The insulation resistance for class I appliances should not be less than

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2 Mega ohm Explanation: According to the Indian Electricity (IE) Rules and relevant BIS standards (such as IS 302 for household appliances), the minimum insulation resistance for Class I appliances must be 2 MΩ. This standard ensures that leakage current is kept within safe limits to prevent electric shock and equipment failure. Working Principle: Insulation resistance serves as a barrier against leakage current flow from live parts to the exposed metal enclosure (earth). By maintaining a resistance of at least 2 MΩ, the potential difference across the insulation is controlled, ensuring the leakage current ($I_l = V/R_{ins}$) remains well below the hazardous human sensation threshold under normal operating voltage. Key Formulas: $R_{ins} = \frac{V_{test}}{I_{leak}}$ — The fundamental relation defining insulation resistance where $V_{test}$ is usually 500V DC for standard testing. Key Points: Class I appliances require an earth connection for safety.; The insulation test is conducted using a Megger or insulation tester.; A value of 2 MΩ is the minimum requirement to prevent tracking and breakdown.; Higher resistance values indicate better quality insulation and safety. Advantages: Ensures safety of personnel by preventing electrocution.; Protects against short-circuit faults during equipment operation. Disadvantages: Insulation degrades over time due to thermal stress and humidity.; Difficult to measure in highly contaminated environments. Applications: Testing of industrial motors and household appliances.; Safety auditing for electrical installations in residential buildings. Additional Info: IE Rules mandate periodic testing of electrical installations to ensure the insulation resistance does not fall below specified safe limits.. Option 7 MΩ is unnecessarily high as a minimum standard; 2 MΩ is the industry-standard threshold for Class I portable appliances. Answer: B is correct — The minimum permissible insulation resistance for Class I appliances is 2 MΩ to ensure operational safety. Exam Tip: Always remember that insulation resistance tests are performed with DC voltage (usually 500V) to avoid capacitive charging currents associated with AC testing.

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