What does the 300V Cat II isolation rating mean?
This means that for a Cat II installation (domestic power outlets, lab use and so on) the USBscope50 can be used with a potential difference between the ground clip and the PC’s ground of 300V RMS (or DC), which in the above types of environment could see voltage transients of up to 2500V. This does not mean the scope will keep working properly with such large voltage spikes across its isolation circuit, but it does mean that the scope will isolate the PC (and you) from seeing the large and dangerous potentials.
