Sorry to say but that's incorrect. Think of all those Ex-type sensors that require a 24V dc supply and have a 4-20 / 4-24 mA output.

It all depends upon the maximum energy level one can obtain, not just voltage or current. At 8 V for zone 0 applications the max. allowable current is way over 60 mA.

The explosion characteristic is a reference chart in which the energy level required to cause a gas/air mixture (typically methane/air) is plotted on a U vs I scale. From this graph, it is possible to determine what the maximum U(open) and I(short) is that one can use BEFORE the explosion happens ;o)