Cable Insulation Resistance Testing (Introduction)
Need of Insulation Testing:
Cable Insulation can be nicked or scraped from individual conductors when pulled through raceways. Insulation can also be damaged when exposed to moisture, excessive heat, or certain chemicals. To verify that insulation has not been damaged during the installation process, an insulation resistance test must be performed after installation.
A cable insulation resistance test is a test that is performed by applying 500 V or more between the conductor and ground and measuring the resulting leakage current. An insulation resistance tester is a test instrument that measures current that leaks through the insulation into ground. Insulation resistance tests can be performed with insulation multi- meters (IMMs) or megohmmeters. Insulation resistance testers are calibrated to display megohms (MΩ) of resistance. A satisfactory insulation resistance measurement should be in millions of ohms. Conductors with damaged insulation must be replaced.
When cable and conductor installations are tested, they should be disconnected from panels and machinery to keep them isolated. The conductors should be tested against each other and against ground. See Figure 1. Minimum resistance readings and specific test methods are determined by acceptance testing specifications for the project.
Procedure for Cable Insulation Test:
Cables and conductors should be tested and maintained on a three-year cycle at minimum. Insulation resistance tests should be performed more frequently for systems that show deterioration of insulation material. When performing insulation tests on conductors and cable, apply the following procedure:
- Inspect exposed sections of cables and conductors for physical damage. Replace or repair sections that exhibit damage.
- Inspect cables and conductors for proper grounding, cable support, and termination. Terminate sections that are not properly terminated.
- If cables and conductors are properly terminated, verify that neutrals and grounds are properly terminated for operation of protective devices.
- Perform an insulation resistance test on each conductor in the cable. Apply 1000 VDC for 1 min (one-minute insulation resistance test) to low-voltage cables (1 kV or less) and use a clamp-on ammeter, DMM, megohmmeter, or IMM to measure the insulation resistance.