In the early days of DCC, power was fed from one source (the "booster") to the whole railway. That was simple, and allowed a lot of wiring to be removed.
Quite quickly however at least three problems were found with this arrangement:
- One booster may not be able to provide enough power for the whole railway;
- A track short in one area - for example caused by a derailed train - caused power to be removed everywhere;
- When there was a short, power was also taken away from accessory decoders, so points could not be changed.
DCC has always allowed several boosters to be used. Separate regions known as "Districts" are electrically isolated by gapping both rails; then a separate booster feeds each District. This allows the total power problem to be resolved: if the separate districts each have several moving trains, the railway as a whole can have more trains operating than if a single booster were used.
Using a separate booster to feed the accessory decoders also became a recognised thing to do. This allows power to the accessory decoders to be always there, even if there are track shorts. However this is an expensive solution; modern accessory decoders (e.g. DAC20, DS64) provide alternative solutions to solve this.
Even if several boosters are used, it is likely that a track power short will affect quite a large area simply because boosters are relatively expensive and likely to cover bid sections of railway. What was needed was the electronic equivalent of a "circuit breaker": enter the Power Manager.
Power Managers act as self resetting electronic fuses. If there is a short, they remove track power; when the short is removed, they re-connect the power. The difference between a power manager and a booster is that the power manager itself doesn't generate the DCC rail signal; you still need a booster for that. The critical advantage over a booster is that they are about a tenth of the price, so you can aspire to have more of them. The output of a power manager feeds a "zone".
The Digitrax PM42 is a quad power manager. Each section behaves independently; that provides up to 4 separate power feeds. Each feed will be isolated from the others, so a short in one section will not affect the others.