Floating neutral in TN-S or TN-C-S earthing system
Speaking about TN-C-S, the N an PE are the same wire on building input-PEN, floating N also means floating PE:

The PEN wire has to be connected to the building ground and/or earthing rods/strips buried in place.

In such way all PEN conductors make a grounding mesh connection, if one fails the other takes over. The most important is the one in place: if the incoming PEN is broken, then you still have a ground connection through your's ground, otherwise all the metallic parts would get on live potential.

For TN-S the PE and N are separated, a floating N isn't an issue.