Tan Delta Method and Insulation Testing of Old Equipment

1) Is Tan Delta Testing of solid insulation materilas for transformers, motors, circuit breakers, cables and Generators is a Destructive type of Test or a Non destructive type. i.e does this test detoriorate the insulation.

2) How frequently or how many times can this test be performed in the life of an equipment?

3) We propose use of this equipment for ascertaining the condition of 132 /11 Kv, 132KV/11 KV oil filled transformers, 11 KV generators, 11 KV motors, 6.6 KV motors etc. In this case to ascertain the condition of the solid insulation and its detoriaration we need testing at regular intervals? What can be the ideal interval