Summary: Dissipation Factor (DF), Capacitance (C), and Dielectric Constant in Cable Systems

1. Minimum Cable Sample Length
For accurate measurement of Dissipation Factor (DF) and Capacitance (C), the cable sample should have a minimum length of **15 feet (4.57 meters)**. Using a shorter sample may result in unreliable measurements due to insufficient signal propagation.

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2. Measurement Procedure

2.1 Instruments and Method
- DF and C are determined at specific frequencies and temperatures using an AC bridge such as:
  - **Schering Bridge**
  - **Transformer-Ratio-Arm Bridge**
- These bridges are used to compare the unknown impedance of the cable with known reference values.
 
2.2 Voltage Application
During measurement, the applied voltage should be:
Where:
- **V_g**: Voltage between conductor and ground in a three-phase system.
- **V_phase**: Rated phase-to-phase voltage of the system.

This ensures that the test simulates real operational conditions for the cable.

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3. Capacitance Calculation

The measured capacitance should be normalized to **picofarads per foot (or meter)** by adjusting for the length of the sample.

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4. Geometric Capacitance (C₀) Calculation

The **equivalent geometric capacitance (C₀)** can be calculated using the formula:


Where:
- **C₀**: Equivalent geometric capacitance in picofarads per foot.
- **D**: Diameter over the insulation in inches.
- **d**: Diameter over the conductor stress control layer (or conductor itself if no stress control layer is present), in inches.

This formula provides the ideal capacitance based solely on the cable's geometry.

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5. Dielectric Constant Calculation

The **dielectric constant** of the insulation material is determined by taking the ratio of the measured capacitance (C) to the geometric capacitance (C₀):


This ratio helps assess how well the insulating material stores electrical energy relative to a vacuum. A higher dielectric constant usually indicates better insulation but may also imply higher energy losses.

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6. Summary of Key Concepts
- **Dissipation Factor (DF)** measures energy loss in the insulation.
- **Capacitance (C)** reflects the cable's ability to store charge.
- **Dielectric Constant** evaluates the insulating efficiency of the cable material.
These parameters are essential for assessing cable performance and ensuring reliability in electrical systems.