A liquid dielectric is a dielectric

Dielectric

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 material in liquid state. Its main purpose is to prevent or rapidly quench electric discharge

Electric discharge

Electric discharge describes any flow of electric charge through a gas, liquid or solid. Electric discharges include:*Electric glow discharge*Electric arc*Electrostatic discharge*Electric discharge in gases*Leader *Partial discharge...

s. Dielectric liquids are used as electrical insulators in high voltage

High voltage

The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

 applications, e.g. transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s, capacitor

Capacitor

A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s, high voltage cable

Cable

A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

s, and switchgear

Switchgear

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults...

 (namely high voltage switchgear). Its function is to provide electrical insulation

Electrical insulation

thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...

, suppress corona

Corona discharge

In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...

 and arcing, and to serve as a coolant

Coolant

A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

.

A good liquid dielectric should have high dielectric strength

Dielectric strength

In physics, the term dielectric strength has the following meanings:*Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties....

, high thermal stability and chemical inertness against the construction materials used, non-flammability and low toxicity

Toxicity

Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

, good heat transfer

Heat transfer

Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 properties, and low cost.

Liquid dielectrics are self-healing; when an electric breakdown occurs, the discharge channel does not leave a permanent conductive trace in the fluid.

The electrical properties tend to be strongly influenced by dissolved gases (e.g. oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 or carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, dust, fibers, and especially ionic impurities and moisture

Moisture

Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

. Electrical discharge may cause production of impurities degrading the dielectric's performance.

Some examples of dielectric liquids are transformer oil

Transformer oil

Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage...

, perfluoroalkanes, and purified water

Purified water

Purified water is water from any source that is physically processed to remove impurities. Distilled water and deionized water have been the most common forms of purified water, but water can also be purified by other processes including reverse osmosis, carbon filtration, microfiltration,...

.
+ Common liquid dielectrics>

Name	Dielectric constant	Max. breakdown strength (MeV/cm)	Properties
Mineral oil Mineral oil A mineral oil is any of various colorless, odorless, light mixtures of alkanes in the C15 to C40 range from a non-vegetable source, particularly a distillate of petroleum....		1.0	Flammable. Common type of transformer oil Transformer oil Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage... .
n-Hexane Hexane Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...		1.1-1.3	Flammable. Used in some capacitors.
n-Heptane Heptane n-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale...			Flammable.
Castor oil Castor oil Castor oil is a vegetable oil obtained from the castor bean . Castor oil is a colorless to very pale yellow liquid with mild or no odor or taste. Its boiling point is and its density is 961 kg/m3...	4.7		High dielectric constant. Flammable. Refined and dried castor oil is used in some high voltage capacitor Capacitor A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated... s.
Silicone oil Silicone oil A silicone oil is any polymerized siloxanes with organic side chains. They are formed of alternating silicon-oxygen atoms or siloxane, rather than carbon atoms . Other species attach to the tetravalent silicon atoms, not to the divalent oxygen atoms which are fully committed to forming the...		1.0-1.2	More expensive than hydrocarbons. Less flammable.
Polychlorinated biphenyl Polychlorinated biphenyl Polychlorinated biphenyls are a class of organic compounds with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx... s			Formerly used in transformers and capacitors. Persistent organic pollutant Persistent organic pollutant thumb|right|275px|State parties to the Stockholm Convention on Persistent Organic PollutantsPersistent organic pollutants are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes... , toxic, now phased out. Low flammability.
Purified water Purified water Purified water is water from any source that is physically processed to remove impurities. Distilled water and deionized water have been the most common forms of purified water, but water can also be purified by other processes including reverse osmosis, carbon filtration, microfiltration,...			High thermal capacity, good cooling properties. Low electrical conductivity when free of ions.
Benzene Benzene Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....		1.1	Toxic, flammable.
Liquid oxygen Liquid oxygen Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...		2.4	Cryogenic. Highly flammable with combustible materials.
Liquid nitrogen Liquid nitrogen Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...		1.6-1.9	Cryogenic. Used as coolant with many low-temperature sensors and high-temperature superconductors.
Liquid hydrogen Liquid hydrogen Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...		1.0	Cryogenic. Flammable.
Liquid helium Liquid helium Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...		0.7	Cryogenic. Used with superconductors.
Liquid argon Argon Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...		1.10-1.42	Cryogenic.