Testing leads play a crucial role in ensuring the accuracy of insulation resistance tests with a Megger instrument.
Additional Notes About Using a Megger Insulation Tester:
Here are some important considerations, engineers must take:
- Inferior Leads: Using low-quality or defective testing leads can lead to erroneous and misleading test results. Always use reliable and well-maintained leads.
- Uninsulated Leads: To prevent errors due to lead insulation, position the Megger instrument near the ungrounded terminal or conductor of the equipment under test. Connect a short piece of light bare wire directly from the Line terminal of the instrument to the equipment. This method eliminates the need to worry about the insulation quality of the Earth or Ground lead.
- Insulated Leads: If you rely on lead insulation, ensure that the leads are durable and made of high-quality insulating material. Recommended leads are oil-resistant, synthetic, rubber-insulated, single-conductor No. 14 stranded wire. They should have a smooth outer jacket with no outer braid. Use lugs for attaching to the instrument terminals and sturdy spring clips for connecting to the equipment or circuit being tested. Avoid joints in the leads.
- Leakage Checks: Before connecting the leads to the equipment, check for any leakage between the leads. Operate the instrument, and it should read Infinity, indicating no leakage. Touch the test ends of the leads together to verify that they are not disconnected or broken.
- Current Testing: When using high-range Megger insulation testers (50,000 megohms), ensure that the Line test lead maintains a high value to prevent it from affecting the measurement. The shielded test lead, with the shield connected to Guard, prevents any leakages over its terminations or through the lead insulating material from being measured.
- Lead Connection: Connect the untagged end of the shielded lead to the Line and Guard terminals of the Megger instrument. The Line end terminal should connect to Line, and the side (shield) terminal should connect to Guard. Attach the clip on the Line lead to the equipment under test. Connect the outboard Guard terminal to the part of the equipment you wish to guard. Ensure that the conductor used for this connection is insulated for the voltage rating of the Megger instrument.
- Capacitance: Capacitance in the equipment under test must be charged up to the rated DC voltage of the Megger insulation tester and maintained for 30 to 60 seconds before taking a final reading. Before connecting the test leads, make sure to discharge the capacitance by short-circuiting and grounding the apparatus. Capacitance can cause the pointer to swing towards zero as the instrument powers up and off-scale beyond infinity as it slows down. This is due to the charge flowing into and out of the capacitance and through the instrument’s ohmmeter coil.
- Capacitance Effects: Capacitance effects are most noticeable in large generators, long power and communication cables, and capacitors. They are generally small for capacitance values below 0.01 F but become more noticeable with increasing capacitance and instrument sensitivity. Heavy-duty Megger insulation testers are suitable for testing large capacitors, especially when operated from a power line rather than hand-cranked.
- Operating Time: The time it takes for insulation resistance readings to reach a maximum is an important factor. Charging the geometric capacitance is quick, typically taking only a few seconds. However, the dielectric absorption effect can cause further delays, taking minutes or even hours to reach an absolute maximum. For short-time readings, operate the instrument for a specified duration (e.g., 30 seconds or 1 minute) and read at the end of that time. Use the same operating time for future tests.
- Time-Resistance Method: With hand-cranked instruments, operate continuously for 1 minute. Take a reading at the end of the first 30 seconds and another at the end of the minute. For motor-driven or rectifier-operated instruments, the time intervals are typically 1 minute and 10 minutes from the time the testing voltage is applied. Longer periods, such as 10 to 30 minutes, may be used for time-resistance curves.
- Voltage Scales: Some insulation testers may come with a voltage scale to check for the absence of voltage before insulation testing. However, it’s essential to follow safety precautions and never connect insulation testers to energized lines or equipment when operating in any Insulation Test or Resistance Test modes.
Ensure you carefully follow these instructions to obtain accurate insulation resistance measurements using your Megger instrument.
Related Articles:
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What Makes Insulation Go Bad?
How Insulation Resistance is Measured.
How to Interpret Resistance Readings.
Factors Affecting Insulation Resistance Readings.
Types of Insulation Resistance Tests.
Test Voltage vs. Equipment Rating.
AC Testing vs. DC.
Use of DC Dielectric Test Set.
Tests During Drying out of Equipment.
Effect of Temperature on insulation Resistance.
Effects of humidity.
Preparation of Apparatus to test.
Safety Precautions.
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Interpretation-Minimum Values.
Minimum Values for Insulation Resistance.
Tests Using Multi-Voltage Megger Insulation Testers.
Step-Voltage Method.
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