It is avtually not grounded, it has a kind of spark gap between th transformer secondary and ground. I have the same system at home, and the had it one place I lived before, there they got a ground fault on on phase, and suddenly I had a 230V and a Neutral. I was a student and the meter did only meter the current trough the line that worked as a neutral so I ran the entire apartment with no metering for a month :-).. (Youngsters :-) )
