Insulation resistance testing is a critical procedure to assess the health of electrical equipment. Proper preparation is key to obtaining accurate results and ensuring the safety of personnel involved in the testing process. Here are essential steps for preparing the apparatus and conducting insulation resistance tests:
1. Take Out of Service:
- Begin by shutting down the apparatus that needs testing.
- Open switches, de-energize the equipment, and disconnect it from other circuits and equipment.
- Ensure that neutral and protective (workmen’s temporary) ground connections are also disconnected.
- Safety precautions should always be followed to minimize electric shock hazards.
2. Determine What’s Included in the Test:
- Carefully inspect the installation to identify all connected equipment and components that will be part of the test.
- Pay special attention to conductors leading away from the installation.
- Understanding what is included in the test is crucial, as the insulation resistance of associated equipment can affect the readings.
- Sometimes, separating components for individual testing may be necessary to obtain accurate results.
- Initially, separate components, record their insulation resistance values, and test them together to establish a baseline.
3. Discharge Capacitance:
- Capacitance must be properly discharged both before and after conducting an insulation resistance test.
- The discharge duration should be approximately four times the duration of the previous test’s applied test voltage.
- Many Megger instruments have built-in discharge circuits for this purpose.
- If not, use a discharge stick to ensure safe discharge.
- High-capacitance equipment should remain short-circuited until ready for re-energization.
4. Address Current Leakage at Switches:
- Verify that current leakage over or through switches, fuse blocks, or other components does not affect the insulation resistance readings.
- Such leakage can obscure the true insulation resistance of the apparatus.
- Take note of any leakage issues that may impact the accuracy of the test results.
- Ensure that all capacitance is discharged before connecting the Megger instrument.
- Never connect a Megger insulation tester to energized lines or equipment, and use it only for its intended purpose.
Proper preparation is essential for conducting insulation resistance tests effectively and safely. It involves de-energizing equipment, understanding what’s included in the test, discharging capacitance, and addressing current leakage issues. Following these steps ensures accurate results and helps maintain the reliability and safety of electrical equipment. Always prioritize safety and adhere to established procedures during insulation resistance testing.
Related Articles:
What is Good insulation?
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.
Safety Precautions.
Connections for testing insulation resistance of electrical equipment.
Additional Notes About using A Megger Insulation Tester.
Interpretation-Minimum Values.
Minimum Values for Insulation Resistance.
Tests Using Multi-Voltage Megger Insulation Testers.
Step-Voltage Method.
Use of a Guard Terminal.
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