In all books written about the reliability of power systems, it is explicitly stated that the reliability of medium voltage networks is lower than similar (overhead/underground) low voltage networks. In the other words, the failure rate in the unit length of a medium voltage network (in a definite period) is more than the similar low voltage network. However, these books does not explains why this is so.
One of the main reasons that cause to decreasing of medium voltage networks’ reliability, is the electric field intensity. Two important phenomena are the main source of faults in the overhead and underground networks respectively, fault path carbonization and water treeing (polymers reaction with water in XLPE cables).
Figure 1 : Water treeing in a XLPE cable joint