage parts. Where higher voltages are present, extra resistance (0.75 megaohm per 250V) is added in the path to ground to protect the wearer from excessive currents; this typically takes the form of a 4 megohm resistor in the coiled cable (or, more usually, a 2 megohm resistor at each end). Very cheap wrist straps do not have conductive fabric and instead use the fabric to hold the metal plate against the skin, which can result in reduced ESD protection over time as the metal corrodes.
Wrist straps in industry usually connect to Earth Bonding Points (part of the grounding system) via either a 4 mm plug or 10 mm press stud, whereas personally owned straps are likely to be connected to ground via a crocodile clip.
In addition to wrist straps, ankle and heel straps are used in industry to bleed away accumulated charge from a body. These devices are usually not tethered to earth ground, but instead incorporate high resistance in their construction, and work by dissipating electrical charge to special floor tiles. Such straps are used when workers need to be mobile in a work area and a grounding cable would get in the way, such as in an operating theatre.
Wireless or Dissipative wrist straps are available, but their level of reliability is unknown.
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Last updated on Wednesday September 03, 2008 at 23:25:42 PDT (GMT -0700)
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