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This stage isn't essential for any railway. However, it will make a big difference to reliability. essentially, it is all about breaking it into little sections so that a short circuit in one zone doesn't affect others.
What are Power Zones?
Power zones are separate sections of railway fed through separate cicuits that act like fuses. A short in one removes its powerr, but doesn't affect power to the others.
The Digitrax PM42 takes as input the DCC "rail" signal from a booster and generates four electrically separate "rail" signals. Critically, each has its own short circuit detection. Now, if a short occurs in one zone, it doesn't affect the others.
The PM42 channels can also be configured to act as reversers for Reverse Loops. In these, the track branches off and folds back onto itself; without special action, a permanent dead short circuit would result. Autoreversers isolate the power feed to such a loop: the track is double gapped at either end, and the polarity of power feed to the central region is set by the reverser. When a train drives in at one end, the polarity may be wrong; if it is, a short circuit happens. The reverser instantly "flips" the polarity, and the short circuit clears. When the train drives out of the other end, the same process happens. In this way, a train can run through unimpeded.
When a PM42 channel acts as an autoreverser, it is no longer able to isolate the track if a short circuit happens. Consequently, the autoreverse channel needs to be fed by another separate PM42 channel that provides the "isolation" capability.
Zones for this Railway
Each end of the railway has separate boosters. This points to a need for each end to have its own PM42 boards; I've located them adjacent to the boosters. Analysis of the track - basically finding areas that make sense to split up - identified the different zones required. It is the fiddle yards (where trains may be added or removed by hand) and the goods yards (where shunting happens, with humans involved) that are priorities for power isolation: these areas are most likely to have difficulties. The "open road" main lines, on the other hand, are less likely to have problems because the train movements are more simple: consequently these don't need to be broken up as much.
The South Downs Railway has four reverse loop zones. One reverse loop - with three parallel storage tracks - is located under Clanfield station. The other, with a single track, is in front of Petersfield station. These tracks each need their own autoreverser. Each reverse loop area - under Clanfield and by Petersfield - has its own PM42 channel for short circuit protection: I have used two because separate boosters are used for each end of the layout.
Power Zones and wiring diagram (PDF file)
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