Howto Measure High Voltages with a Multimeter

[joeqsmith]

From what volts does it regarded as the high voltage which needs extra care and probes and devices in measurements?

I don't think there is a one answer fits all.   It will depend on what you are trying to measure and what equipment you are using.   

For example, I have a UNI-T UT90A that can read I think can read close to 2KV before it over ranges.   I think Dave's 121 over ranges around 600V.  I personally wouldn't put these meters across a 600V high energy bus but I have certainly demonstrated them running with low energy sources.