The testing of insulation resistance is important in preventing electrical shock, particularly in electrical wire. A number of factors can contribute to insulation failure, including nicks on the wire itself, moisture or extreme heat or cold. The standard tool used for testing insulation values is called a megohmmeter. Megohmmeters are devices that measure high resistances, such as those found in insulated wiring, like sparkplug wires.
Completely remove the insulated wire you are going to test from all power sources.
Connect the two positive and negative leads of the megohmmeter to the two ends of the wire you are going to test.
Turn the megohmmeter to the on position.
Apply the test voltage by pressing the charge button on your megohmmeter for about a minute.
Examine the readings on the screen of your meter. The resistance value of your insulation is measured on your meter in megohms. A resistance value (or Megohmmeter reading) of 999 megohms means that your wire has perfect insulation. If the resistance value falls anywhere below 999 megohms to 1.5 megohms, it means the insulation of your wire is degraded but still functional. A resistance value of 1.5 megohms or below means the wire's insulation is bad and that the wire should be completely replaced.