Burnt Cable ?
 

Right !!!

I have two ring circuits, and one leg on each of them has burnt out but the other is sound (this is only on the phase.... Neutral and earth are both sound).
e.g. in-coming legs from both rings are burnt.
Yet when i undid the sockets both cables were burnt inside on about three quaters of the sockets and a couple were okay, and this problem was on both of the rings.
One was on an rcbo.
so why did this happen ?

thanks

Normally each socket is connected through both sides of the loop in parallel. The current roughly divides between them

If there is a bad connection somewhere this place will heat up locally and most current will flow through the other path. You will not notice this as everything still works normally

If there is a complete open somewhere you won't notice this too. the current just flows completely through the other side

In any of the said cases all of the current flows through one side of the loop instead of dividing. This may be just too much for the single wire

If you have a clamp current meter you may measure the current in each wire and see if it is nearly balanced or allmost all the current flows through one side. If this happens it means something is wrong on the other side