How Does a Megohmmeter Test Insulation?

What a Megohmmeter Measures

• A megohmmeter is a portable electrical testing instrument that measures electrical resistance in measurement units called ohms. In principle it is like a home handyman's electric multimeter set to its ohmmeter function. But a megohmmeter generates much higher voltages. Where a standard multimeter uses voltages below 250 volts for testing, a megohmmeter uses voltages that range from 500 volts to as high as 10,000 volts to conduct tests. Further, a megohmmeter measures millions of ohms, whereas a multimeter only measures thousands of ohms. One megohm equals 1 million ohms, and a megohmeter can measure hundreds of megohms.

Insulation Resistance

• A megohmmeter conducts three basic tests of electrical insulation integrity. The first is insulation resistance. In this test, you attach the test leads as appropriate for the equipment being tested and take a reading by applying the test voltage for 60 seconds. During that time, the megohm reading should stay steady or drop slightly. This indicates good insulation. If the reading falls below minimum values set by the International Electrical Testing Association for the equipment being tested or is substantially higher or lower than previous insulation resistance readings taken on the equipment, further investigation is indicated.

Dielectric Absorption

• The dielectric absorption ratio test relies on the principle that good insulation shows increasing resistance over a long span of time. In this test, you apply the test voltage and take a reading after 60 seconds then another reading after 10 minutes. You divide the 10-minute reading by the 60-second reading to find the change ratio. You then compare that value against standard tables of acceptable values for the equipment being tested. If the change ratio figure is markedly different from acceptable values, you need to investigate further. A quick rule-of-thumb test is to take readings at 30 seconds and 60 seconds then divide the 60-second reading by the 30-second reading. If the result is less than 1, the insulation is bad.

Step Voltage

• The step voltage test checks for insulation deterioration due to age or damage. In this test, you first take an insulation resistance reading at 500 volts on the equipment being tested and record that reading. You then repeat the insulation resistance test at 2,500 volts and compare that reading against the 500-volt reading. Good insulation should provide roughly the same resistance readings, regardless of test voltage. If the two readings differ by more than 25 percent, you should suspect failing or damaged insulation.