Why to earth radiators?

Metal central heating pipework requires supplementary bonding in the bathroom. It is may also be earthed through other points, where it contacts main equipotentially bonded services, such as a non-disconnected filling loop (to water), or a gas boiler (to gas pipework). There is no harm in bonding it to the main terminal as well. There is no need to bond every radiator separately. Indeed, if the pipework is plastic, this will reduce the safety of the installation.

Christian.

In some circumstances, yes.

It might be -- your description is a bit on the thin side to give a definative answer.

If the wire's old enough to be merely green and lacks yellow stripes it too is probably a bit on the thin side :-)