Aging of Electrical Insulation

The aging of electrical equipment is caused principally by the aging of the insulation.  The insulation made of materials such as rubber, paper or polymers deteriorates over a period of time.  The molecules of the material break down.  The material loses its mechanical and electrical properties.  It finally gives in, leading to failure. 
Aging of insulation in Electric machines has a number of reasons.  Some of them are
  1. Partial Discharges
  2. Tracking
  3. Treeing
  4. Electrolysis
  5. Dielectric Heating
  6. Space Charges

Aging of Electrical Insulation can be monitored by periodically testing the insulation such as Insulation Resistance, Polarization Index, Tan Delta test, Surge Test, etc.