IR Testing: How and What is Measured
In routine testing direct current (DC) voltage is normally used, as with a megohmmeter. However continuous monitoring with an IR tester, such as an insulation monitoring device (IMD), usually utilises some form of alternating current (AC) waveform. This is because DC measurements are susceptible to interference from noise or stray currents. The testing discussed herein predominantly concerns DC voltage testing.
When a voltage E (Volts, V) is applied between a metal conductor and earth, current flows through the cable insulation. This is known as leakage current. However, the current measured by an IR tester is the total current which includes capacitive and absorption currents. Capacitive current is associated with the charging of a capacitor. A capacitor consists of two conductive mediums separated by a dielectric material (an insulator), such as an insulated copper cable submerged in seawater. When a DC voltage is applied between the two conductors separated by an insulator, a build up of equal and opposite charge at the facing conductor surfaces occurs. The dielectric on the other hand develops an electric field with which molecular dipoles in the dielectric align. It is this movement of chargecarriers to/from the surface of the conductors which produces capacitive leakage current. Absorption currents stem from the movement of dipoles within the insulator due to the presence of an electric field.
So, how do capacitive and absorptive currents affect an IR test? At the start of a DC voltage IR test, capacitive, absorption and leakage currents are present. Capacitive current dominates at the start and is typically much larger than leakage and absorption currents. With time, capacitance and absorption currents will dissipate, dictated by the ability of a material to store charge (which insulators do more effectively). Capacitive current (associated with the conductors) dissipates quickly compared to absorption current (associated with the insulator). The reduction in current therefore causes a continual increase in IR during an IR test. Most IR testing methods, however, take these effects into account.