ESD Systems - Questions And Answers

We recently applied ESD wax in our Manufacturing area and it tests below 1X10^-8. We require our technicians to wear 2 heelstraps with 1 megohm resistors. While wiping some stainless steel panels wrapped in plastic the techs complained about getting repeatedly zapped. The panel and plastic were on a stainless steel gurney with rubber wheels that did not have a drag chain. My understanding was that the 1 megohm resistor should prevent the techs from being zapped so badly. I measured the plastic and it had over 19K volts of static electricity. Please advise

If your techs are grounded properly, the charge on them should be near zero. The 1 Megohm current limiting resistor is to protect them, but only from inadvertent contact via the foot grounders with line voltage.

Per ESD Glossary ESD ADV1.0 Electrostatic Discharge (ESD) is defined as "The rapid, spontaneous transfer of electrostatic charge induced by a high electrostatic field. Note: Usually, the charge flows through a spark between two bodies at different electrostatic potentials as they approach one another."

When the plastic or a gurney with insulative wheels has a charge generated on it, it will basically stay there as static (at rest) electricity. Insulators are non-conductors and cannot be grounded by definition. The tech at zero volts and the plastic at 19,000 volts when they approach one another will discharge, an ESD event.

Per ANSI/ESD S20.20 Paragraph 6.2.3.1. Protected Areas Requirement "All nonessential insulators must be removed from the [ESD] workstation. Ionization or other charge mitigating techniques shall be used at the workstation to neutralize electrostatic fields on all process essential insulators if the electrostatic field is considered a threat."

So replace the high charging plastic with low charging, dissipative plastic. Using conductive casters or drag chains, ground all portions of the gurney. Otherwise, use ionizers to neutralize the charges.

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