The word "centralina" is used for a place where there is a control. There are two types of controls:
- a place where switch on or off some lines (both in a dedicated place how in power plants or only a box where there are the controls). This is just control and doesn't need big place;
- a place where reduce or increase the power of line (in Italy, the big lines of power use many KV but in the homes we receive only 230V, so they are necessary some places where transformer the voltage of input line to the local voltage). This type of "centralina" can be very little if the new line is little or if just isolation is necessary but also big size where new lines with big power are created starting from main power line. For example, we can think to power line as a tree and every time that we add a branch we need a "centralina" to control the power conversion and isolate the main line from local line. This until the leaf that for power are homes.
For this reason, the definition found in dictionary is correct: we can have just control (first point) or part of a power plant (second point).
It's possible to use "centralina", especially with the first meaning, for phone lines, thermal or hydraulic plants. But also in these cases, both types of "centralina" are necessary (for example, for phone lines are necessary places where local lines are connected with main line: some servers done this work).
The word "centralino", normally, is used with a little different meaning: in the past it was necessary a person that manually switch phones lines. "Centralino" is used for this person or this group of people that control switch in phone lines. Changing the way how switch is done (not a long manually but automatically) the word "centralino" has been used also for a control part that done switches and, with this means, it's possible to use it for every place as in first point also for not phone lines.
I'm sorry if the English is not good, but I hope that explanation is clear.