Dangerous step potential with low (line) voltage possible?
Where is two different things. Electrical ground and earth surface aka ground is not the same things. But if manhole is grounded electrically, ground fault protection should work. Otherwise electrical potential spreads over surface depends on soil conditions. And step voltage is potential difference, so the value depends of length of step and potential spreading. But if manhole cover lying on some concrete structure, not electrically grounded and live wire touch it you may get full voltage between cover and soil.