Any item that carries electrical current is covered with some type of insulation.  The insulation restricts the flow of current between different conductors and between conductors and ground.  The insulation system is subject to stress, and contamination over time.  If the insulation system fails it can be very costly.  An IR Test measures the electrical insulation resistance of the system.

The test may be done as a quick check after an item is manufactured, installed or repaired.  It is also a useful test to perform when doing preventative maintenance on an item over time.  Changes in the insulation measurement can help to predict an incipient failure of an item such as a motor, cable or component.

The IR Test is performed with a DC potential.  This DC potential is impressed across the insulation system between circuits and/or ground and the resulting currents are measured.  The meter is usually scaled in Megohms which is the ratio of the DC voltage versus the measured current (R=E/1).  The two most common IR Test's are Polarization Index and Dielectric Absorption.