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Is it required for a wrist strap to have a resistance of 1 megohm? Where does this value come from?
The wrist strap is to have a 1 megohm current limiting resistor at the cuff end of the wrist strap cord. However, another value can be used, but a Tailoring statement is to be included in the ESD control plan document explaining the rationale.
The ESD Association Wrist Strap standard is ANSI/ESD S1.1. The Wrist Strap System required limit per ANSI/ESD S20.20 Table 3 is 0.8 to 1.2 x 10^6 ohms with product qualification testing per ANSI/ESD S1.1.
Per ANSI/ESD S1.1 Table 2. Acceptance Testing, the Wrist Strap Resistance limit is to be 1 megohm +/- 20% or user defined value.
Construction Guidelines “1. Current-Limiting Resistance, A resistance of sufficient resistance to limit current to less than 0.0005 amps (0.5 mA), at the highest voltage that may be encountered, should be incorporated into the wrist strap. Nominally, 800,000 ohms (800 kilohms) are sufficient for voltages of up to 240 volts alternating current (AC). The value of 1 megohm is specified because it is a standard value discrete resistor.
Special situations may dictate the use of values above or below the 1 megohm value. Wrist straps with nominal resistances other than 1 megohm should be marked in accordance with paragraph 5.9.
Discrete current-limiting resistors should be located near the connection between the ground cord and the cuff.” [ANSI/ESD S1.1 ANNEX B]
Note: “Wrist straps incorporating a resistance of non-standard value (other than 1.0 megohm +/-20%) shall be identified by having a prominent feature, red in color, on the wrist strap. Additionally, the value of the non-standard resistance shall be marked on the portion of the wrist strap that contains the resistance. Only wrist straps incorporating a non-standard value shall use the color red on any portion of the wrist strap.” [ANSI/ESD S1.1 section 5.9 identification of NonStandard Resistance Value]
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