Indeed this is a TN-C system. According to today's regulations it's only allowed using wire sizes of 10mm2 and up, basically restricting it to feeders in a TN-C-S system, i.e. TN-C up to the splitting point, TN-S all the way to the sockets. In Western germany TN-C in residential wiring was prohibited in 1973, in the GDR in 1990. In both parts of germany many old systems remain in use. The use of an RCD is impossible with TN-C and if the PEN breaks grounded parts can be live at 230V. That's also the reason for the >10mm2 rule, they say such a big wire is far less likely to break than a 1.5mm2 wire.
Fortunately we never had TN-C in Austria, it's a really ugly system.
