ian said:
If the set has a fully isolated SMPS it would be easier and neater to fit
a 3-core mains lead and earth the chassis plane on the SMPS secondary
side. The cable installer would probably walk away from a set with a
random looking earth wire hanging from the co-ax socket.
I got the impression that it was a cable system throughout the house with
splitters etc. and so nobody would go looking in (for example) a wall
cavity for an earth connection. Say for example that there was a splitter
with a metal casing and it just happened to be bolted to a piece of metal
that just happened to be earthed.
Chris