What Is It Used For?

The surge comparison test is used to find shorts and insulation weaknesses in coils, windings, electric motors, generators, alternators and transformers. These faults are typically turn to turn, coil to coil, or phase to phase. Other problems found include wrong internal connections, wrong turn counts, and more for DC motors.

Weak turn to turn insulation is the start of most winding failures. Because it is the only test that can find weak insulation at elevated voltages, the surge comparison test is important for motor reliability and maintenance programs, for diagnosing problems and for quality control. It is an especially powerful tool in combination with Partial Discharge measurements.

Surge tests are done on individual coils, stators, wound rotors and fully assembled motors and generators without the need to turn the rotor.

How Surge Comparison Tests Work


Good surge test.

A set of fast rising pulses are passed through the coil or motor (the devise under test – DUT).  The voltage of the surge test pulses depends on what voltage the test operator sets, or the standard that is used. The test voltage can range from the peak operating voltage of the DUT to around 3.5 times the operating voltage of the DUT.  2E+1000V where E is the operating RMS voltage of the DUT is most common.


Failed surge test.

The surge pulses produce a decaying wave in the circuit that includes the tester and the DUT (see picture). The wave is compared to the wave from another coil or to the waves from the other motor phases. All are displayed on the screen.

The waves will be nearly identical if the coils or windings are identical. If one has a fault or insulation weakness, the wave will have a different frequency from the others and separate from them. The iTIG II testers will calculate the wave differences (%WD), also called %EAR or Error Area Ratio).

For more details on surge comparison tests, see Surge Tests under Test Technologies.

Pulse to Pulse Surge Test

The pulse to pulse surge test is used in applications where there are normal differences in the surge waves, but when the tolerance for Pass/Fail is unknown such as with certain assembled motors and many concentric wound stators. It is also used when there are no other coils/phases to compare to.

For more information on Pulse to Pulse Surge Tests and Surge Pass/Fail recommendations, see the Test Technology – Surge Tests section.

Are Surge Tests Destructive?

Surge comparison tests are most often done at a voltage higher than the peak operating voltage of the DUT and therefore referred to as over-voltage tests. They are not destructive. The main reasons for this are that the test voltage is far below the design voltage of the insulation, and the energy involved in an arc is low. A good analogy is an arc from your finger to a door handle as a result of static electricity. The voltage involved can be in the 12kV to 20kV range, but the energy is low and thus not deadly.

An arc in a winding from a surge test has a low energy given by the discharge capacitor in the tester and the voltage applied, and it will not damage the insulation as long as the number of pulses used in the surge comparison test is limited, and the test is done under conditions when over-voltage tests are recommended.

The Electrom iTIG II series of motor testers come with Surge Guard™ and Quick Surge™ technology to eliminate damage from arcs during a surge test. Click here for more details on surge tests and proper conditions for over-voltage testing.