For more detailed procedures and testing examples and testing forms please download the PDF below.
Insulation resistance testing is a non-destructive test procedure. The test measures the insulation resistance between the phases and/or between phase and ground. It is commonly used in the industry for acceptance testing prior to energizing the cable and for maintenance testing programs.
General Testing Information
• For single conductor non-shielded cable on a reel, insulation resistance testing cannot be performed due to the fact that low voltage single conductors do not have a grounding conductor, shield or ground plane.
• For other cable on a reel, insulation resistance testing can be performed provided the sealing caps are removed. The procedure to test these cables is outlined below.
• NOTE: It is important to remove sealing caps from both ends of the cable to be tested. Residue inside the sealing cap can be conductive and lead to false readings.
• DC Insulation Tester (Megger)
• Before conducting tests on cables, verify circuits are de-energized. Follow all safety practices according to NFPA 70E and NESC C2-2012.
• Follow insulation tester instructions and guidelines.
• Only qualified persons should perform this testing.
• Ensure that test equipment is in good working order and has been calibrated within the last year.
• Clean up the area. Good housekeeping and environmental conditions are important since insulation resistance readings are affected by dirt, moisture and heat.
• Disconnect cables from equipment on both ends.
• Clean cable ends/lugs with appropriate cleaning solution.
• If lugs or accessories are not being used, cutback the cable as described below.
• On the end not being connected to the test equipment, make sure sufficient clearance (six inches or more) is maintained between cable ends and the ground plane.
For 600–5,000 Volt Non-Shielded Cable
• Measure 0.5” from cable end.
• Make a 360° ring cut on insulation without cutting into the conductor.
• Make a longitude cut on insulation from the ring cut to cable end.
• Remove the cables insulation from the end of the cable.
• Clean the cable end to remove dirt and contaminates that may be present.
For 2,400 - 35,000 Volt Shielded Cable
• Mark the cable jacket 8" from cable end.
• At this location, make a 360° ring cut on cable jacket. Next make a longitude cut on the jacket from ring cut to cable end. Remove this section of the jacket, taking care not to nick tape shield below.
• Measure 1” from the cable jacket toward cable end and apply a constant force spring at that mark, remove the tape shield by using the spring as a guide. Tear the tape off against the constant force spring.
• Measure 2” from cable jacket to the semicon and apply a constant force spring at that mark. Make a 360° ring cut
at this location. Now make two to three longitude cuts from the ring cut to cable end, taking care not nick insulation
below. Remove the semi-con.
• Measure and mark the insulation 0.5” from cable end. Apply a constant force spring at the mark. Make a 360°
ring cut at the mark, then make a longitude cut from the ring cut to cable end. Be careful not nick conductor below.
Remove the insulation.