The first parameter for the V-I curve is VSO (Standoff Voltage). This is the maximum continuous voltage that can be applied to the TVS diode that will not cause it to begin to conduct. VBR (Breakdown Voltage) should then be a higher number than the highest working voltage in the protected system. The next slide has more detail on working voltages for AC systems. VSO is measured by running a very small test current, called IR (Reverse Current) through the device and measuring the resulting voltage. For the AK series, IR is only twenty micro amperes or one fifty thousandth of an ampere. The intent is to assure that the power dissipation at VSO is minimal. This means that the AK device will not heat itself during normal operation. For instance, an AK10-190C device will dissipate less than four thousandths of a watt when subjected to VSO.