Step 4 of 5•4 minutes read
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 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.
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:
It is possible for a brush discharge to ignite gases and vapours.
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:
Sparks can be incendive if various requirements are met. These include:
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:
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.