The “standard” resistance value for a wrist strap is to be 1 megohm (+/-20%). Touch-Testers test for this, so should your single-wire constant monitor. One of the ways that manufacturers of low cost continuous monitors cut corners is to not monitor for a current-limiting resistor in the wrist strap system. While there is no industry standard for continuous or constant monitors (although there is useful information in ESD TR 12-01 Survey of Constant (Continuous) Monitors for Wrist Straps), the ESD Association says the following about current limiting resistors in the operator’s ground cord.
“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.” [ANSI/ESD S1.1 section 5.9 identification of NonStandard Resistance Value]
“A resistance of sufficient resistance to limit current to less than 0.0005 amps (0.5mA), 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 megohms 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 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 B1 Construction Guidelines Current-Limiting Resistance]
Compared to a normal “hard ground” with very low resistance in the path-to-ground, the vast majority of wrist straps used for ESD control use a ground or coil cord with resistance also called a “soft ground.” The typical 1 megohm of in-line resistance is designed to limit any potential current the operator may come in contact with if exposed to 110 VAC and up to 250 VAC maximum. Underwriters Laboratories recommends that the electrical current that the operator be exposed to be limited to 0.25 milliamp at 250 volts, the 1 megohm resistor does this.
If you choose to use continuous monitoring for operator grounding, your continuous monitor should verify this “soft ground” in order to verify that the operators’ wrist straps have a functioning 1 megohm resistor in them as well as a path-to-ground.
To confirm that a continuous monitor verifies a lower limit follow this simple test procedure:
- Use a ground cord with a no resistor in it, and connect the snap to a wristband.
- Make sure the wearer of the wrist strap is isolated from ground other than the wrist strap.
- Then plug the banana end of the cord into the monitor.
If the continuous monitor alarms (Red) with the ground cord without a resistor the continuous monitor is verifying for a lower limit.
If the continuous monitor does not alarm (Green) with the ground cord without a resistor the monitor is not verifying for lower limit. These types of monitors do not verify that there is a resistor in the cord and may allow the operator to be “hard grounded” without a current limiting resistor.
Click HERE to request a sample of a coil cord made with no resistor.
To verify that ground cord has no resistance between the snap and banana, conduct this test:
- Use or a multimeter (set above 1 megohm range and test for continuity)
- If no resistor = <~5 ohms, however if includes 1 megohm resistor = ~ 0.8 to 1.2 megohm