absorption

When you apply voltage across a dielectric material, energy doesn't distribute evenly through it. Some energy gets trapped in the molecular structure of the insulation itself, held there by polarized atoms and molecules that have been forced into alignment. This retention of electrical energy after the voltage source is removed is absorption, and it appears across cables, capacitors, bushings, and transformer windings in every power system.

The effect is measurable and significant. A high-voltage cable that has carried current will continue to discharge energy slowly after you disconnect it, even with no external circuit present. The time constant for this discharge ranges from seconds to hours depending on the material: modern plastics like polyethylene discharge relatively quickly, while older paper-oil systems in transformers can hold charge for days. This is why de-energized equipment can remain hazardous long after you switch it off.

Why absorption matters in testing and operation

Absorption creates two practical problems. First, it masks the real insulation resistance when you measure it. A megohm meter will show artificially low resistance readings immediately after disconnecting high voltage because absorption discharge is still flowing. You must wait a fixed interval, typically 60 seconds, between removing voltage and taking the measurement, to let absorption decay and get a stable number. Second, absorbed energy can re-discharge unexpectedly during maintenance, creating a shock hazard. A capacitor bank or long cable that reads zero volts can deliver a dangerous jolt if disturbed after the discharge path is broken.

The amount of energy absorbed depends on the dielectric material's polarizability, the voltage applied, the duration of application, and the temperature. Mineral oil in transformers exhibits strong absorption, which is why transformer insulation testing procedures include standardized waiting periods. Polymer films show less absorption than paper, making them attractive for modern switchgear and cable insulation. Moisture in insulation dramatically increases absorption and also indicates degradation, so absorption measurements during commissioning and condition assessment tests reveal the health of the dielectric system.

In electrical standards, absorption is quantified as part of the polarization index test, which compares insulation resistance measurements taken at 10 seconds and 60 seconds after applying test voltage. A ratio above 1.4 typically indicates clean, dry insulation; ratios below 1.1 suggest moisture, contamination, or aging. This simple measurement has caught countless failing transformers and cables before catastrophic failure occurred.