The crucial PLUS in safety:

“PRCD-S perform a measurement through the users body during switch-on operation. If the user for example wears gloves, the masurement can not be performed well, and the unit shows “everything alright” even there is no safety function active. That means, that PRCD-S cannot recoginze dangerous voltage on ground conductor (PE). Housing parts of connected devices can be energized under perilous voltage.”

Source: DGUV Deutsche Gesetzliche Unfallversicherung