During a power surge such as lightning or switching overvoltage transformers experience elevated terminal voltages and steep wave fronts. Insulation and bushings are stressed and internal electric fields increase. Surge arresters and protective devices limit peak voltage and protect the insulation system. If protection is inadequate or insulation already weakened, partial discharges or flashover can occur, leading to local damage or more severe faults. Repeated surges accelerate aging even if immediate failure does not occur.