HV test should be conducted if the insulation resistance is ____________ the value defined in IS 472

Question

HV test should be conducted if the insulation resistance is ____________ the value defined in IS 4722.

Examoogle · Accepted Answer

greater than

Explanation: According to IS 4722, a High Voltage (HV) test is a dielectric withstand test used to verify the integrity of the insulation system of rotating electrical machines. This test should only be performed after the insulation resistance (IR) has been measured and confirmed to be above the minimum permissible value defined by the standard to prevent damage to faulty or moisture-laden insulation during the high-voltage stress.

Working Principle: The insulation resistance test (Megger test) is a non-destructive check that ensures the insulation is dry and free from surface contamination. Applying high voltage to insulation with low resistance (e.g., due to moisture or physical breakdown) can cause destructive arcing or puncture. Therefore, the IR measurement serves as a prerequisite safety gate before subjecting the machine windings to the stress of the HV proof test.

Key Formulas: $R_{IR} \ge \frac{V_{rated(kV)} + 1}{1} M\Omega$ — Minimum IR value requirement (Approximate for many machines as per IS standards)

Key Points: IS 4722 regulates the specifications for rotating electrical machines.; IR test is mandatory to prevent breakdown during high-potential testing.; HV test validates the machine's ability to withstand transient overvoltages.; The HV test voltage is generally defined as $(2 	imes V_{rated} + 1000)V$ for a duration of one minute.

Advantages: Ensures machine safety before applying high voltage stress.; Identifies insulation degradation early.; Prevents catastrophic failure during commissioning.

Disadvantages: HV test can potentially weaken 'borderline' insulation if repeated excessively.; Requires specialized high-voltage test equipment.

Applications: Routine testing of AC and DC motors.; Commissioning of generators and large industrial alternators.; Post-repair quality assurance of electrical windings.

Additional Info: If the IR value is found to be below the limit, the machine must be dried out or cleaned before performing the HV test to avoid winding puncture.. Option B (less than) is incorrect because performing an HV test on low-resistance insulation usually results in a permanent short-circuit or insulation failure.

Answer: A is correct — HV testing is a proof test that should only be performed after verifying that the insulation is dry and has sufficient resistance (exceeding the IS 4722 limit).

Exam Tip: Always remember that in electrical testing, non-destructive tests (IR, Polarization Index) must precede destructive or high-stress tests (HV, Surge comparison) to safeguard the equipment.

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