Introduction to static electricity

Static electricity is a well-known but little understood phenomenon that affects

many industries and diverse environments. It can be something of a phantom -

impossible to see, touch, or smell. However, the results of static charge buildup

are quite noticeable. These results include potentially dangerous electrical shocks

which can cause decreases in productivity, machinery jams, fires, and explosions.

Static charges can also cause severe damage to sensitive electronic components,

requiring costly rework and/or field service repair.

Figure 1: Triboelectric charging (a transfer of electrons)

occurs when two materials are in contact and are then

separated. One material acquires an excess of negative ions

and the other an excess of positive ions.

Static charge buildup is caused by one of two processes: either by friction between two surfaces

(called triboelectrification) or by proximity to an electrostatic field (called induction charging).

When substances become charged by triboelectrification, electrons migrate from the surface of one
material to the surface of the other (see Figures 1 and 2). Upon separation of the two surfaces,

one surface loses electrons and becomes positively charged.

The other surface gains electrons and becomes negatively charged.

Figure 2:Friction, pressure, and separation are the major causes of industrial static electricity.

As the pressure or speed of contact and

separation (friction) increases, the amount of

the static charge buildup (voltage level) increases. Rapidly moving materials - such

as plastic trim in a pneumatic conveyor or a converted film web - can quickly

develop charges of more than 25,000 volts.

The second means by which an object or material may be charged is by induction.

A static charge field surrounds a highly charged object. If an isolated or

ungrounded conductive object enters into this static field, it too will becomecharged.

This creates the possibility of electrostatic discharge to some other

conductive object, which could result in an arc of sufficient energy to ignite

combustibles or destroy sensitive electronic components.

Conductors and Insulators

Materials are divided into two basic groups: conductors and insulators. Within a

conductor, electrons move freely throughout the entire substance. Therefore, when

an ungrounded conductor becomes charged, the entire volume of the conductive

body assumes a charge of the same voltage and polarity. A charged conductor

can be neutralized by connecting it to earth ground.

A charged insulator can remain charged for many hours.

Opposite polarity charges can exist on an insulator at the same time.

Charges will not migrate on insulators.

Grounding insulators neither removes nor prevents surface charges.

Charge of one polarity can remain on a conductor long as it is isolated from ground.

A Charged conductor will discharge completely when grounded.

Figure 3: Insulators and conductors have different surface charge and grounding capabilities.

An insulator reacts much differently to static electricity and cannot be

neutralized by simple grounding techniques (see Figure 3). Within an

insulator, the flow of electrons is very limited. Because of this, an

insulator may retain several static charges of different polarities and

potentials at various areas on its surface. This accounts for why

certain areas of a material may stick together and others may repel

each other. Connecting the insulator to ground will not result in an

exchange of electrons as is the case with conductive substances;

therefore, other means must be used for neutralizing static on

insulators. The Triboelectric Series chart (see Figure 4) shows the

relative positive or negative charge of various materials.

Figure 4: The Triboelectric Series chart shows the relative positive or negative

               charge of various materials.