In heavily built up areas ,such as London and other large cities, the public distribution system is in the form of a network and each distribution cable in the streets is fed from a
sub-station at each end .The supply system itself thus contains its own strandby provision .The only addition the building developer can make is to duplicate the short length of cable from the distributor in the road into the building ,and it may be doubted whether the risk of this cable failing is sufficiently great to justify the cost of duplicating it . In rural areas the service
cable to individual buildings may be quite long ,and may take the form of an overhead line rather than an underground cable .The risk of damage is thus greater than in urban areas and there is much more reason for installing a duplicate cable.