Molly Ayala
Electrical breakdown, also known as dielectric breakdown, is a process that occurs when an electrical insulating material becomes an electrical conductor. This happens when the electric field in the material exceeds a certain threshold, causing a sudden increase in current and a loss of the material's ability to prevent the flow of electric current. This can lead to the failure of electrical devices, short circuits, and electrical discharges such as arcing, sparks, or even lightning. The voltage at which this occurs is called the breakdown voltage and depends on the size and shape of the object, as well as the location on the object where the voltage is applied.
Characteristics and Values of Electrical Breakdown
| Characteristics | Values |
|---|---|
| Definition | A process that occurs when an electrical insulating material, or medium that normally does not conduct electricity, becomes conductive |
| Occurrence | Can occur within solids, liquids, gases, or vacuum |
| Cause | A high enough voltage is applied, causing a sudden large increase in current |
| Impact | Can lead to the failure of electrical devices, short circuits, or electrical discharges such as arcing, sparks, or lightning |
| Insulating Materials | Epoxy resins, mica, paper, glass-fiber backing, polyethylene |
| Dielectric Breakdown | Failure of an insulating material to prevent the flow of current under an applied electrical stress |
| Electric Field Strength | Voltage applied across the material, measured in volts per metre |
| Breakdown Voltage | Voltage at which the insulating object becomes conductive, dependent on size and shape |
| Partial Discharge | Breakdown limited to a small region, such as a crack or bubble in a solid insulator |
| Corona Discharge | Local breakdown process allowing current to leak off a conductor into a gas as ions |
| Specific Mechanisms | Different for each kind of dielectric medium |
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What You'll Learn
Electrical breakdown in solids, liquids, gases, and vacuum
Electrical breakdown, or dielectric breakdown, is a process that occurs when an electrically insulating material (a dielectric) is subjected to a high enough voltage and suddenly becomes a conductor, allowing a current to flow through it. This can occur within solids, liquids, or gases, and even in a vacuum, but the specific breakdown mechanisms differ for each kind of dielectric medium.
In solids, electrical breakdown occurs when the electric field becomes strong enough to pull outer valence electrons away from their atoms, causing them to become mobile. The resulting collisions with other atoms generate heat, which in turn releases additional electrons. This process can lead to a decrease in resistance, making it easier for electricity to flow through the material. Solid insulators, such as ceramic, can experience breakdown starting at local defects, like cracks or bubbles.
For liquids, the specific breakdown mechanism is not clearly defined in the sources provided. However, it is known that electrical breakdown occurs when the electric field caused by an applied voltage exceeds the material's dielectric strength, which is the maximum electric field that the insulating material can withstand.
In gases, electrical breakdown is associated with the growth of ionization. The electric field accelerates the small number of naturally occurring free electrons to high speeds. When these electrons collide with gas molecules, they knock out additional electrons through a process called ionization, creating a chain reaction of more free electrons and ions. This breakdown of the insulating properties of the gas is fundamental to the operation of high-power gas switches.
Although not commonly discussed, electrical breakdown can theoretically occur in a vacuum as well. This indicates that even in the absence of matter, a sufficiently high voltage can lead to electrical conduction.
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Causes of electrical breakdown
Electrical breakdown occurs when an electrical insulating material or medium that normally does not conduct electricity becomes conductive. This happens when a high enough voltage is applied to the insulating substance, causing the electric field in the material to exceed a certain threshold. The specific breakdown mechanisms differ depending on the type of dielectric medium, which can be solids, liquids, or gases.
In solids, electrical breakdown occurs when the electric field becomes strong enough to pull outer valence electrons away from their atoms, making them mobile. The resulting collisions with other atoms release additional electrons in a chain reaction. This process is influenced by the presence of protruding parts, sharp points, and edges, where the electric field is strongest. Additionally, local defects in the material, such as cracks or bubbles, can initiate the breakdown process.
In gases, the electric field accelerates the small number of naturally present free electrons to high speeds. When these electrons collide with gas molecules, they knock out additional electrons through a process called ionization, creating more free electrons and ions. This chain reaction can lead to a continuous electric arc if protective devices fail to interrupt the current.
The breakdown process can also be influenced by the inclusion of certain materials. For example, the incorporation of glass-fiber matting within an epoxy resin has been found to reduce the electric strength of the material, making it more susceptible to electrical breakdown.
Furthermore, the shape of the conductors and the location of voltage application can impact the breakdown voltage. A flat sheet of insulator between two flat metal electrodes, for instance, will have a different breakdown voltage compared to conductors with different shapes.
Understanding the causes of electrical breakdown is crucial to prevent the failure of electrical equipment, short circuits, and fire hazards. By recognizing the factors that contribute to this phenomenon, engineers can design and implement protective measures to ensure the safe operation of electrical systems.
