The insulating materials used in power and distribution transformers must possess the following properties:
- Good electrical properties to withstand the power frequency (50 Hz) voltages and impulse overvoltages.
- Good mechanical properties to withstand fabrication and handling during manufacture and electromagnetic forces during overload, short circuit, and normal operating conditions.
- Good thermal stability and low ageing effect.
The insulating materials for different parts are indicated below.
| Low voltage coil to ground and high voltage coil to low voltage coil | Thick radial spacers or tubes made up of pressboard, paper, glass fabric, paper filled plastic laminates, or porcelain |
| Turn to Turn | Organic enamel, papers, glass tapes |
| Layer to Layer, Coil to Coil | Kraft paper, glass fabric, press board, varnished paper. |
| Fluid (used as insulating and cooling medium) | Mineral oil, air, nitrogen |
| Bushings | Porcelain, phenolic bonded tubes |
