Conductance measurements in the investigation of short circuit arcs, in miniature circuit breakers

The arc formed in a current limiting miniature circuit breaker during the interruption of a short circuit fault current produces high temperature ionised gases in the arc chamber. The presence of high ionisation levels increases the probability of undesirable arc re-ignition in the contact region or other parts of the circuit breaker where small gaps occur. Control of the gas temperature, composition and movement is therefore an important factor in improving current limiting performance. Information on the state of the gas can be obtained by measuring its electrical properties. This paper presents new results from a study of gas conductance in the arc chamber of a miniature circuit breaker by two different methods. The conductance of the gap between a pair of probes placed either side of the arc chamber is measured. The conductance of the gas between the plates in the arc stack is obtained from measurements of the voltage on the splitter plates. Results from short circuit tests (3 kA) are related to gas conductivity and temperature and are supported by are position measurements obtained using the high speed imaging system described previously. The measurements indicate that gas temperatures rarely drop below 2000 K during the arcing period. Information on the rate of rise of gas conductivity as the contacts open and residual conductivity after current zero are presented.