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Catastrophic failures of modern SF6 circuit breakers have been reported during shunt reactor and capacitor bank de-energisation. In those cases, evidence of destruction of the interrupter nozzle by cumulative re-strikes is found. Monitoring of voltage waveforms during switching using established method would provide information about the magnitude and frequency of small re-ignitions and re-strikes. However, waveform measurements at a moderately high frequency require plant outages to connect equipment. A non-intrusive measurement technique has been developed to measure the high and low frequency voltage waveforms during switching operations without the need of an outage. The principles of this measuring technique are discussed and an example of its application in both shunt reactor and capacitor switching is presented.