VS is the peak voltage that will appear across the SIDACtor device during its transition into the crowbar state. During power cross and other slow overvoltage events, VS will be the same as VBO. However, the thryistor structure takes some time to transition into the crowbar mode. This means that if the voltage is rising quickly, VS will be a few volts higher than VBO. Since a curve tracer only shows slow events, VS is not typically visible on a curve tracer for fast waveforms. The SIDACtor device must be selected so that VS is below the voltage that could damage the protected circuit. Keep in mind that VS is only present for a microsecond or two while many protected circuits are only rated for continuous exposure to high voltages.