This paper presents life test results obtained on twisted pair specimens, that are insulated by three different corona resistant and one conventional polyamide-imide materials, subjected to sinusoidal and square waveforms at supply and high frequencies. Life reduction as frequency increases is discussed both in the presence and in the absence of partial discharges. Life tests showed a strong dependence of life on peak-to-peak voltage and pulse repetition frequency. These results indicate that high frequency waveforms are able to affect severely conventional enamel insulation, while CR wires can provide longer lives depending on materials. It is observed that ageing acceleration under high-frequency pulses occurs even in the absence of partial discharges and in this condition failure times follow a rule roughly proportional to the ratio of test to supply frequency. Under partial discharge regime, breakdown is much more accelerated, and failure times follow a superlinear law as function of frequency.