Hi prc, your link explains the concept that in none electrical engineering field. It most suitable for mechanical, structural and material engineering. A good example for it is a car crash test. However, in power electrical engineering especially in high voltage test technology, destructive considers whether the test can lead a possible dielectric breakdown or the test conditions to certain degree that will add to dielectric degradation.
To better explain, the concept of "polarization" is very important. Most structure of gas molecules are symmetrical like O2,N2,CO2 and SF6. They are in non-polarized nature and thus considered insulation gas. Under a certain degree of electric field they become momentarily ionized and lose their dielectric characters as a result a breakdown could happen. After the energy dissipated gas molecules de-ionized or neutralized and self-restore its insulation nature. This type of breakdown is called "electrical breakdown", like lightning breaks down air space.
However, in liquid and solid dielectric materials, molecules are in polarized nature but they are scatter to all directions, in overall or in bulk they are still non-polarized. Applying electric field to stress them up, molecules of solid or liquid dielectric material start to line up and you will see capacitive leakage current whether thru the bulk martial or along the surface, and here is the megger test come across. At this point if the electric field is removed then molecules become messy again and in bulk wise it still non-polarized. Therefore solid or liquid dielectric material have some self-restoring nature and that is why megger is not consider a destructive.
Further increasing the field stress pass a certain threshold the molecules will be twisted too much and they cannot restore even the electric field is removed. This type of polarization is permanent. At this point breakdowns may not happen immediately as the polarization path may not form completely, but the damage has been made permanently and as the result the insulation has been weakened. That is why impulse test is destructive. Most breakdown in solid dielectric materials is the combination of "thermal breakdown" and "mechanical and chemical breakdown", like aging, overloading and excessive temperature rise, moisture, chemical reaction etc. Pure electrical breakdowns seldom happen.
Dissipation factor or tangent delta are not destructive because the material under test only stress up to a low level that enough to cause the capacitive/resistive leakage current to be detectable but far away to cause the permanent polarization. PD is not destructive because it only stress up the gas molecules in the voids of the solid or liquid dielectrics not the material itself. Once the field source is removed materials go back to their previous characteristic.
mesutphen - we are talking all about the probabilities, the probability for numbers of lightning strikes per year, numbers of strikes to the power lines or stations, chance of lightning strikes are not protected by skywires and LA's, poor design and poor grounding, accumulated apparatus aging or damaging etc, all probabilities multiply together makes you feel transformers do not fail very often, but they do fail under lightning.