Hose
The hose on a central vacuum cleaner is much longer than other kinds of vacuums. Hoses of 30 feet are not uncommon. The hose does the usual job of conducting the dirt being picked up by the vacuum cleaner and sucking it or transporting it into the bag and filters. In addition to this, a central vacuum hose features a metal ring or a pair of contacts.
Wall Receptacle
Unlike ordinary or conventional vacuum cleaners, a central vacuum typically has one or more wall receptacles (sometimes called valves) into which the hose is connected when the vacuum is in use. There is usually a small spring-loaded flap or door covering the opening to these receptacles when the vacuum cleaner is not in use. Connected to the wall receptacle behind the wall is plastic piping that makes its way to the central vacuum location.
Main Unit
Just as with a conventional vacuum cleaner, the central vacuum has a main unit. It houses the motor that powers the fan that produces the suction. The filtration system is also in this unit. Typically this part of the vacuum cleaner is hidden away in a basement or garage or some other such area. In addition to the plastic piping that connects the wall receptacle to the main unit, there is wiring connected to two contacts inside the wall receptacle.
Transformer
Inside the central vacuum's main unit is wiring that brings current in through a cord. This cord plugs into an outlet supplying regular household AC voltage. This ordinary household current powers the motor that runs the suction part of the vacuum cleaner. One key additional feature is a transformer that takes some of the electrical current and converts it to low voltage. It's this low voltage feature that allows the vacuum to seem to start by itself.
Hidden Switch
As mentioned, built into the wall receptacle is wiring that runs to the main vacuum unit. And there is a pair of metal contacts in the part of the hose connector that gets inserted into the wall receptacle. When you put the hose into the wall slot, these metal parts make contact with their counterparts in the receptacle. When the central vacuum hose is inserted into the outlet, the metal parts make contact with those metal surfaces in the wall receptacle and complete the circuit. This turns the vacuum cleaner on. When the hose is not inserted into the wall receptacle, the two metal plates inside the wall receptacle are separated. This functions as an open switch, breaking the circuit as long as the hose is not inserted into the outlet.
The reason for the transformer built into the main unit is so that that the current going to these metal surfaces inside the wall receptacle can be converted to low-voltage. This is a safety feature that prevents electrical shock. Opening the flap door on the wall receptacle and touching the metal plates is no more harmful than touching a flashlight battery.