Static electricity and protection

Introduction

Static electricity has been the earliest known form of electricity to mankind dating back to sixth century BC. Though in terms of power output, static electricity does not come anywhere near the electromagnetic form of electricity with which we are all familiar, it can be as much of a hazard as its other forms, and well-planned protection is needed against the hazards posed by it. In this section, we will discuss the physics of static electricity, its effects on equipment and humans and how to overcome the problems likely to be caused by it.

What is static electricity?

We know that all substances known to us are composed of molecules which are themselves formed from the atoms of a few basic elements and that all atoms are made up of subatomic particles, namely protons (positive charge), neutrons (neutral uncharged) and electrons (negative charge). In an atom, protons and electrons are equal in number thus making the atom electrically neutral and stable. Application of energy to a substance can cause separation of electrons from the parent atom. Electrons are free to move from the confines of their parent atoms whereas protons are bound to and move with the parent atoms and are therefore of limited mobility particularly so when the substance is a solid.

Any substance which is deficient in electrons even marginally (1 in 100 000) exhibits a strong electric charge.

When two bodies of dissimilar materials are in contact with each other, electrons migrate from one body to the other through the contact surface. If the two bodies are suddenly separated, the electrons try to return back to their parent substance. In case the substances are electrically conductive, the electrons are able to do so, but if one of the substances or both are insulating materials this does not happen. The electrons get trapped in the surface of the material to which they have migrated. The surfaces of both substances now exhibit electrical charge due to excess or deficiency of electrons.

The voltage of a charged body can be calculated using the formula:

V = Q / C

…where V is the voltage in volts, Q is the charge in coulombs and C is the capacitance of the body in Farads with reference to the surface of measurement.

The accumulated charge leaks away from the charged body gradually but if the rate of charge generation is higher than the rate of leakage, the voltage of the charged body increases till a breakdown of the surrounding medium takes place. This causes a spark to jump across the medium.

Generation of charge

Charge generation happens under various conditions. These are described below.

Type of materials Static buildup needs two dissimilar materials, at least one of which should be an insulator, with differing dielectric constants to be in contact with each other.

Large contact area

The contact area between these dissimilar materials should be as high as possible to facilitate migration of electrons between the materials.

Speed of separation

The higher the speed of separation, less is the opportunity for the electrons to move back to the parent body and therefore higher is the charge buildup.

Motion between the substances

Though this is not a necessary condition, charge buildup is facilitated by motion between the surfaces in two ways. The friction and heat produced as a result of this friction increase the energy level of the atoms making escape of electrons easier. Secondly, the movement causes better surface contact by bringing the microscopic irregularities on both surfaces to come within contact of each other thus increasing migration.

Atmospheric condition

Humidity increases the leakage of charge through the air surrounding the charged body and reduces the buildup. Conversely the drier the atmosphere, the better is the ability to retain charge.

Some common examples of static buildup

A belted drive, especially of the flat variety on a pair of metallic pulleys rotating at high speed is one of the most common examples of charge buildup. The belt should be of non conducting rubber material. Interestingly, conveyor belts made of rubber, which are used in material-handling systems extensively don’t produce high static electricity due to the low linear speeds at which the conveyors work.

The speed of separation is not high enough to cause appreciable charge buildup.

• Flow of materials such as pulverized non-conducting solid particles, gas, air, etc. through a chute or an orifice results in charge generation.

• Rubber castors of chairs moving over non-conducting flooring and automobile tires running over road surface produce static charges.

• Motion of liquid falling into a tank during filling operations can produce static charges.

• Human workers can have several kilovolts of charge buildup on their bodies by the friction of rubber shoes or by charge-producing fabrics used in their dress.

The IEEE 142 (green book) gives the values of accumulation of charges for some of the commonly encountered conditions and the DC breakdown voltages and are reproduced in FIG. 1 and 2 for comparison. It may be seen that the static voltage produced in these example situations is sufficient to cause a spark breakdown across gaps of up to 90 mm (3.5 in.). Also, it can be seen that a pointed object can cause spark at lower-voltage levels.

Type of Equipment/Process Voltage Buildup (kV)

Belted drives 60-100 Fabric handling 15-50 Paper machines 5-100 Tanker trucks <25 Belt conveyors handling grain <45

FIG. 1 Static voltage generation by various processes

Distance (mm)

Breakdown Voltage (kV) From a Point Breakdown Voltage (kV) From a Plane

FIG. 2 DC breakdown voltage

Energy of spark and its ignition capability

The energy of spark discharge can be calculated using the formula:

_ 29 = 0.5 ( · 10 ) ECV ×

…where C is the capacitance of the body, which stores the charge in pF (picofarads), V is the voltage in volts and E is the energy in milliJoules.

The capacitances of some of the bodies discussed in the examples above are given below:

• Human body 100-400 pF

• Tank truck 1000 pF

• Tank with rubber lining 100 000 pF

The energy levels required to cause ignition depend on the flammability of the materials present in the environment and whether they form an explosive mixture. Too rich or too lean mixtures don’t ignite easily.

Dangers of static electricity buildup

The following are the dangers posed by static electricity:

• Ignition causing fire or explosion

• Damage to sensitive electronic components

• Electric shock to humans followed by accidents such as a fall

• Damage to mechanical components such as bearings due to sparking through the oil films on bearing surfaces.

It’s necessary to study the static buildup potential of any workplace and institute protective measures to control such buildup.

