Galvanic isolation - threshold for considering a voltage to be dangerous
In Europe, the Low Voltage directive points to documents (UL 60950-1) that define the Safety Extra low voltage (SELV) as being: -


  The voltage between any two accessible parts/conductors or between a
  single accessible part/conductor and earth must not exceed a safe
  value, which is defined as 42.4 VAC peak or 60VDC for no longer than
  200 ms during normal operation. Under a single fault condition, these
  limits are allowed to go higher to 71VAC peak or 120VDC for no longer
  than 20 ms


Definition not taken directly from standard but from here. But there are other definitions that are slightly different. "ELV" covered by Wiki is found here.

So, do some research on it because there isn't common agreement in the world.