A universal standard? Now you are an optimist!
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The easiest is the LV->MV: In the metric world, the threshold is usually 1000V. In North America and areas under its influence, it seems that 600V is the limit. This is reflected in the electric codes, the voltage ratings of cables and tools.
The transisition from MV to HV is more uncertain. It seems it is usually somewhere between 35 and 70kV but even the 33kV systems are sometimes considered to be HV.
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As to the limits of EHV and so on I don't know.
