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A megger is an electrical instrument that is used to measure the resistance of insulation by applying a DC voltage to the conductor. One lead is connected to the conductor and the other lead is connected to ground. Safe insulation reading to ground should be above 2,000,000 ohms or 2 megs.
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Why digital multimeter fails to read low resistance values?
They read low resistance values very well. I measured several dead shorts today accurately with a digital voltmeter. However, when attempting to measure LARGE amount of resistance, the current generator in a standard digital voltmeter may fail to effectively penetrate the medium being measured. It is for this reason that Meggers (Mega-Ohmmeters) are used to measure high amounts of resistance. They usually employ a hand crank to develop the current needed to effectively flow through the medium being measured. Sometimes digital voltmeters can be very inaccurate if they are measuring something that matches the impedance of the meter (loading effects of the voltmeter), but that is another story.
What is megger and how does it work?
Purpose: The megger tests the integrity of the insulation between conductors that run side by side and do not physically meet or connect. Meggers or Mega-ohm testers also are typically used to test the dielectric in coaxial cable. Meggers are used to test the insulation of buried cables by placing a lead to earth ground and one lead to the outer conductor of a coax cable or any or all conductors of multiple (more than one) buried wires. Method: introduce a high enough voltage to the conductors eventually the current will travel betwen the conductors. Read the indicator to see the amount of resistence to current flow on the meter indicator. the reading will generally be in millions of Ohms (megohms)
What is the standard reading in megger testing to conclude that the equipment is passed and or failed?
Megger testing, also known as insulation resistance testing, is used to measure the electrical insulation resistance of equipment. The standard readings to determine whether equipment has passed or failed depend on the type of equipment, the voltage rating, and industry standards or regulations. However, general guidelines can be provided. General Guidelines for Insulation Resistance 1. Low Voltage Equipment (Up to 1000V): o Minimum Insulation Resistance: Typically, a minimum of 1 MΩ per kV of operating voltage is considered acceptable. For example, for a 240V system, a minimum reading of 0.24 MΩ might be acceptable, but higher values are generally expected. o Preferred Insulation Resistance: Values greater than 1 MΩ are generally preferred, and values in the range of tens to hundreds of megaohms are considered very good. 2. Medium and High Voltage Equipment (Above 1000V): o Minimum Insulation Resistance: Standards often require a minimum of 1 MΩ per kV of operating voltage plus 1 MΩ. For example, for a 10 kV system, the minimum insulation resistance would be 10 MΩ + 1 MΩ = 11 MΩ. o Preferred Insulation Resistance: For high voltage equipment, values much higher than the minimum are typically expected. Readings in the range of hundreds to thousands of megaohms are common for good insulation. Industry Standards and Practices 1. IEEE Standards: o IEEE Standard 43 recommends a minimum insulation resistance of 1 MΩ for equipment rated below 1 kV and 100 MΩ for equipment rated above 1 kV. 2. IEC Standards: o IEC 60034-1 recommends that the insulation resistance of rotating electrical machines be at least R=kV+1R = kV + 1R=kV+1 MΩ, where kVkVkV is the rated voltage of the equipment. 3. Other Industry Practices: o Some industries may have more stringent requirements based on safety, environmental conditions, or criticality of the equipment. For example, oil and gas, mining, and critical infrastructure might require higher minimum values. Pass/Fail Criteria • Pass: If the insulation resistance reading meets or exceeds the minimum required value specified by the relevant standard or guideline, the equipment is generally considered to have passed the test. • Fail: If the insulation resistance reading is below the minimum required value, the equipment is considered to have failed the test, indicating potential insulation degradation, moisture ingress, or other insulation issues. Conclusion The specific pass/fail criteria can vary, so it's important to refer to the relevant industry standards, manufacturer's recommendations, and specific application requirements when interpreting megger test results. Generally, higher insulation resistance values indicate better insulation health and lower risk of electrical failures.
