The pressure of vacuum switching elements after production is checked normally by Penning or magnetron methods (combined electrical and magnetic field). Vacuum in the range of 10/sup -1/ to 10/sup -4/ Pa can be measured in this way. After assembly into circuit breakers however, these methods are not applicable. HF interruption performance during the make operation was proposed earlier as a possible alternative. Further investigations show that differences in the number of HF prestrike current loops can be found in the pressure range of 10/sup -1/ to 10/sup 5/ Pa. Current chopping of dc arcs between 5 and 30 A during the opening operation may be another option for determination of the pressure range by measuring the lifetime of the arc, but the resolution in the vacuum range >10/sup -1/ Pa is too poor.
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