The next parameter is VBR. This is the breakdown voltage of the TVS diode at which real activity begins. VBR is measured by passing a test current called IT (Test Current) through the device and measuring the resulting voltage. That sounds a lot like VR and, at least on this simplified graph, IR and IT look like they may be very similar. In fact IT is 500 times higher than IR in the AK series. Note that IR is measured in micro amps (μA) but IT is measured in milli-amps (mA). For the AK Series IT is 10mA. Remember that the power dissipation is the voltage times the current, so VBR times IT represents some real heating energy, especially on the higher voltage products. An AK10-430C will be dissipating at least 440V times 10mA or 4.4W of power. So do not be tempted to use VBR as a substitute for VSO as users may be surprised to hear of field problems due to the AK device getting hot.