transformer windings load losses aluminum windings transformer testing thermal design cooling systems test validates acceptable limits switching surges insulation system tests verify test helps core materials excitation current core losses test determines calculating losses dc resistance test verifies insulation resistance test measures rrr electricals dependability keywords transformers ability applied voltage transformers impulse test high level test high quality copper vital electrical components crucial process ensuring transformers temperature rise induced voltage tests output voltage matches withstand transient overvoltage relevant key concepts