The definition of static electricity is an electrical charge caused by an imbalance of electrons on the surface of a material. All matter is composed of atoms which are made of charged subatomic particles known as electrons and protons. Protons carry a positive charge (+), and electrons carry a negative charge (-). Ordinarily every object carries equal numbers of protons and electrons and is said to have a neutral charge.
When two non-conductive surfaces come into contact, a bond takes place with their surface molecules. When separated or slid across one another, the bond is broken leaving behind a shared electron to one surface or the other. When this happens, one material ends up wtih an excess of electrons and becomes negatively charged, while the other ends up with a deficiency of electrons and becomes positively charged. This accumulation of imbalanced charges on objects results in the phenomena we commonly refer to as static electricity.
It is called "static" because there is no current flowing as in AC or DC electricity is just an electrical potential which is measured in volts. As with all potentials, there is an attraction to the opposite charge. This attractive phenomenon can be benficial or detrimental.
In the process of pollination by bees, it helps to attract and hold pollen to the bee's charged body.
In some printing applications the attractive properties may be desirable and may be purposely induced.
But in the process of feeding from stacked sheets, static cling prevents the sheets from separating and jams the feeder systems.
With vision systems, dust attracted to lenses impedes the performance of the reader.
Dust attracted to parts prior to paint ruins the finish.
The attractive property of static electricity presents another unique problem, electrostatic charge (ESD). We have all felt the annoying effects of getting zapped as we slide out of our seats or walk across a carpeted area. The rubbing of our clothing creates a static charge. When we come close to a conductive surface, we hear the snap, and feel the pain of electrons rushing out of our bodes.
In the electronics environment this can wipe out circuit boards, overload circuits, and cause faults. In flammable environments it can be a source of ignition. In the production environment, while it may not be life threatening, the reaction of the operator could put them into jeopardy.
Controlling electrostatic discharge begins with understanding how electrostatic charge occurs in the first place. Electrostatic charge is most commonly created by the contact and separation of two materials. For example, a person walking across the floor generates static electricity as shoe soles contact and then separate from the floor surface. An electronic device sliding into or out of a bag, magazine or tube generates an electrostatic charge as the device's housing and metal leads make multiple contacts and separations with the surface of the container. While the magnitude of electrostatic charge may be different in these examples, static electricity is indeed generated.
Figure 1. The Triboelectric Charge. Materials Make Intimate Contact.
Figure 2. The Triboelectric Charge - Separation
Creating electrostatic charge by contact and separation of materials is known as "triboelectric charging". It involves the transfer of electrons between materials. The atoms of a material with no static charge have an equal number of positive (+) protons in their nucleus and negative (-) electrons orbiting the nucleus. In Figure 1, Material "A" consists of atoms with equal numbers of protons and electrons. Material "B" also consists of atoms with equal (though perhaps different) numbers of protons and electrons. Both materials are electrically neutral.
When the two materials are placed in contact and then separated, negatively charged electrons are transferred from the surface of one material to the surface of the other material. Which material loses electrons and which gains electrons will depend on the nature of the two materials. The material that loses electrons becomes positively charged, while the material that gains electrons is negatively charged. This is shown in Figure 2.
This process of material contact, electron transfer and separation is really a more complex mechanism than described here. The amount of charge created by triboelectric generation is affected by the area of contact, the speed of separation, relative humidity, and other factors. Once the charge is created on a material, it becomes an "electrostatic" charge (if it remains on the material). This charge may be transferred from the material, creating an electrostatic discharge, or ESD, event. Additional factors such as the resistance of the actual discharge circuit and the contact resistance at the interface between contacting surfaces also affect the actual charge that can cause damage.
Examples of Static Generation
Typical Voltage Levels
|Means of Generation
|Walking across carpet
|Walking across vinyl tile
|Worker at bench
|Poly bag picked up from bench
|Chair with urethane foam
Reference Midwest Chapter ESD Association
Measuring Static Electricity
Static Electricity is measured in coulombs. The charge "q" on an object is determined by the product of the capacitance of the object "C" and the voltage potential on the object "V":
More commonly we refer to an electrostatic potential expressed in voltage.
Electroscopes measure the amount of static electricity stored in an object (Figure 7a). An electroscope works because like charges repel. As an electroscope is filled with a particular charge, positive or negative, all of those like charges begin to repel. If there are enough repelling charges the ball at the end of the electroscope is repelled away from the rest of the electroscope (Figure 7b). In addition, the electroscope is calibrated so you can actually measure the amount of static electricity stored in an object.
Reference The Bakken Library and Museum
Digital Static Meter
While electroscopes are fun projects for science classes they do not have a practical application in industrial applications. What is needed is a non-contact diagnostic tool like a modern professional static meter. This tool will indicate the level of charge as well as its polarity. Being hand held, it is portable and easily directed toward the suspected source of static generation.
1. Eliminate materials that generate static charges from the process. Rubber rollers, Teflon slides, PVC hoses etc are all non-conductive surfaces that when rubbed will generate a static charge.
2. Ground or electrically bond all conductive and dissipative materials to earth ground. This will provide a discharge path to reduce charge generation and accumulation. For plastics, paper, and other non-conductive materials, grounding does not remove an electrostatic charge because there is no conductive pathway.
3. Typically, ionization is used to neutralize charges on these insulating materials. The ionization process generates negative and positive ions that are attracted to the surface of a charged object, thereby effectively neutralizing the charge.
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