Also you should be very careful when measuring something you know have a high ground offset voltage, for example when doing phase to phase measurement. I would simply say this should be avoided when using the nano if the phase to ground voltage is high where you intend to ground the probe. Also the back plate on the nano is made of metal and is very close to the battery cord (which has a ground wire) and I can say that the battery cord is not meant to isolate like 400V so it is not safe to hold the nano while having the probe ground connected to high voltages.