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We have the standard cloth roll-a-round adjustable type for all the production lines. We also have constant ESD monitors installed at all the ESD workstations. I was asked to check on the chairs that are listed as static controlled with a drag chain. We don’t have any ESD wax or tile on the floors, just a grey painted surface. I know that with conductive casters the chairs won’t generate static like the typical chairs do, but there are some that think you don’t need to wear a Wrist strap if you are sitting in one of these “ESD Safe chairs. I’m having a hard time convincing them otherwise, can you help me out? -Anonymous, Radio Frequency Systems, Phoenix, Arizona
For your ESD Safe chairs, it’s not necessarily the conductive casters that minimize the tribocharging, but the whole system. The material of the seat and back must be electrically dissipative (conductive) and tied to ground via the chair structure. Whether the chair is grounded or not, the large capacitance change on a human body via sitting (increased surface area) can still cause large charge imbalances on the human skin that needs to be handled via a direct and reliable ground path, e.g. a wrist strap.
One way to prove this to someone is to have them sit in a chair (any chair) and not be grounded (i.e., with wrist strap or foot grounder) but connected to a charge plate analyzer. Then have them stand up while watching the large voltage put on the charge plate, even if only for a second or two.
In lieu of a charge plate, as they are standing, if they were to touch a metal plate or surface, they may experience a discharge (one they can feel), if not a high-speed oscilloscope with a loop antenna hooked into the high-impedance input would pick up an EMI from the ESD to visually see.
This just stresses the fact that having a high-integrity ground path for the human skin is extremely important for any good ESD control program