Factor affecting deterioration of insulation resistance of insulating materials?

Question

Examoogle · Accepted Answer

Option D: All of these

Explanation & Concepts:
Insulation resistance (IR) is a measure of the health of dielectric materials used in electrical systems. Over time, several environmental, mechanical, and electrical factors degrade the chemical structure and physical integrity of the insulation, leading to a decrease in its resistance to current leakage.

Working Principle: 1. Ageing: Thermal degradation causes oxidation and cracking of polymers. 2. Over-voltage: High voltage transients cause partial discharge and corona, creating carbon paths (treeing) in the material. 3. Grease/Oil Contamination: Foreign particles reduce surface resistivity, allowing moisture and contaminants to penetrate the material, significantly increasing leakage current.

Key Formulas: $R_{ins} = \rho \frac{l}{A}$ — Relation between resistance, resistivity, length, and cross-section; $I_{leakage} = \frac{V}{R_{ins}}$ — Relationship showing that lower $R_{ins}$ leads to higher leakage current

Key Points: The insulation resistance value is usually measured in MΩ using a Megger.; Ageing is primarily a thermal process accelerated by operating temperatures.; Contamination by grease or conductive dust lowers the surface resistance, leading to tracking.; Dielectric stress beyond the rated level causes irreversible breakdown of the insulation molecular bonds.

Advantages: Timely testing prevents catastrophic failure; Predictive maintenance allows for replacement planning

Disadvantages: Tests cannot always simulate multi-stress conditions; Surface conditions may mask internal insulation health

Applications: Testing cables and busbars; Transformer winding assessment; Electrical motor insulation monitoring

Additional Info: As insulation deteriorates, the leakage current increases and the insulation resistance value drops according to the Inverse Square Law of leakage.. Option C is specifically problematic because grease is often hygroscopic and traps dust, creating low-resistance conductive paths.

Answer: D is correct — The deterioration of insulation resistance is a cumulative effect caused by a combination of thermal ageing, electrical stress, and contamination by foreign substances.

Exam Tip: Always remember that for electrical insulation, 'clean, cool, and dry' is the golden rule; any violation of these conditions leads to reduced insulation resistance.

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Option A: Ageing

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Option B: Over voltage

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Option C: Absorption of grease by material

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