For example, in a simple spot network with Feeder A supplying Transformer A, and Feeder B supplying Transformer B, a fault on Feeder A will operate the Feeder A breaker at the substation, but the fault will still be powered by Feeder B through Transformer B and (backfed) Transformer A, and the current on Feeder B will not be high enough (due to the impedances of the two transformers back-to-back) to trip the Feeder B breaker. So, without the network protector to disconnect Transformer A from the bus, the fault will continue to burn.
Here the expectation would be that the forward protection on the LV side of transformer B, either an oil circuit breaker or a fuse, would then operate, as it sees both the transformed feeder A fault current and its normal load current, plus quite possibly the load current of any other substations connected to feeder A. More commonly here Transformers A and B would not be co-located, but might feed opposite ends of perhaps 200m of cable with various loads along it, protected each end by a fuse in the region of 400 A. Transformers A and B might have other transformers in the same substation, but often feeding different cables, and perhaps meshed with different substations, from other sections of a split LV board.
A typical UK arrangement in a high-load area might have two transformers of perhaps 1 MVA in each substation, feeding two sides of a split LV distribution board with a normally-open switch in the middle of the busbars. There may be an oil circuit breaker on the LV side of the transformer before the board, but commonly there isn't and the only protection for those cables and busbars is on the HV side. From each half of the board cables go out in many directions, most with various branches, and loads tapped off them along their length. Typically these are 3 phase cables of around 95-400 mm
2 protected by fuses in the range 300-500A, perhaps something in the region of 20 such feeders. Many of those cables connect to underground linkboxes where fuses or most-often solid links can be fitted, and cables from other substations also come to the same boxes. A given length of cable and its various loads can then be fed from either end, or from both with a break at some point (not necessarily in the centre), or from two or more places in areas with very dense loads (referred to as meshed operation).
In most of the UK loads are generally fed radially from one substation but exactly which can be changed by physically moving links/fuses, which is often done during maintenance to limit the number of customers off-supply, or to manually re-configure to link out faulted sections or even whole substations. It is also done occasionally to move loads from a more to a less loaded substation, and occasionally both substation LV boards and underground linkboxes are used as convinient points to connect diesel generators. Generally links are fitted and removed live, with substations paralleled for a small number of minutes to avoid loosing power to the section that's being reconfigured. In some urban areas (possibly only central London and Liverpool) the LV network is operated meshed, and many substations are connected together at LV via these cables and linkboxes, giving rise to the backfeeding faults and in some cases fairly specialised protection is in place to limit their duration or improve fault clearance times.
In a low load area a substation might have a single transformer of 250-500 kVA, and single, small LV board with perhaps 4-6 outgoing ways. The LV cabling is still likely to have a mesh-like structure, but generally with open points at linkboxes.