Under What Conditions Will an Anti-Static Wrist Strap Trigger an Alarm?
Under normal operating conditions, both anti-static wrist straps and their accompanying online monitors must be designed with ESD safety performance in mind, adhering to the anti-static standard specified in Section 6.1.3.2 of ANSI ESD S20.20.
1. When the anti-static wrist strap is plugged in, and the resistance between the human body and the ESD ground is less than 0.75 MΩ:
Indicator Light: Illuminates, accompanied by an audible alarm.
Cause: This indicates that the wrist strap's wire is broken (disconnected) or that the strap is not being worn correctly.
Explanation: The hazard associated with a resistance of less than 0.75 MΩ between the human body and the ESD ground is as follows: if the body comes into contact with faulty equipment that is leaking current or with a charged object, there is a risk of electric shock! (Anti-Static Safety Tip: A current of 20 mA passing through the human body can cause a sensation of electric shock, while a current of 50 mA can lead to shock or death.) Therefore, the 0.75 MΩ threshold serves as a safety margin. All anti-static wrist straps are internally equipped with a resistor having a value greater than 0.75 MΩ but less than 2 MΩ.
2. When the anti-static wrist strap is plugged in, and the resistance between the human body and the ESD ground is greater than 9.999 MΩ:
Indicator Light: Illuminates, accompanied by an audible alarm.
Cause: This indicates that the wrist strap's internal resistance is too low, or that the resistance threshold set on the online monitor is too low.
Explanation: The hazard associated with a resistance of greater than 9.999 MΩ between the human body and the ESD ground is as follows: the human body continuously generates static electricity through movement; when the resistance exceeds 9.999 MΩ, the system is unable to effectively dissipate the static electricity accumulated on the body. (Static Electricity Hazards: For example, electronic components have a withstand voltage of only a few volts or tens of volts. When the static electricity from the human body exceeds the component's withstand voltage, the contacted electronic component will be damaged and rendered unusable. This is especially true for IC integrated circuits. For example, on plastic casing production lines, static electricity can cause rough surfaces, numerous pinholes, and poor feel during plastic casing painting and electroplating. etc.
3. When the Pass indicator light is on or only one indicator light is off
[Note: Online monitors from different manufacturers have different models: some have only one indicator light; some have three indicator lights: LO/low resistance; PASS/normal resistance; HI/high resistance]. If the alarm does not sound, it indicates that the user has made close contact with the ESD ground, indicating that the wrist strap is properly worn.]