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Electrical breakdown in composites
Electrical breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. This can lead to the failure of electrical devices, short circuits, or electrical discharges such as arcing, sparks, or lightning.
Insulating composites are characterised by a dielectric strength (Eb), which is a measure of the electric field necessary to induce an electrical breakdown. The value of the dielectric strength defines the necessary thickness of the insulation and is a fundamental parameter in electrical design.
The topic of electrical breakdown in composites is technologically important as most insulation in large electrical machines is based on epoxy resins, mica, paper, and glass-fibre backing. Similar systems are used in other high-voltage engineering applications.
The incorporation of glass-fibre matting within an epoxy resin has been found to reduce the electric strength of the material, especially when mechanically deformed. This is due to the poor bonding at interfaces and dielectric mismatches within the system, which can lead to local stress enhancements.
Thermal-aging of epoxy/glass fibre composites can also reduce their electrical breakdown strength. During thermal-aging, the epoxy resin molecular chains undergo continuous oxidation and chain scission, which generates numerous polar functional groups and short chains. This results in an increase in the free volume and enhanced chain segmental dynamics, which significantly reduces the activation energy of the epoxy resin.
In insulating paper, the use of layered construction is important because the paper thickness varies, resulting in a non-homogeneous dielectric strength across its surface. The rough surface of the paper can produce an electric field stress comparable to that of the discharge channel, which can cause breakdown at lower voltages.
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Electrical breakdown in atomic and molecular gases
Electrical breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes a conductor and electric current flows through it. This can occur within solids, liquids, gases, or even in a vacuum. However, the specific breakdown mechanisms differ for each type of dielectric medium.
In its normal state, a gas is almost a perfect insulator. However, under certain conditions, when an electric field is established between two electrodes, the gas can become a conductor. This transition from an insulating to a conducting state is an electrical discharge known as the electrical breakdown of the gas or a spark. The critical potential difference at which this transition occurs is known as the breakdown potential of the gas for that particular electrode system.
The breakdown of atomic gases is dominated by elastic collisions, where the energy transfer from electrons to heavy particles is facilitated by these collisions. On the other hand, molecular gases exhibit vibrational excitation, which can be dominant or at least significant, depending on the temperature of the electrons.
The electrical breakdown in gases can occur through various mechanisms, including spark breakdown in uniform and non-uniform fields, corona discharge, and laser-induced electrical breakdown. Additionally, photo-ionization can lead to electrical breakdown under specific restricted conditions. This involves the interaction of ionization coefficients and atomic cross-sections for photon-molecule interactions, requiring a high penetration of the gas by ionizing photons.
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Electrical breakdown and electrical discharges
Electrical breakdown, also known as dielectric breakdown, is a process that occurs when an electrical insulating material becomes an electrical conductor. This happens when the material is subjected to a high enough voltage, causing a current to flow through it. All insulating materials have what is called a dielectric strength, which is the electric field strength that they can withstand. When an applied voltage exceeds this threshold, the material undergoes a breakdown and becomes conductive.
The voltage at which this occurs is called the breakdown voltage and depends on the size and shape of the object, as well as the location on the object where the voltage is applied. The breakdown process can occur within solids, liquids, gases, or even in a vacuum, but the specific mechanisms differ for each type of dielectric medium. For example, in a solid, the electric field pulls outer valence electrons away from their atoms, while in a gas, the electric field accelerates free electrons to knock additional electrons out of gas molecules in a process called ionization.
Electrical breakdown can lead to several issues, including the failure of electrical devices and short circuits. It can also result in electrical discharges such as arcing, sparks, or lightning. Arcing, specifically, is a continuous electric arc that occurs when protective devices fail to interrupt the current in a power circuit. Partial discharges can also occur, where breakdown remains limited to a small region of the material.
To prevent electrical breakdown, certain measures can be taken, such as the incorporation of barriers like mica to retard the growth of electrical trees. However, poor bonding at interfaces and dielectric mismatches can lead to local stress enhancements that may contribute to electrical breakdown. Understanding the mechanisms and factors influencing electrical breakdown is crucial for the development and maintenance of electrical systems and devices.
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Frequently asked questions
Electrical breakdown is when an insulating material or medium that does not conduct electricity becomes conductive. This happens when the electric field in the material exceeds a certain threshold, causing a breakdown of insulation and a surge in electrical conductivity.
Electrical breakdown is caused when an insulating material is subjected to a high enough voltage, causing a sudden increase in current.
Electrical breakdown can lead to the failure of electrical devices, short circuits, or electrical discharges such as sparks, arcing, or lightning. It can also cause fire hazards.
The breakdown voltage is the voltage at which the insulating object becomes conductive. It depends on the size, shape, and location of the electrical contacts on the object.
Dielectric breakdown is the failure of an insulating material to prevent the flow of current under an applied electrical stress. All insulating materials have a breakdown voltage, and when this is exceeded, the material is no longer insulating and current flows through it.
