-An important part of the photovoltaic power plants are cable systems. The dielectric properties of cables, reliability and durability depend on quality of production processes, operating conditions and degradation factors, as well. Expected lifetime of the cable systems is more than 20-30 years in general. Their failure free operation and longterm stability of properties has a direct impact on the economic return of the investments. According to our experiences the tests in compliance with valid standards are not adequate to verify the real life time during operation. The photovoltaic cables intended for use in outdoor applications for the connection between the solar panels and possible connection between panels and inverter were chosen for our experiments. The changes of the insulation resistance and breakdown voltage caused by some degradation factors, mainly water, are presented. This research was inspired by a real failure in operation.