In the UK it is fine to construct a ring with either a standard T and E,  which of course has the reduced diameter bare CPC., or equally  a cable with 3 equal cores - no-one would blink if NYY-J or similar was used. A mixture is unusual, but unless the installation fails the test for ring-round' continuity, there is nothing to worry about.

It could be  a concern, perhaps, if both legs of the ring had been extended to a remote peninsula, in a single 4 core and earth cable where the 4 cores were the 2 lives and 2 neutrals. such an arrangement might be unacceptable if loads with high earth leakage were expected, as there is then  not the redundant earthing that the ring architecture provides.

Note that many continental countries have a blanket ban on cables where the earth is smaller than the live, and the Southern Irish have come into line with that, by having their T and E with a full size CPC.  The objection is based on the far end touch voltage during a fault - rather than half the mains voltage as the resistance of the two cores is equal, the voltage the case of the equipment rises to until the ADS kicks in is more like 150-170.  Actually it is particularly noticeable on 10mm.

For two and three conductor cables:

• 1.0 sqmm cable has 1.0 sqmm CPC
• 1.5 sqmm cable has 1.0 sqmm CPC
• 2.5 sqmm cable has 1.5 sqmm CPC
• 4 sqmm cable has 1.5 sqmm CPC
• 6 sqmm cable has 2.5 sqmm CPC
• 10 sqmm cable has 4 sqmm CPC
• 16 sqmm cable has 6 sqmm CPC

Of course that is only a question in a TN system, in TT the voltage rise is higher due to the electrode resistances but the permitted disconnection times are significantly shorter.

In the UK it is fine to construct a ring with either a standard T and E,  which of course has the reduced diameter bare CPC., or equally  a cable with 3 equal cores - no-one would blink if NYY-J or similar was used. A mixture is unusual, but unless the installation fails the test for ring-round' continuity, there is nothing to worry about.

It could be  a concern, perhaps, if both legs of the ring had been extended to a remote peninsula, in a single 4 core and earth cable where the 4 cores were the 2 lives and 2 neutrals. such an arrangement might be unacceptable if loads with high earth leakage were expected, as there is then  not the redundant earthing that the ring architecture provides.

Note that many continental countries have a blanket ban on cables where the earth is smaller than the live, and the Southern Irish have come into line with that, by having their T and E with a full size CPC.  The objection is based on the far end touch voltage during a fault - rather than half the mains voltage as the resistance of the two cores is equal, the voltage the case of the equipment rises to until the ADS kicks in is more like 150-170.  Actually it is particularly noticeable on 10mm.

For two and three conductor cables:

• 1.0 sqmm cable has 1.0 sqmm CPC
• 1.5 sqmm cable has 1.0 sqmm CPC
• 2.5 sqmm cable has 1.5 sqmm CPC
• 4 sqmm cable has 1.5 sqmm CPC
• 6 sqmm cable has 2.5 sqmm CPC
• 10 sqmm cable has 4 sqmm CPC
• 16 sqmm cable has 6 sqmm CPC

Of course that is only a question in a TN system, in TT the voltage rise is higher due to the electrode resistances but the permitted disconnection times are significantly shorter.