Proper combination of fast, high-resolution analogue to digital converters, optical signal transfer and digital signal processing tools with advanced current and voltage measurement sensors has led to the development of a compact current zero measurement system for application in high-power laboratories as an aid in testing SF/sub 6/ circuit breakers. The application of this system in the renovated high-power laboratory of Toshiba Co is described, including the results of short-line fault tests in the range 16-70 kA on a 300 kV single break model circuit breaker. Three levels of information on the 'quality of interruption', obtained from current zero measurements are discussed: (1) direct observation of post-arc current, arc voltage collapse phenomena, commutation process of arc current into (stray-) capacitance etc.; (2) the arc conductivity very shortly (200 ns) before current zero, an indicator of the performance of the breaker under test. A new quantitative method of electrical endurance mapping is presented; and (3) arc model parameters are extracted out of every individual test, that enable prediction of the performance under other conditions than those tested (e.g. with an internal capacitor).