How to measure insulation resistance correctly - Solutions - Huaqiang Electronic Network

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Electrical safety is a critical concern during the use of electrical equipment, including motors, cables, and household appliances. One key factor in ensuring safe operation is the insulation resistance of insulating materials. When exposed to heat or moisture, insulation can degrade, reducing its resistance and potentially leading to leakage currents or short circuits. To prevent such incidents, it's essential to regularly measure the insulation resistance of all electrical devices to ensure they meet required safety standards.

Common methods for measuring insulation resistance involve low-voltage and high-voltage testing. However, low-voltage measurements often fail to reflect the true resistance values under operating conditions. A megohmmeter, also known as an insulation resistance tester, is specifically designed for this purpose. It uses a high-voltage power supply, making it more accurate than standard multimeters for insulation checks. Despite its reliability, improper use can lead to measurement errors, so following correct procedures is crucial.

Before using a megohmmeter, several precautions should be taken:

• Power off the device: Ensure the equipment being tested is completely disconnected from power and grounded to avoid electric shock or damage.
• Eliminate induced voltage: If the device may generate high voltage, make sure it’s fully discharged before testing.
• Clean the surface: Remove dirt or moisture from the test object to reduce contact resistance and improve accuracy.
• Check the megohmmeter: Verify that the meter is functioning properly by checking the "0" and "∞" positions when short-circuited and open.
• Stable placement: Keep the megohmmeter away from strong magnetic fields and external currents for consistent readings.

The megohmmeter typically has three terminals: "L" (line), "E" (earth), and "G" (guard). For most insulation tests, connect "L" to the conductor and "E" to ground. However, if the surface of the insulation is contaminated or moist, the "G" terminal should be used to bypass surface leakage current, ensuring accurate results. This is especially important when testing cables, where humidity or dirt can cause significant surface leakage.

Correct wiring is vital. The "L" terminal should always be connected to the live part of the device, while "E" is grounded to the casing. Reversing these connections can introduce errors, as the leakage current may flow through the wrong path, affecting the measurement. Additionally, the "E" terminal has lower insulation compared to "L," so reversing them could lead to inaccurate readings due to parallel resistance effects.

In summary, proper use of the megohmmeter is essential for accurate and reliable insulation resistance measurements. Any mistake in setup or procedure can compromise the safety and performance of electrical systems.

1. It is recommended to keep all three copies of test records to ensure full documentation. Archiving only the final engineering test results might not provide a complete picture, as it may only show the grounding resistance of the entire lightning protection system at that stage. Without detailed records, it’s hard to identify issues between the lightning protection system, down conductors, and foundation grounding.

2. Insulation resistance testing should be conducted after wiring and joint completion, and after electrical appliances are installed to avoid damage. For cables, testing should be done both before and after installation to ensure integrity throughout the process.