What's the Difference Between a Megger & Hi-Pot Test?

Dielectic Withstand Test

• "Megger" and "hi-pot" tests both determine insulation resistance, the measure of current leakage in a conductor. A "hi-pot" is used primarily to test the capacity of voltage that insulation can withstand before failing. In a "dielectric withstand test," voltage is applied to a conductor and current leakage is measured over time to determine insulation integrity. The leakage is compared to a limit based on the size of the component being tested. Voltage is determined by using the formula "2 x U + 1,000 volts", with the letter U representing the operating voltage of the conductor or component being tested.

Dielectric Breakdown Test

• A "hi-pot" tester also performs a dielectric breakdown test. In this test, voltage is increased on a conductor or component until the insulation fails. This test is performed primarily for sample or demonstration purposes at the point of manufacture, as it often destroys the component being tested. A "megger" cannot perform the dielectric withstand or breakdown test.

Differences in Voltage and Test Time

• "Megger" and "hi-pot" insulation resistance tests differ in applied voltage and in test duration. "Meggers" test low and medium voltage with a charge between 600 to 2,000 volts over the span of a minute. "Hi-pot" testers apply a much higher voltage in the range of 15,000 volts and above, to a maximum of 300 volts per mil of insulation. "Hi-pot" tests are performed over 15 minutes with readings taken every minute.

Thumping

• "Hi-pot" testers can also be used to detect faults in underground cable with a process called "thumping," in which voltage is applied to create an arc over the gap of the wire that is damaged. The sound of the arc jumping over the gap in the damaged wire makes an audible sound, like a thump, which helps identify the area where the damage is present.