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Question Hi,

I read in the "ESD Mathematics" training guide the example where an operator is wearing a wriststrap with a 1 Megohm resistor and touches 120v rms AC chassis. It says this is safe and he can touch up to 250v. 

Is this information correct ?   

Safety questions should be directed to the facility’s safety officer.


However, this is what it states in ESD Association ESD Handbook ESD TR20.20 and other documents:


Per ESD Handbook ESD TR20.20-2008 Wrist Strap section “For personnel safety, wrist straps should not be used in situations where there is an exposed electrical circuit of 250 volts or higher.

NOTE: Several publications exist (beyond UL) that provide varying limits of allowable current for personnel safety. For example, most power supplies are current limited to 5 mA. For this reason, the user should check with local safety personnel to determine requirements for their particular area. Refer to the personnel safety section of this handbook for further guidance.”


ANSI/ESD S20.20 section 5. “PERSONNEL SAFETY, The Procedures and equipment described in this document may expose personnel to hazardous electrical conditions. Users of this document are responsible for selecting equipment that complies with applicable laws, regulatory codes and both external and internal policy. Users are cautioned that this document cannot replace or supersede any requirements for personnel safety. Ground fault circuit interrupters (GFCI) and other safety protection should be considered wherever personnel might come into contact with electrical sources. Electrical hazard reduction practices should be exercised and proper grounding instructions for equipment must be followed.”


Our Wrist Straps are Underwriter Laboratories listed. UL cautions that these products are not recommended for use on equipment with operating voltage exceeding 250 volts recommending that the electrical current that the operator be exposed to be limited to 0.70 milliamp.  At 250 volts, a 1 megohm [1,000,000 ohms] resistor limits current to 0.25 milliamp.


UL Marking for Wrist Straps:


This product is not recommended for use on equipment with operating voltage exceeding 250 volts.

A one megohm resistor is molded into Wrist Strap Coil Cords. DO NOT REMOVE. If it becomes damaged, replace Coil Cord immediately. These products are not to be used in areas where the individual may come in contact with exposed electrical circuitry exceeding 250 volts.

These products are for ElectroStatic control. They will not reduce or increase your risk of receiving electrical shock when using or working on electrical equipment. Follow the same precautions you would use without wrist straps, including:

• Make certain that equipment having a grounding type plug is properly grounded.

• Make certain that you are not in contact with grounded objects other than through the Wrist Strap.”


Per ESD Handbook ESD TR20.20 section Current Limiting “Most wrist straps have a current limiting resistor molded into the ground cord head on the end that connects to the cuff. The resistor most commonly used is a one megohm, 1/4 watt with a working voltage rating of 250 volts. Resistors limit current as defined by Ohm's Law, which states the current is equal to the voltage divided by the resistance.

In a practical application, the maximum amount of current through a wrist strap ground cord if it was placed across a 250 Volt source is 250 microamps or 0.25 milliamps. This amount of current is well below the 0.7 milliamps that Underwriters Laboratories uses as the peak current in a LIMITED CURRENT CIRCUIT. See document UL 1950.

For personnel safety, wrist straps should not be used in situations where there is an exposed electrical circuit of 250 volts or higher.”


Per Wrist Strap standard ANSI/ESD S1.1 section 7.1 Construction Guidelines, Current-Limiting Resistance “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 240VAC. 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 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.”

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