When do partial discharges typically occur in electrical systems?

Partial discharges in electrical systems generally occur when the supply voltage reaches its positive or negative maximums. This is due to the electrical stresses that are present at these peaks in the voltage cycle. During these moments, insulation systems can become overstressed, potentially leading to ionization and partial discharge events, which can indicate degradation or failure in insulation materials.

As the voltage rises and peaks, it can exceed certain thresholds in specific areas of the insulation, particularly where there are imperfections or voids, resulting in partial discharge. This phenomenon is critical to monitor, as regular occurrences can lead to insulation breakdown and ultimately system failure.

The other options do not directly relate to the primary causes of partial discharge occurrences. Variations in ambient temperature might influence overall system performance but are not direct triggers for partial discharges. While systems may experience disturbances during offline modes or power interruptions, these events do not create the same electrical stresses related to voltage peaks necessary for partial discharges to occur.