What can cause a medium voltage cable to fail?

There are a lot of possible ways that a MV cable can fail. It all varies depending on the constructive type, and the topology of grid it's being used in. Both terminal boxes and the underground section can fail.

Terminals can fail if
1) subjected to moisture - high humidity of the air is enough to produce failure in time;
2) PILC cables naturally lose the oil and thus insulating properties from the terminal during years of service, because of its vertical position
3) partial discharge because of poorly executed terminal, for PVC or XLPE cables

Underground section of the cable can fail if
1) PILC cable on an incline (hill) with oil migrating naturally to the portion situated at lower altitude, leads to damaging of the insulation properties of the paper
2) sheath damage during installation can lead to water trees, partial discharges and finally, failure point
3) for PVC and XLPE cables, manufacturing flaws mean air gaps within the insulation which ultimately lead to partial discharges, electrical treeing and failure

For all cables, if the grid allows continuous operation with an active ground fault point (usually when neutral is compensated via a Petersen coil), the 2 healthy phases are subjected to overvoltage which accelerates the aging of the insulation and quickly transforms electrical trees into failure points on the "healthy" cables.

High voltage Partial discharge, water treeing, insulation failure, poor jointing, poor termination, jacket damage on Installation, inadequate bedding, thermal & magneto tracking, substrate abrasion perforated jacket, poor manufacturing ie: irregular extrusion of sheathing and outer jacket, irregular drawing of conductor, feroresonats, thermal overload due to harmonics, thermal overload due to poor power factor, thermal/current overload due to switchgear failure, insulation break down between phases..... Could be many things, a proper investigation should reveal the cause. Get the supplier/manufacturer involved to assist with investigation, there should be a warranty of sorts.

Cable

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