Electrical isolation test

An electrical isolation test is a Direct current (DC) or Alternating current (AC) resistance test that is performed between sub-circuit common and subsystem chassis to verify that a specified level of isolation resistance is met. Isolation testing may also be conducted between one or more electrical circuits of the same subsystem. The test often reveals problems that occurred during assembly, such as defective/wrong component, improper component placement/orientation and wire insulation or insulator defects that may cause inadvertent shorting or grounding to chassis, in turn, compromising electrical circuit quality and product safety. 
Isolation resistance measurements may be achieved using a high input impedance ohmmeter, digital multimeter (DMM) or current-limited Hipot test instrument. The selected equipment should not over stress sensitive electronic components comprising the subsystem. The test limits should also consider semiconductor components within the subsystem that may be activated by the potentials imposed by each type of test instrumentation. A minimum acceptable resistance value is usually specified (typically in the mega ohm (MΩ) range per circuit tested). Multiple circuits having a common return may be tested simultaneously, provided the minimum allowable resistance value is based on the number of circuits in parallel.
Five basic isolation test configurations exist:
Isolation measurements are made with the assembly or subsystem unpowered and disconnected from any support equipment.