The regs actually state that it should be the same size as the phase conductor ( 6.12.1.1 e) but may be smaller if CB instantaneous tripping current is less than 10 ( 6.12.1.1 f )

If the earth wire forms part of the cable then it needs to comply to that spec ( 6.12.1.1 b ) - So in Twin and Earth , Armour cable ECC , surfix etc the earth is part of the cable and conforms to that standard.When the cable is manufactured/ designed it takes into account ( 6.12.1.1 d ) be able to carry the prospective fault current without excessive temperature rise of the conductor, within the disconnecting time,





6.12.1 Earth continuity conductors
6.12.1.1 An earth continuity conductor shall
a) consist of compatible conductors,
b) if it forms part of a cable other than a flexible cable, comply with the relevant
requirements of the standard for the cable,
c) if it forms part of a flexible cable, be of the same material as, and have a
nominal cross-sectional area at least equal to, that of the largest phase
conductor,
d) be able to carry the prospective fault current without excessive temperature
rise of the conductor, within the disconnecting time,
e) if it does not form part of a cable or flexible cable, have a nominal
cross-sectional area in accordance with table 6.25 and if it equals to the
phase conductor size refer to table 6.28(a), as follows:


f) using the same formulae above where the earth continuity conductor cross
sectional area is 50 % of the phase conductor, thus m = 2, table 6.28(b) for
the maximum length of copper earth continuity conductor for general
purpose circuit breaker apply.
Table 6.28(b) — Minimum size and maximum length of copper earth
contuinity conductors rated at 50 % of phase conductor diameter, for
general purpose circuit-breakers with instantaneous tripping current
Im = 10 × In

g) Where the proposed PE conductor with increased cross-section becomes
uneconomical, an earth-leakage protection device may be added or
deployed as additional protection, where the tripping threshold would be
substantially lower.
h) For aluminium conductors apply the formulae above with ρ = 37 × 10-3
ohm.mm2/metre