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Insulation Resistance Testing

Insulation Resistance Testing. Resistance 101. Resistance is the opposition to current flow Resistance is used to lower the amount of current and/or voltage flowing through a circuit Unit of resistance is the ohm Ohms symbol is omega: . What is megohm testing?.

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Insulation Resistance Testing

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  1. Insulation Resistance Testing

  2. Resistance 101 • Resistance is the opposition to current flow • Resistance is used to lower the amount of current and/or voltage flowing through a circuit • Unit of resistance is the ohm • Ohms symbol is omega: 

  3. What is megohm testing? • It’s testing for electrical insulation failure under controlled conditions • Used to determine the integrity of • Cables • Windings in motors and transformers • Switching gear • Electrical installations • Similar to pressure-testing a water pipe

  4. Why different instruments read different ohms values • Ohmmeter (DMM) • Low voltage (~1-2V). • Low current (~1-10mA) . • MegOhmMeter • High voltages to stress the insulation. • Insulation Resistance Test (IRT). • Low Ohms Tester • Relatively high current (>200 mA). • Greater accuracy for low-resistance devices or conductors.

  5. Leakage current basics • All insulators leak because all resistances conduct some current, however small • High voltages produce currents in insulators • The amount of current depends on • Applied voltage (Ohm’s Law) • System capacitance • Total resistance • Temperature of material

  6. Components of leakage current • Polarization absorption leakage (IA) • Conductive leakage (IL) • Capacitive charging leakage (Ic)

  7. Measuring insulation resistance • Customary: between two conductors “insulated” from each other • Worst case: tie one conductor to the conduit

  8. Measuring insulation resistance Insulation resistance test Insulation resistance test over time for a motor

  9. Be careful: I bite! • A 5 KV tester = up to 5200V DC output • Nonlethal, but potent • Limited to 2mA

  10. Installation testing • Performed on these conductors • De-energized current-carrying (live) • Grounded • Grounding • When • Construction and maintenance • After initial installation • Why • #1 reason: Safety • Establish baseline

  11. Maintenance testing Reasons for regular monitoring/testing • Predicts and identifies imminent failure • Prevents disaster • Lowers total cost of ownership Performed on critical conductors

  12. Common voltages for testing

  13. Testing procedure To ensure proper installation and integrity: • Verify supply power is isolated/disconnected from system under test. • Select the appropriate voltage level. • Connect leads. • Take reading. • Allow discharge of test voltage • Test each conductor.

  14. Using test results • Keep records • Analyze readings • Resolve issues • Monitor suspect cables

  15. Spot-reading/Short-time test • Connection time: 60 seconds • Applies to low-capacitance equipment • Steady increase in resistance • One Megohm Rule of Thumb • Trending rates of decrease

  16. Step voltage test Resistance testing at discrete voltage settings • Connection time: 60 seconds • When: After spot test • Looking for inconsistencies • Flaws show at steps

  17. Step voltage test, good results

  18. Step voltage test failure

  19. Dielectric-absorption /time-resistance test Absorption characteristics: contaminated vs. good. • During first minute apply voltage every 10 seconds • Apply once a minute for 9 remaining minutes

  20. Generator, motor, or transformer windings Check: • Winding to winding • Winding to ground • Phase to phase

  21. Generator or motor windings • Disconnect stator winding and phases • Raise brushes for DC

  22. Stop the presses! Newspaper printing press example • Initially:>500 M. • After a day or two: 10 M. • At about 1 M: rebuild the motor.

  23. Low ohms testing Connection quality check • Measure across connection • Typically less than 1 ohm • Requires lead compensation

  24. Electrical contractors IR test applications: • Leakage • Cable insulation integrity • Windings and switches • Connections, fuses, breakers, and switches

  25. Motor technicians IR test applications: • Leakage between windings • Leakage between windings and motor frame • When to test?

  26. Plant maintenance IR test applications: • Wiring and cable • Contacts and connections • Grounding system integrity

  27. Other users • Electrical Utility Workers • Low-voltage equipment • High voltage equipment • Elevator Technicians • Motors • Wiring • Controls

  28. Final tips • Use the right tester • Follow the procedures • Analyze the results • Use proper trending • Take corrective action

  29. 1520/1550 MegOhmMeteroptional hands-on exercises • Basic checks • Normal resistance test • Low resistance test • High resistance test

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