Q: If unsealed concrete has a resistance in the 1.0 x 10⁸ range and dusting is not a problem, can this be considered a static control floor?

A: There are several potential problems in considering this concrete floor a static protective floor, mostly related to lack of adequate control over the floor's properties.

First, resistance alone doesn't always indicate the voltage that will be generated on people moving across the floor. Tests for body-voltage generation are better indicators of performance.

Second, the resistance of a concrete floor can vary considerably over time. Because the resistance of concrete is highly dependent upon its moisture content, the resistance can change depending upon the amount of moisture present in the concrete. Newly poured concrete will normally show a significantly lower resistance than concrete that has "dried out" over a couple of years. Concrete can also absorb moisture from the ground so that changes in the water table that often occur during rainy or dry seasons will affect the concrete.

Third, it is difficult to provide a proper ESD ground with a concrete floor because of its size, the metal reinforcing, and the moisture content. Items in direct contact with the concrete, such as grounded workbenches, may not be properly isolated from the concrete. The concrete thus could provide an alternate path to ground, bypassing the ESD ground on the workbench.

Q: How do we control high levels of static charge that occur on plastic parts dropped into bins?

A: In part, ionization may be required to neutralize the charge on plastic parts that are dumped into or out of bins. Many plastic parts tribocharge quite easily as they are dumped into the bin and as they slide against each other within it. Also, they are usually insulators that cannot be grounded to remove the static charge. Ionization used in the area can neutralize these charges.

The second part of the solution is using the proper type of bin. When charged insulative materials are dumped into conductive or dissipative bins, the bins tend to suppress the field associated with the static charge. If the conductive or dissipative bin is on an insulative surface, the bin itself may become charged by field induction. If the conductive or dissipative bin is placed on a conductive or dissipative surface, the induced charge on the bin can be drained away, even though the plastic parts will retain a charge.

Bins made of insulative materials do not suppress the electrostatic field, but tend to charge up and retain that charge.

Additional information may be obtained by contacting the ESD Association, 7900 Turin Rd., Bldg. 3, Ste. 2, Rome, NY 13440; phone: 315/339-6937; fax: 315/339-6793; Internet: http://www.eosesd.org; e-mail: eosesd@aol.com.