The Polarisation Index (PI) is a measure used in the field of electrical insulation testing to assess the condition of insulation in electrical equipment, such as motors, transformers, cables, and switchgear. It is also known as the Insulation Resistance (IR) ratio.

The PI is calculated by comparing the insulation resistance measured at two different time intervals, typically at 1 minute and 10 minutes. The ratio of the insulation resistance measured at 10 minutes to that measured at 1 minute provides an indication of the insulation’s ability to withstand voltage stress over time.

A high PI value indicates good insulation condition, suggesting that the insulation is dry and capable of maintaining its resistance over time. Conversely, a low PI value may indicate moisture ingress, contamination, or degradation of the insulation material, which can compromise its effectiveness and lead to potential electrical failures.

In summary, the Polarisation Index is a diagnostic tool used by electrical engineers and technicians to evaluate the health and integrity of insulation systems in electrical equipment, helping to identify potential issues before they lead to equipment failure or safety hazards.