Introduction Partial Discharges

Partial discharges are small electrical breakdowns within the insulation of the power cable

Partial discharge activity can degrade the insulation of the cable to such an extent that there will be a short circuit. In other words, cables with a high partial discharge activity have a greater risk of failure than cables with no partial discharge activity.
The problem with partial discharge activity is that it is not continuously present, some cables have a “self-healing” effect by migrating oil due to loads and cable temperatures. Other cables can show partial discharge activity rising to critical levels in days. Partial discharges affect all cable and cable accessories above 6.6 kV.

To prevent blackouts the network owner can do preventive measurements with an occasional offline snapshot kind of measurement, this might look like a good precaution matter, but as explained earlier, partial discharge activity in a joint or cable can build up to a critical level in days or weeks, or was visible yesterday but not today.
Using an offline monitoring system is like a security camera that takes a picture once a year, while online monitoring involves constant surveillance.