Questions And Answers

Here is the problem that I am facing. Keep in mind that this is a Non ESD factory.

We have 3 material handlers that have wire rack carts w/laptop computers on top.

They push these carts across carpet (a huge static generator) into an elevator then out of the elevator onto a Non ESD linoleum floor. They gown up for the clean room. They push their carts across blue sticky mats and as they enter the clean room the static discharge is enough to cause their computers to reboot etc..

Are there any ESD products that we could use to help dissipate this static?

What if I place an ESD mat under the computer and ground the cart via a drag chain. As the cart passes across the threshold of the elevator the drag chain would then have a path to ground and hopefully discharge some or all of the static generated from walking across the carpet.

Any help on this matter would greatly be appreciated.

Yes, either one or two megohms in each foot grounder will meet ESD Association requirements and pass an ESD footwear tester. The upper limit on most ESD footwear is 100 megohms; some users have upper limit set to 35 megohms. With either the person wearing the foot grounder should pass. The lower limit of 800 kilohms is to verify the presence of the current limiting resistor in the path-to-ground.

Per ESD Handbook TR 20.20 paragraph 5.3.2.6 Current Limiting “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.” [this information is applicable to foot grounders as well].

Yes, ESD Shoes and Foot Grounders have basically the same requirement. But the answer is a bit complicated. Table 1 of ANSI/ESD S20.20, lists ESD Footwear technical range as <1 x 10E9 ohms referring ANSI/ESD STM9.1 test method which is for ESD shoes. Foot Grounders do not have an ESD Association standard test method. A standard practice was released Foot Grounder Citations from ESD SP 9.2 Footwear - Foot Grounders Resistive Characterization (not to include static control shoes). It’s “APPENDIX E -Foot Grounder Classification Electrical Resistance Value
Foot Grounder Standard Use Type A 8.0 X 10^ 5 to 3.5 X 10^7 Ohms
Foot Grounder Special Use High Type B 3.5 X 10^7 to 1.0 X 10^9 Ohms
Foot Grounder Special Use Low Type C <8.0 X10^5 ohms”

To limit the voltage on a person to under 100 volt charge, the “Standard Use” Foot Grounder with 3.5 X 10^7 Ohms upper limit should be specified. This would also be true of ESD Shoes.