The Various Types of Electrostatic Discharges

Why Do We Need to Worry about Static Electricity?

Why Do We Need to Worry about Static Electricity?

In this lesson, you will learn more about how static electricity is formed and why you must be aware of static electricity risks when handling flammable fluids.

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How Does Different Material Behave in Terms of Static Electricity?

How Does Different Material Behave in Terms of Static Electricity?

Conductivity differs between different materials. To know how to handle your equipment onboard, you will learn more about the classification of materials in this lesson.

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What Are the Sources of Static Electricity Onboard?

What Are the Sources of Static Electricity Onboard?

In this lesson, you will learn more about static accumulators and the importance of interting when handling flammable fluids.

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What Are the Hazards Posed by Static Electricity Onboard?

What Are the Hazards Posed by Static Electricity Onboard?

In this lesson, you will learn more about what are the hazards caused by static electricity.

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What Precautions Can Be Implemented to Mitigate the Risks Posed By Static Electricity?

What Precautions Can Be Implemented to Mitigate the Risks Posed By Static Electricity?

What can you do to minimize the risks of static electricity? In this lesson, you will learn more about precautions to prevent incidents that electrostatic discharges may cause.

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Step 4 of 54 minutes read

The Various Types of Electrostatic Discharges

Electrostatic discharge events do not necessarily occur with a visible or audible spark. There are various types of electrostatic discharges that can take the form of a ‘corona’, a ‘brush discharge’, a ‘spark’, or a ‘propagating brush discharge’.

These terms differentiate electrostatic discharges depending on the voltage difference, the shape of the surface from which the discharge originates, etc.

Corona Discharge

Corona is a diffuse discharge from a single sharp conductor that slowly releases some of the available energy. A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying or holding a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona occurs at locations where the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages and emits light by the same property as a gas discharge lamp.

Generally, corona on its own is incapable of igniting a flammable atmosphere, however, one should still take it into account.

Corona discharge on corona ring of 500 kV overhead power line Nitromethane, CC BY-SA 3.0 Photo: Wikimedia Commons

Brush Discharge

Brush Discharge is a diffuse discharge from a highly charged non-conductive object to a single blunt conductor that is more rapid than corona and releases more energy. Examples of a brush discharge are:

  • Between a conductive sampling, apparatus lowered into a tank and the surface of a charged liquid.
  • Between a conductive protrusion (e.g. fixed tank washing machine) or structural member and a charged liquid being loaded at a high rate.

It is possible for a brush discharge to ignite gases and vapours. 

Comparison of brush discharge (left) and corona discharges (right) from a Tesla coil Airarcs, CC BY-SA 3.0 Photo: Wikimedia Commons

Spark

Spark is an almost instantaneous discharge between two conductors where almost all of the energy in the electrostatic field is converted into heat that is available to ignite a flammable atmosphere. Examples of sparks are:

  • Between an unearthed conductive object floating on the surface of a charged liquid and the adjacent tank structure.
  • Between unearthed conductive equipment suspended in a tank and the adjacent tank structure.
  • Between conductive tools or materials left behind after maintenance when insulated by a rag or piece of lagging.

Sparks can be incendive if various requirements are met. These include:

  • A discharge gap short enough to allow the discharge to take place with the voltage difference present, but not so short that any resulting flame is quenched.
  • Sufficient electrical energy to supply the minimum amount of energy to initiate combustion.

Propagating Brush Discharge

Propagating Brush Discharge is a rapid, high energy discharge from a sheet of material of high resistivity and high dielectric strength with the two surfaces highly charged but of opposite polarity. The discharge is initiated by an electrical connection (short circuit) between the two surfaces. The bipolar sheet can be in ‘free space’ or, as is more normal, have one surface in intimate contact with a conducting material (normally earthed). The short circuit can be achieved:

  • By piercing the surface (mechanically or by an electrical break-through).
  • By approaching both surfaces simultaneously with two electrodes electrically connected.
  • When one of the surfaces is earthed, by touching the other surface with an earthed conductor.

A propagating brush discharge can be highly energetic (1 joule or more) and so will readily ignite a flammable mixture.

Scientific studies have shown that epoxy coatings greater than 2 mm thick on tanks, filling pipes, and fittings may give rise to conditions whereby there is a possibility of a propagating brush discharge. In these cases, there would be a need to seek expert advice on requirements to explicitly earth the cargo. However, on most tankers, the thickness of epoxy coatings is not generally greater than 2 mm.

Propagating Brush Discharge. I.D. Pavey, Propagating brush discharges in flexible hoses, Journal of Electrostatics, Volume 67, Issues 2–3, 2009, Pages 251-255, ISSN 0304-3886, https://doi.org/10.1016/j.elstat.2008.12.008.