Mike Dolt Enterprises
Mike Dolt Enterprises
User: HotConductor, Location: Philadelphia
Asked on: Tuesday, March 15, 2011 at 8:54 pm EDT
Answered on: Tuesday, May 14, 2024 at 2:51 am EDT
Question: I meggered two #8 thhn conductors in an underground conduit. I tested them(Fluke 1587)against each other @500v for a one minute test period and they tested fine(550 meggohms). I then tested them each to a 277v good source with my Knopp K60 and they were both grounded. I've had the same thing happen in the past. I tested one conductor at a time with every other conductor connected together and they insulation tested fine. They weren't though.Am I doing something wrong with the insulation test?
Potential Causes and Troubleshooting Steps
Incorrect Grounding
Ensure that the conduit and the Fluke 1587 instrument are properly grounded. Insufficient grounding can lead to false grounding readings.
Damaged Insulation
Despite passing the insulation test, the conductors may have minor damage that allows for small grounding currents. Check for any visible damage or imperfections along the conductor's length.
Test Instrument Error
While unlikely, it's possible that the Fluke 1587 or Knopp K60 instruments may not be calibrated or functioning correctly. Consider using a different insulation tester for verification.
Capacitance Effects
The Knopp K60 is designed to detect low-level grounding currents. Underground conductors may exhibit capacitance effects that can introduce small amounts of current during the test. This can result in false positive grounding readings.*
Electromagnetic Interference (EMI)
External electromagnetic fields can interfere with insulation testing. Ensure that the test is conducted in an environment free from electromagnetic sources such as power lines, transformers, or motors.
Troubleshooting Procedure
1.
Check Grounding
Verify that the conduit and test instruments are properly grounded using a low-resistance ohmmeter.2.
Inspect Conductors
Visually examine the conductors for any damage, cuts, or nicks.3.
Calibrate Instruments
Ensure that both the Fluke 1587 and Knopp K60 instruments are calibrated according to the manufacturer's instructions.4.
Test Individual Conductors
Isolate each conductor and repeat the insulation test with the Fluke 1587. If a conductor fails the test individually, it indicates insulation damage.
5.
Disable Capacitance Effects
If capacitance is suspected, connect a shunt capacitor (typically 1-2 μF) across the conductors during the Knopp K60 test. This will help suppress capacitance effects.6.
Minimize EMI: Conduct the test in an environment with minimal electromagnetic interference. If possible, move the test location away from potential sources of EMI.If the above troubleshooting steps do not resolve the issue, consider consulting with a qualified electrician or testing specialist for further assistance. Flag for review