Static electricity—what it is, how it is generated, and the problems associated with it

What is static electricity

Static electricity occurs when intraatomic or intramolecular equilibrium is disturbed due to the gain or loss of an electron. Normally, an atom is in equilibrium because of the same number of positive and negative particles—protons and electrons. Electrons can easily move from one atom to another. At the same time, they form positive (where there is no electron) or negative (a single electron or an atom with an extra electron) ions. When this imbalance occurs, static electricity is generated.

For more details see here: About static electricity in pictures

Electric charge on an electron — ( -) 1.6 x 10-19 pendant. A proton with the same charge has a positive polarity. The static charge in coulombs is directly proportional to the excess or deficiency of electrons, i.e. the number of unstable ions.

The pendant is the basic unit of static charge, which defines the amount of electricity passing through the cross-section of a wire in 1 second at 1 ampere.

A positive ion does not have one electron, therefore, it can easily accept an electron from a negatively charged particle. A negative ion, in turn, can be either a single electron or an atom/molecule with a large number of electrons. In both cases, there is an electron that can neutralize the positive charge.

How static electricity is generated

The main causes of static electricity:

• Contact between two materials and their separation from each other (including rubbing, rolling / unwinding, etc.).

• A rapid drop in temperature (for example, when the material is placed in the oven).

• High energy radiation, ultraviolet radiation, X-rays, strong electric fields (not common in industrial applications).

• Cutting operations (eg on cutting machines or paper cutting machines).

• Manual (Generated Static Electricity).

Surface contact and separation of materials are probably the most common causes of static electricity in the roll film and plastic sheet industry. Static charge is generated during unwinding / rewinding of materials or movement of different layers of materials relative to each other.

This process is not completely clear, but the truest explanation for the appearance of static electricity in this case can be obtained by analogy with a flat capacitor, in which mechanical energy is converted into electrical energy when the plates are separated:

Resulting stress = initial stress x (final plate spacing / initial plate spacing).

When the synthetic film touches the feed / take-up roller, a slight charge flowing from the material to the roller causes an imbalance. As the material overcomes the contact area with the shaft, the voltage rises in the same way as in the case of the capacitor plates at the moment of their separation.

Practice shows that the amplitude of the resulting voltage is limited due to electrical breakdown that occurs in the gap between adjacent materials, surface conductivity and other factors. At the exit of the film from the contact area, you can often hear a slight crackle or observe sparks. This happens at the moment when the static charge reaches a value sufficient to break down the surrounding air.

Before contact with the roll, the synthetic film is electrically neutral, but in the process of movement and contact with the feeding surfaces, a flow of electrons is directed to the film and charges it with a negative charge. If the shaft is metal and grounded, its positive charge will quickly drain.

Most equipment has many shafts, so the amount of charge and its polarity can change frequently. The best way to control static charge is to accurately measure it in the area just in front of the problem area. If the charge is neutralized too early, it may recover before the film reaches this problem area.

If the object has the ability to store a significant charge and if there is a high voltage, static electricity will cause serious problems such as arcing, electrostatic repulsion / attraction or electric shock to personnel.

Charge the polarity

Static charge can be positive or negative.For direct current (AC) and passive limiters (brushes), charge polarity is usually not important.

Static electricity problems

Static discharge in electronics

It is necessary to pay attention to this problem, as it often happens when working with electronic blocks and components used in modern control and measurement devices.

In electronics, the main danger associated with static electricity comes from the person carrying the charge and should not be ignored. The discharge current generates heat, which leads to broken connections, broken contacts, and broken microcircuit traces. The high voltage also destroys the thin oxide film on the field effect transistors and other coated elements.

Often, components do not fail completely, which can be considered even more dangerous, since the malfunction does not appear immediately, but at an unpredictable moment during the operation of the device.

As a general rule, when working with static-sensitive parts and devices, you should always take steps to neutralize the built-up charge on your body.

Electrostatic attraction / repulsion

This is perhaps the most common problem in the plastics, paper, textile and related industries. It manifests itself in the fact that the materials independently change their behavior — they stick together or, conversely, repel, stick to the equipment, attract dust, irregular wind on the receiving device, etc.

Attraction / repulsion occurs in accordance with Coulomb's law, which is based on the principle of the opposite of the square. In its simplest form, it is expressed as follows:

The force of attraction or repulsion (in Newtons) = Charge (A) x Charge (B) / (Distance between objects 2 (in meters)).

