Surge Tests – Overview

Below is an overview of surge testing. For details on surge test subjects, see the red links on the left side of this page under “BROWSE MORE”.

Why surge tests are critical

Surge tests are critical because it is the only test that can find turn to turn insulation weaknesses. These weaknesses found at voltages above the operating voltage of the devise under test (DUT) are precursors to serious failures and shutdown of a motor. Surge tests are also used to find hard shorts and a number of other mistakes in windings and coils.

Good Surge Test, surge test waves

Three nearly identical waves from a 3-phase motor.

Most winding failures, including shorts to ground, start with weak turn-to-turn insulation. Once the weakness causes arcs turn to turn, an electrical closed loop is created. Due to transformer action, current starts flowing in the loop. This current is dissipated as heat and creates a hotspot. More turns short out because of the hotspot, and subsequently more heat is created and eventually the winding shorts to ground.

Surge tests are sometimes called surge comparison tests since the result from surge testing a coil or phase is compared to the result from another coil or phase. When coils are designed to be identical, the surge test results should be close to the same. In cases where the windings/phases under test are not designed to be identical, or there is nothing to compare to, the Pulse to Pulse surge test is used.

What can be tested by a surge tester?

Just about any type of coil can be tested, from tiny sensors, antennas, and actuating coils in relays or solenoids, to the biggest electric motors and generators. The surge test is a load dependent test, so there are limitations to take into account. These will be explained later on under Test Voltages and also under Low Inductance Surge Tests.

What typical failures can and cannot be detected with a surge test?

Which issues can only be found with a surge test?

Weaknesses and Faults Found With Surge Test

  • It is the only test that can find weak turn to turn insulation because of the higher voltages used in a surge test. Low voltage tests do not stress the insulation, and consequently dielectric strength weaknesses are not found.
  • Weak coil to coil and phase to phase insulation, unless the coils and phases are hipot tested individually against the other coils and phases. Doing such hipot tests are not always practical.
  • Some connection mistakes may only be found with a surge test or inductance test when the resistance is correct.


For a summary of what surge tests are used for and the short version of how it works, see Surge Comparison Test.