Hi all. What does the voltage rating in each category I/II/III/IV actually mean? For example, my multimeter is Cat III rated to 300V. Is this RMS? Peak? Peak-peak? Our supply here is 230V RMS AC, but this means the momentary peak is about + and - 330V (relative to neutral/ground) and the peak-peak is of course 660V. Can I therefore use it to measure mains voltage at a socket? My interpretation is: 1. If RMS, yes. 2. If peak, no. 3. If peak-peak, definitely no.