An interesting thought.CPCs can over heat due to high currents created by the CPC connecting two different "Earths" together when those two "Earths" are at different potentials. A cable bonding a metallic supply pipe to the MET of a PME system can carry very high currents when the PME Neutral is pulled a few volts above local ground potential by un-balanced loading on the three phases in the local netwrork. ... Connecting the CPC of a PME system to a boiler, electric shower or other device that has metallic connection to a low impedance Ground will result in current in the the CPC when the Neutral is not at local Ground potential.
However, I would have thought that, in the absence of a 'lost neutral' in the supply network (which would only be brief, until it was rectified), and in the presence of (deemed to be) 'adequate' main bonding, it would take a fairly extraordinary set of circumstances for the current through a (say) 1.5mm² CPC to be high enough to raise its temperature enough to melt PVC. After all, the path through that CPC to some 'true earth' would, in such a situation, be in parallel with both the supply neutral and the main bonding, so only a fairly small fraction of the total current would flow through the CPC.
Kind Regards, John