Static electricity poses a major risk in explosive atmospheres. Its generation is unavoidable, but its accumulation and discharge can be controlled. This article discusses the causes, dangers and control measures to prevent electrostatic discharges.

Contact charge (triboelectricity)

This is the most common mechanism and occurs when two materials come into contact with each other and are then separated. Electrons are transferred from one material to the other, causing both to retain a net charge. This process takes place in various industrial applications:

• Fluid processing:

  Charge can be generated during flow through pipes, valves and filters, especially with fluids with low conductivity. Pouring from containers, stirring fluids (such as water in hydrocarbons) and atomising conductive fluids (such as paint or water) can also generate significant charges. The settling of solid particles from a suspension also contributes to this.

• Powder processing:

  Grinding, spray drying, pneumatic transport, sieving and the filling and emptying of bags or drums can build up electrostatic charge in powders.

• Daily processes:

  Contact charges can also occur when rubbing insulating surfaces, walking on insulating floors, removing synthetic clothing, when conveyor belts move over rollers and when unwinding plastic film.

Induction charge

Induction charging occurs when a charged object creates an electrical field that influences the charge in a nearby conductor without direct contact. This can cause the nearby object to retain a net charge if it is poorly grounded or remains insulated. This effect is particularly important for poorly grounded metal objects in a charged environment. People can also become charged by induction if they are in the vicinity of a highly charged object.

Charge transfer (Conduction)

When an object is physically connected to an already charged surface, charge can travel along this conductive path and charge the connected object. This can occur, for example, with equipment or tools that come into contact with a charged material.

These three mechanisms allow electrostatic charge to build up in industrial environments, which can lead to dangerous discharges if the correct control measures are not in place.

When does static electricity become dangerous?

The danger arises when the rate of charge build-up is greater than the rate of discharge. This can lead to spark discharges that ignite flammable atmospheres. Some common scenarios in which charge can accumulate:

Insulated metal parts

Metal parts without proper grounding can retain significant charges. The extent to which this happens depends on the resistance to ground. In general, when the resistance is above 10⁶ – 10⁸ ohms, potentially dangerous voltages can arise. Spark discharges from isolated metal parts can reach energies of 50-500 mJ, which is more than enough to ignite a flammable atmosphere.

Personnel

People can build up a static charge by wearing insulating footwear or by standing on a non-conductive floor. If the resistance to earth is above 10⁸ ohms, a person can hold a considerable charge. This is particularly dangerous in environments where flammable gases or vapours are present with a minimum ignition energy (MIE) below 100 mJ. Sparks from isolated persons or metal parts are some of the most dangerous types of discharge.

Non-conductive materials

Materials such as rubber, PVC, polyethylene, PTFE, glass and fibre-reinforced plastics have a high volume or surface resistance (>10⁹ ohms·m). This means that they can easily accumulate an electrostatic charge. Unlike wood and paper, which often have some conductivity, plastics can retain significant charges and suddenly discharge them, which poses a risk of ignition in flammable vapour and dust environments.

Low conductivity liquids

Electrostatic charge can build up when processing and storing liquids with low conductivity (<50 pS/m). This happens mainly at high flow rates or turbulent flow. Turbulent liquid flows generate more charge than laminar flow. Charge can build up in storage tanks if the discharge is not fast enough. If the conductivity of a liquid is above 50 pS/m, the charge is usually quickly discharged and does not pose a danger. Examples of low-conductivity liquids are toluene, xylene, hexane, heptane, carbon disulphide and petroleum products.

Powders

The degree of charge build-up in powders depends on their volume resistance and the speed at which charge can leak away (relaxation time). Powders are divided into three categories based on their volume resistance:

• Low resistance (<10⁶ ohm·m): These powders conduct charge well and hardly build up dangerous voltages.
• Medium resistance (10⁶ – 10¹⁰ ohm·m): Can retain a charge and pose a potential risk.
• High resistance (>10¹⁰ ohm·m): Can generate dangerous voltages, especially if they are stored in non-conductive containers.

With a volume resistance above 10⁹ ohm·m and a MIE above 10 mJ, there is usually no electrostatic hazard in a grounded metal installation. However, powders in plastic containers can lead to very energetic discharges.

Liquid mists and dust clouds

When liquids are atomised or powders are suspended in the air, these particles can become charged. This often happens during spraying, cleaning or industrial processes that create an aerosol. Hazardous discharges can occur in tanks where charged mist or dust accumulates, especially in the centre of the tank where the highest voltages occur. Mist and dust discharges can ignite sensitive flammable vapours and gases, unless their MIE is higher than 25 mJ.

Gases

Pure gases do not generate a charge themselves, but if a gas contains suspended particles or droplets, this can lead to electrostatic charges. This can occur with hydrocarbon mists or gas mixtures with fine particles, which can pose an ignition hazard.

What discharge mechanisms are there?

The energy of a discharge can vary from <<0.1 mJ to several joules, depending on the type and the circumstances.

How do you prevent electrostatic discharges?

The primary objective of most control measures is to create a way in which separated charges can safely recombine before dangerous potentials are reached, or to avoid sparking where dangerous discharges can occur. The most important measures:

•  Grounding and conductive components

  • Fixed metal components must be earthed (<10 ohm); in some cases up to 10⁶ ohm is acceptable.
  • Mobile equipment must be connected with flexible earthing cables.
  • Flexible IBCs (type C) must be earthed at the point of use.
  • Regular checking of the earthing is essential.

• Hoses and flexible connections

  • Conductive hoses (<10³ ohm/m) prevent electrostatic discharge, but can transmit stray currents.
  • Semi-conductive hoses (10³-10⁶ ohm/m) offer protection against both charge build-up and inductive sparks.
  • Non-conductive tubing is not recommended in potentially explosive environments.

• Personnel and floors

  • Personnel must be earthed via suitable footwear and conductive floors.
  • Antistatic shoes (5 x 10⁴ – 10⁸ ohm) prevent charge build-up.
  • Floors must have a resistance to earth of <10⁸ ohm.
  • Clothing may not be removed in flammable atmospheres and must fit properly.

• Liquids and flow rates

  • Liquids with a low conductivity (<50 pS/m) may not flow through pipes faster than 1 m/s.
  • A limit of 7 m/s is recommended for solvents with a higher conductivity.
  • Avoid spraying and splashing to prevent charge build-up and mist formation.
  • In storage tanks >30 m³, the flow velocity must be limited to 2 m/s (1 m/s in multi-phase systems).

• Powders and pneumatic transport

  • Limiting mechanical processing reduces charge build-up.
  • Powders with MIE <10 mJ must be grounded to prevent risks.
  • In grounded pneumatic pipes (<1 m diameter) there is no risk with an MIE >0.2 mJ.

• Non-conductive components

  • Plastics and other insulators can retain a charge and suddenly discharge it.
  • No insulating plastics may be used in Zone 0 and Zone 1.
  • Avoid insulating materials in the event of dust clouds with MIE <10 mJ or applications with a high charging speed.

• Barrels and storage

  • Contents must make contact with the ground to prevent charge accumulation.
  • Unlined metal drums require external earthing.
  • Drums with insulating coatings must be earthed both internally and externally.
  • Glass-lined drums must be fitted with a grounded tantalum plug or tip.

Conclusion

Static electricity is an unavoidable phenomenon, but the risks can be limited with the right measures. Hazardous sparks can be prevented through reliable earthing, controlled choice of materials and process management. Regular checks of earthing systems and staff awareness are crucial for a safe working environment.