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The DNO are replacing the old separate cables (was a TT system) with ABC (aerial bunched conductors) as part of the conversion from TT to a PME earthing system, they are bunched together to help prevent the dreaded loss of Neutral (breaking of a single wire) condition that is more dangerous with a PME system, as has been mentioned on here before, being as how the N is also the Earthing conductor (TNC) on the DNO supply side, and then becomes TNC-S on the consumer side.

Edit: PME isn't the only TNC-S system, there is also PNB, or protective Neutral Bonding system although this is comparatively rarer and is usually restricted to pole mounted transformer arrangements as far as DNO supplies are concerned .