Therefore, the intensity of this effect is directly related to the amplitude of the static charge and the distance between attractive or repulsive objects. Attraction and repulsion occur in the direction of the electric field lines.

If two charges have the same polarity, they repel; if the opposite, they attract each other. If one of the objects is charged, it will cause an attraction, creating a mirror image of the charge on neutral objects.

Risk of fire

Fire risk is not a common problem for all industries. But the likelihood of fire is very high in printing and other businesses that use flammable solvents.

In hazardous areas, the most common sources of ignition are ungrounded equipment and moving wires. If an operator in a hazardous area wears sports shoes or shoes with non-conductive soles, there is a risk that his body will generate a charge that can ignite solvents. Ungrounded conductive parts of the machine are also dangerous. Everything in the danger zone must be properly grounded.

The following information provides a brief explanation of the ignition potential of static electricity in flammable environments. It is important that inexperienced traders are aware of the types of equipment in advance in order to avoid mistakes in the selection of devices for use in such conditions.

The ability of a discharge to cause a fire depends on many variables:

• type of disposal;

• discharge power;

• discharge source;

• discharge energy;

• the presence of a flammable environment (solvents in the gas phase, dust or flammable liquids);

• minimum ignition energy (MEW) of a flammable medium.

Types of discharge

There are three main types—spark, brush, and slide brushes. In this case, the coronary discharge is not taken into account, since it is not very energetic and happens quite slowly. Corona discharge is generally harmless and should only be considered in areas of very high fire and explosion hazard.

A sincere discharge

It comes mainly from a moderately conductive, electrically insulated object. It can be a human body, a part of a machine or a tool. It is assumed that all the energy of the charge is dissipated at the moment of sparking. If the energy is higher than the MEW of the solvent vapor, ignition may occur.

The spark energy is calculated as follows: E (in Joules) = ½ C U2.

Discharge from the hands

Brush discharge occurs when sharp pieces of equipment concentrate the charge on the surfaces of dielectric materials whose insulating properties cause it to accumulate. A brush discharge has a lower energy than a spark discharge and therefore presents less of an ignition hazard.

Spread with a sliding brush

Sliding brush spraying occurs on sheets or rolls of high resistivity synthetic materials with increased charge density and different charge polarities on each side of the web. This phenomenon can be caused by rubbing or spraying the powder coating. The effect is comparable to the discharge of a flat capacitor and can be just as dangerous as a spark discharge.

Source of power and energy

The size and geometry of the charge distribution are important factors. The larger the volume of the body, the more energy it contains. Sharp corners increase field strength and sustain discharges.

Discharge power

If an object with energy does not behave well electricityeg a human body, the object's resistance will weaken the ejection and reduce the hazard. For the human body, there is a basic rule: assume that all solvents with an internal minimum ignition energy of less than 100 mJ can ignite, despite the fact that the energy contained in the body can be 2 to 3 times high.

Minimum Ignition Energy MEW

The minimum ignition energy of the solvents and their concentration in the hazardous area are very important factors. If the minimum ignition energy is lower than the discharge energy, there is a risk of fire.

Electric shock

More and more attention is paid to the question of the risk of static shock in an industrial enterprise. This is due to a significant increase in occupational health and safety requirements.

An electric shock caused by static electricity is generally not particularly dangerous. It's just unpleasant and often causes severe reactions.

There are two common causes of static shock:

Induced charge

If a person is in an electric field and holds a charged object, such as a reel of film, it is possible for their body to become charged.

The charge remains in the operator's body if he is wearing shoes with insulating soles until he touches the grounded equipment. The charge flows down to the ground and hits the person. This also happens when the operator touches charged objects or materials — due to the insulating shoes, the charge accumulates in the body. When the operator touches the metal parts of the equipment, the charge can be discharged and cause an electric shock.

When people walk on synthetic carpets, static electricity is generated by contact between the carpet and the shoes. The electric shocks drivers get when they get out of their cars are triggered by a charge built up between the seat and their clothes when they get up. The solution to this problem is to touch a metal part of the car, such as a door frame, before lifting from the seat. This allows the charge to safely drain to the ground through the vehicle body and tires.

Equipment induced electric shock

Such an electric shock is possible, although it occurs much less often than damage provoked by the material.

If the take-up reel has a significant charge, it happens that the operator's fingers concentrate the charge to such an extent that it reaches the breaking point and a discharge occurs. Also, if an ungrounded metal object is in an electric field, it can become charged with an induced charge. Since a metal object is conductive, the mobile charge will discharge into the person touching the object.