Question

Define dielectric failure of electrical insulating material.

Answer 1

Definition: Dielectric Failure

Dielectric failure, also known as dielectric breakdown or insulation failure, is the event where an electrical insulating material, subjected to a sufficiently high voltage, abruptly loses its insulating properties and begins to conduct electricity. At this point, the material is no longer an effective insulator and a disruptive electrical discharge (a spark or arc) passes through it.

In simpler terms, it is the point at which an insulator "breaks" and becomes a conductor.

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How and Why It Happens

1. Normal State of an Insulator: An electrical insulator (a dielectric material) works because its electrons are tightly bound to their atoms. Unlike conductors, these electrons are not free to move and carry a current when a normal voltage is applied.

2. Applying Electrical Stress: When a voltage is applied across an insulator, it creates an electric field. This field exerts a force on the bound electrons, pulling them in one direction and the atomic nuclei in the other. This stretching or polarization is called electrical stress.

3. The Breakdown Point: As the voltage increases, the electric field becomes stronger, and the stress on the material's atoms intensifies. There is a critical point, unique to each material, where the electric field is so strong that it rips electrons away from their atoms.

4. The Avalanche Effect: Once a few electrons are freed, they are rapidly accelerated by the powerful electric field. These high-energy electrons collide with other atoms, knocking more electrons free. This creates a cascade or "avalanche" of free electrons, forming a conductive path through the material.

5. Failure: This newly formed conductive channel allows a large current to flow, resulting in an arc or spark. The material has now failed. This failure is almost always permanent and results in physical damage, such as a puncture, charring, or melting.

Analogy: Think of a dam holding back water. The water pressure is the voltage, and the dam is the insulator. The dam can withstand a certain amount of pressure. If the water level (voltage) gets too high, the pressure will eventually break the dam, and water will rush through. This breach is the dielectric failure.

Key Associated Term: Dielectric Strength

The property that quantifies a material's ability to resist dielectric failure is its dielectric strength.

• Definition: Dielectric strength is the maximum electric field that a material can withstand without breaking down.
• Units: It is typically measured in volts per unit of thickness, such as kilovolts per millimeter (kV/mm) or volts per mil (a mil is a thousandth of an inch).
• Example: If a plastic has a dielectric strength of 20 kV/mm, a 1 mm thick sheet of that plastic can theoretically withstand 20,000 volts before it fails. A 2 mm thick sheet could withstand 40,000 volts.

Common Causes of Dielectric Failure in Practice

While exceeding the material's inherent dielectric strength is the fundamental cause, failure in real-world applications is often accelerated by other factors:

• Overvoltage: A sudden voltage spike from a lightning strike or switching surge that exceeds the material's rating.
• Aging and Degradation: Over time, exposure to heat, UV radiation, and chemicals can weaken the material's molecular structure, lowering its dielectric strength.
• Contamination: The presence of moisture, dust, oil, or other conductive contaminants on the surface of an insulator can create a path for current to flow (known as a "flashover"), leading to failure at a much lower voltage.
• Physical Damage: Cracks, voids, pinholes, or scratches in the insulation create points of high electrical stress, which can become the starting point for a breakdown.
• Thermal Stress: Excessive heat can cause the insulating material to degrade chemically and physically, reducing its ability to insulate.

Consequences of Dielectric Failure

• Short Circuit: Creates a direct path for current to flow, often bypassing the intended load.
• Arcing and Flashover: A visible, high-temperature plasma discharge.
• Permanent Damage: The insulator is typically punctured, carbonized (charred), or melted, rendering it useless.
• Equipment Damage: Can lead to catastrophic failure of transformers, motors, cables, and other electrical equipment.
• Safety Hazards: Poses a significant risk of fire and electric shock.