Conductors and Insulators
by Jerry R. Giuliano
President, Julie Industries, Inc.
By definition, static electricity is electricity at rest. By experience, we know that static electricity may rest now and then, but it never rests completely.
At one time or another, static electricity affects everyone. The results are sometimes amusing: like trying to get the plastic wrap from a package of crackers off your hands. And sometimes not: getting struck by lightening, for instance.
If the plastic wrap were made of metal (a conductor) and the earth were made of plastic (an insulator), perhaps the problems just described would not exist. But our world consists of a host of materials, like plastic, which generates, store and discharge static electricity; these are called insulators.
Conductors
A conductor is a material that allows electricity to pass through it. Copper, steel and most metals are good conductors. Silver is an excellent conductor. It is important to remember that conductors allow electricity to pass through them only if they are connected to good electrical ground.
A simple experiment illustrates that point: Isolate a ten inch square metal plate from "ground" by attaching a string to one end and suspending it in air. Rub the metal plate (a conductor) vigorously with an insulative material--a piece of dry wool, plastic or a balloon--being careful to let only the insulator touch the metal.
The friction between the two materials will cause both to become electrostatically charged and both the insulator and the conductor will be surrounded by a detectable electrostatic field. (An electrostatic fieldmeter would display the intensity of that field in volts per distance. Some meters are calibrated in inches and feet, others in centimeters and meters.)
Now, attach one end of a ground wire to the metal plate and the other end to electrical ground. The static charge will immediately flow through the plate to ground, the electrostatic field will disappear, and the meter would measure zero. Grounding metal dissipates a static charge.
As we've seen, friction, or direct contact and separation, charges non-grounded or isolated metal objects. Those same metal objects can be electro-statically charged by a method known as induction, a phenomenon whereby an object picks up a charge from the electrostatic field surrounding another charged object; the two objects need not touch.
Here's an example: Rub the insulator, generation a charge on its surface, then bring it close to, but not touching, the metal plate. By induction, the metal plate will become electrostatically charged.
Once again, the charge on the metal plate can be transferred easily by grounding. Academic? Yes! Yet, a significant number of "static problems" that exist in a variety of manufacturing processes could be solved easily and inexpensively with proper grounding techniques.
Do not assume that metal conveyor belts, tension drums and idler rollers can not becomeelectrified. Plastic gears can isolate conveyor belts from ground and greased bearings can isolate metal drums and rollers.
A spark from an ungrounded metal roller to either a grounded machine frame or a machine operator may not be especially harmful. Add an explosive solvent to the equation and the result, from a relatively insignificant spark, could be catastrophic. In two words, the solutions for preventing the electrification or static charge accumalation on conductors is: ground them!
Insulators
An insulator is a material such as plastic, rubber, glass or ceramic that prevents the flow or transfer of electricity. Insulative materials can not be grounded. Attaching a ground wire to an insulator would have no effect.
Insulative materials have a proclivity to either give-up or accept electrons. Rubbing insulative materials together, friction, or contacting and separating them generates a static charge. That charge results from the interchange of electrons between the two materials.
The material that accepts electrons becomes negatively charged; the one that gives-up electrons is left positively charged.
Many years ago, a group of materials was listed in order of their propensity to become electrostatically charged. This list, called the Triboelectric Series, ranks different materials according to their ability to accept or reject electrons.
Theoretically, those materials at the top of the list charge positively, those at the bottom take on a negative charge. The farther apart two materials are in the series, the greater the magnitude of charge, or potential difference, there exists between them.
The closer they come in the series, the lower the magnitude or potential difference. With steel at the neutral mid-point in the series, the closer the materials are to steel, the less likely they are to charge at all.
Air, human hands, asbestos, rabbit fur and glass top the list. Polypropylene, /PVC, Del-F, Silicone and Teflon are at the bottom. According to theory, the contact and separation of dry air and Teflon would result in positively charged air and negatively charged Teflon; and the magnitude of charge would be greatest since those materials are the farthest apart in the series.
The results of Triboelectric series experimentation conducted in a laboratory environment are interesting. In reality, factors such as contact area and pressure, temperature, relative humidity and moisture content, speed and the coefficient of friction of the material all work to influence the magnitude of electrostatic charge susceptibility, generation and accumulation.
As we said, electricity does not pass through insulative materials. Positive of negative charges accumulate on insulators and can cause a myiad of problems ranging in scope from nuisance (fly-away hair) to devastation (fire, injury and even death have resulted from industrial accidents attributed to uncontrolled static electricity).
During the normal process of manufacturing, converting and handling of insulative materials, uncontrolled static electricity creates problems that interrupt production, lower yield and waste time, energy and money.