Control of static electricity

1 Grounding and bonding

As we have seen earlier in this section that charge buildup takes place when two surfaces, which are in contact and across which electrons migrate, get suddenly separated.

Connecting such surfaces together with a conducting medium prevents charge accumulation by providing a leakage path. This is called bonding and can be achieved by using a bare or insulated conductor of adequate mechanical strength. Electric current flow due to charge leakage being of very low magnitudes, the size of the conductor is immaterial and so is the resistance of this conductor.

For moving objects, a ground brush of metal, brass or carbon can be used to provide the required leakage path. This method is commonly used for shafts of rotating machines to prevent bearing surface damages (refer FIG. 3). For objects, which are in contact with ground already, no separate grounding or bonding is necessary.

Grounding cannot, however, provide a solution in all cases, especially where a bulky non-conducting material is involved. In this case, the part of the substance, which is a distance away from the grounded portion, can retain sufficient charge, since movement of charge won’t be fast enough in an insulating material. This charge can result in a spark.

2 Control by humidity

Many insulating materials such as fabric, paper, etc. can absorb small quantities of water when the atmospheric humidity is sufficiently high. Even in the case of materials that don’t absorb water, a thin layer of moisture gets deposited on the surface due to humidity (e.g., plate glass). If the environment has a humidity of over 50% moist insulating, materials can leak charges as fast as they are produced. This prevents high charge buildup thus avoiding sparks.

FIG. 3 Example of the use of grounding brush

Conversely, most of the materials become dry when the humidity becomes lower than 30% since they tend to lose moisture to the atmosphere. This results in increased charge accumulation, which can cause sparking. Keeping humidity levels at 60-70% can solve static problems in many cases such as industries handling paper and fibers where charge buildup causes unwanted adhesion. In some cases, localized humidification using steam ejectors can be useful particularly where the large space involved makes increase of humidity in the entire space a difficult proposition.

This method is however unsuitable where:

• The processed material can be adversely affected due to high humidity.

• If the area involved is air conditioned or humidity controlled for process reasons or human comfort.

• In cases where humidity increase does not cause appreciable drop in resistivity.

In all such cases, other methods of static control may have to be resorted to.

3 Ionization

Ionization consists of forced separation of electrons from air molecules by application of electric stress or other forms of energy. The air thus ionized becomes conductive and can drain charges from charged bodies with which it’s in contact. The positive ions and electrons are also attracted by the negative and positive charges respectively thus resulting in charge neutralization. Ionization can be produced by high-voltage electricity, by ultraviolet light or by open flames. Various devices using a step up transformer operating on mains supply and producing high electric fields are commercially available. Due care is needed however to address safety issues arising out of the use of high voltage. Such devices find application in paper and fabric-processing plants. They are, however, unsuitable for use in situations where the environment contains inflammable gas mixtures.

A simpler device is the static comb, which does not use electricity at all. It consists of a metallic bar with a row of sharp points projecting from it and bonded to ground. When this device is placed near the charged surface, the electric stress due to accumulation of charge near the sharp points causes ionization and helps to drain the charge from the surface. This method is commonly used in belt-driven equipment near the point of separation of the belt and pulley.

FIG. 4 Use of static comb, an example

Another method of ionization is by using a row of small open flames. This method, however, requires caution where combustible materials are handled.

4 Use of anti-static (conductive) materials

Since the buildup of charge requires at least one of the surfaces to be non-conductive, it follows that by making the non-conductive surfaces conductive even slightly would reduce charge buildup. For example, coating a belt using conductive film on the side where it’s in contact with the metallic pulley can prevent charge buildup.

Use of conductive flooring or conductive floor covering can reduce charge buildup. The resistance of the floor should be less than 1 M-ohm when measured between points approximately 1 m apart for this method to be effective. At the same time, the resistance should be more than 25 000 ohm to avoid shock to personnel.

Similarly, conductive footwear and suits will prevent static buildup in the workplace.

Also, materials with lower static producing properties should be used wherever necessary.

Static accumulation and discharge can destroy integrated circuit (IC) devices. Facilities, which handle these devices, or components that are made using them, should be designed with adequate precautions. Conducting cuffs connected with ground using metallic bonding conductors is a common device used in assembly shops to avoid transfer of charges from the operators' body to the circuit components.

Assessment of static risks and planning prevention

Workplace risks in respect of static charge buildup should be carefully assessed while planning a production facility.

The following questions need to be posed and answers sought.

• Is there equipment, which can cause generation of static charge in the planned facility?

• Does the workplace handle static-prone materials?

• Does the workplace involve processes where static generation is inherent?

• Are there flammable mixtures of gases or combustible materials present in the workplace?

Depending on the answers to the questions, the design of the facility should be reviewed and potential areas of static generation identified. Methods of static protection appropriate to the process in question should be selected and implemented. Wherever necessary, safe operating practices should be evolved, built into the working system and enforced. The equipment used for static prevention must be inspected from time to time, measurements made to verify their effectiveness and maintenance practices put in place to ensure their proper upkeep. A review should be carried out whenever process changes are effected.

These measures will greatly help in reducing the potential risks from static electricity.

Summary

In this section, we learnt about the physics of static electricity generation and the various factors that cause buildup of static charges. Conditions for spark generation from static voltage buildup were discussed. The methods of control of static buildup and preventing static spark discharge were also reviewed. The importance of making an assessment of static risk while planning a facility was also discussed.