@SteveH posted:@gunrunnerjohn Good explanation. For anyone without an oscilliscope, can you think of any other way to effectively test if a TVS zener diode is conducting current when it's rated clamping voltage is exceeded?
The way they're tested in an industry setting, say for certification of the component for use in a critical system, is basically what I said. The part is placed in a test stand, and a current limited pulse supply is connected across it. The resultant voltage waveform is analyzed to see if the TVS device meets it's specifications. I believe I've seen some mention for bipolar devices of using AC so you can test both polarities at a time.
A simple bench test can be done using a high voltage current limited bench supply. Say you have a diode with a 36V breakdown rating, and you have a 50VDC power supply with constant current capability handy (doesn't everyone?). 
Connect the TVS directly across the power supply feeds, monitor the current through the TVS device as you slowly increase the voltage. At some point, usually somewhat above the breakdown rating, the current will start to go up fairly quickly with voltage. You can check that "knee" in response against the data sheet for the TVS device.
Don't forget to do the test again with the TVS device flipped around, it's a bi-polar device and you need to test both polarities.
