What's involved in testing insulation with a megger. I'm not looking for an instruction manual, but approx. method and time to test 20 residential circuits. Also any preferred equipment?
Thanks
Dave
Dave: Here's what is required based upon the NEC including the commentary from the 2005 NECH.
110.7 Insulation Integrity
Completed wiring installations shall be free from short circuits and from grounds other than as required or permitted in Article 250. Commentary from NECH
Insulation is the material that prevents the flow of electricity between points of different potential in an electrical system.
Failure of the insulation system is one of the most common causes of problems in electrical installations, in both high-voltage and low-voltage systems.
Insulation tests are performed on new or existing installations to determine the quality or condition of the insulation of conductors and equipment. The principal causes of insulation failures are heat, moisture, dirt, and physical damage (abrasion or nicks) occurring during and after installation. Insulation can also fail due to chemical attack, sunlight, and excessive voltage stresses.
Insulation integrity must be maintained during overcurrent conditions. Overcurrent protective devices must be selected and coordinated using tables of insulation thermal-withstand ability to ensure that the damage point of an insulated conductor is never reached. These tables, entitled "Allowable Short-Circuit Currents for Insulated Copper (or Aluminum) Conductors," are contained in the Insulated Cable Engineers Association's publication ICEA P-32-382. See 110.10 for other circuit components.
In an insulation resistance test, a voltage ranging from 100 to 5000 (usually 500 to 1000 volts for systems of 600 volts or less), supplied from a source of constant potential, is applied across the insulation.
A megohmmeter is usually the potential source, and it indicates the insulation resistance directly on a scale calibrated in megohms (M). The quality of the insulation is evaluated based on the level of the insulation resistance.
The insulation resistance of many types of insulation varies with temperature, so the field data obtained should be corrected to the standard temperature for the class of equipment being tested. The megohm value of insulation resistance obtained is inversely proportional to the volume of insulation tested. For example, a cable 1000 ft long would be expected to have one-tenth the insulation resistance of a cable 100 ft long, if all other conditions are identical.
The insulation resistance test is relatively easy to perform and is useful on all types and classes of electrical equipment. Its main value lies in the charting of data from periodic tests, corrected for temperature, over a long period so that deteriorative trends can be detected.
Manuals on this subject are available from instrument manufacturers.
Thorough knowledge in the use of insulation testers is essential if the test results are to be meaningful.
