EMI – Static Electricity



 

 


Static electricity discharge is a major contributor to EMI problems, especially in the autumn when the relative humidity begins to drop. This includes lightning, discharge from the human body and furniture discharge which, because it does not travel through your body, is the invisible form of static discharge and in reality the most common and insidious.

The higher moisture content of the atmosphere during the summer months in temperate zones of the globe helps to drain away static charges as quickly as they are generated. In the fall and winter however, the humidity drops and the static charges build to very high levels. When you touch something metallic you may then discharge the accumulated charge causing a spark, a slightly audible zap and a stinging, tingling sensation on your body.

The static discharges that you feel can cause a problem for your computer, but the ones you don’t feel because they are between your office furniture and the computer usually cause the worst problems. The static you don’t feel is caused when one object discharges to another. When you sit in a chair, for example, your clothing is in contact with the seat, back and arms of the chair. As you stand up from the chair your clothing is pulled away from the fabric of the chair. It is this pulling apart of two dissimilar fabrics or surfaces that generates the static buildup on the chair.

Static charges on a standard office chair can reach as high as 75,000 volts; yes that is seventy five thousand. The casters of the chair are rubber or plastic and tend to be good insulators. The charge on the chair does not dissipate at all or dissipates only very slowly. The charge on your body may dissipate more quickly if your shoes are leather and better conductors than the chair casters, and because you come in contact with other objects that help to drain the charge away from your body.

Entry Paths

Static discharge has two basic components, radio frequency interference, RFI, and the pulse of electricity conducted into the surface of your computer when you touch the case.

EMI can gain access through openings in the computer case. These access points can be caused by running the computer without the side of the case in order to facilitate easy access for various reasons and cover plates left off of the bus slots in the back of the computer. EMI can also infiltrate a computer even through a cover that is merely set in place and not fastened down with the appropriate screws.

Cables connected at one end to the computer but left dangling at the other end while a peripheral is disconnected can act as antennae to capture signals and draw them into the computer.