What is a single phase circuit

Only one of the three phases of the three-phase current is connected to the domestic socket





All electrical circuits in a household come together at the fuse box. Since each only carries one phase of the three-phase current, it is important to ensure that the three phases are loaded as evenly as possible when installing the circuits.

The private end user normally has no three-phase current. Rather, the three-phase alternating current ends at the house connection. Within the house, only one of the three phases is connected to the sockets.

Accordingly, the usual socket in the household has two contacts: One is the "phase", more precisely one of the three phases of the three-phase system; the other is the earthed "neutral conductor", which also functions as the center conductor and is connected to the neutral point of the low-voltage transformer. This second conductor is also known as the "neutral conductor".

"Neutral conductor" ensures balance

Each of the individual electrical circuits in a household can have a different phase. For example, the "kitchen and bathroom" circuit can be supplied with phase 1, "living room and bedroom" with phase 2, "hobby room / garage" with phase 3 and "guest room" with phase 2. You could also supply all electrical circuits in a household or several houses with one and the same phase. However, this would lead to increased asymmetries in the loading of the three phases in the low-voltage network. The electrical installers are therefore required to distribute the three phases as evenly as possible over the circuits.

A completely even load on the three phases will never be possible with this switching method. A corresponding equalizing current therefore flows via the neutral conductor.

230 volts also to "earth"

Since the neutral conductor is earthed, the voltage of 230 volts also exists against "earth" (particularly pronounced e.g. against water pipes or central heating). It also exists when a person comes into contact with the phase conductor and at the same time has a more or less conductive connection to "earth". A current flows through the human body to earth, which, depending on the quality of the conductive connection, can exceed a strength of 0.025 amperes and thus be life-threatening.

It is not possible to see which of the two contacts of the socket is carrying the phase and can therefore be dangerous. In contrast to three-phase sockets, normal sockets also have no leading "nose". It depends on the respective plug-in of the plug where in the connected cables or devices the phase or neutral conductor is located. The phase contact can be determined with the help of a pole finder or voltage tester. This is usually in the form of a screwdriver, in the insulated handle of which a glow lamp lights up when the blade comes into contact with the phase contact. The current that flows through the human body to earth and makes the glow lamp glow is completely harmless.


Schematic representation of the single-phase power supply in the household: Each circuit - with the exception of the stove, which is connected in three phases as a strong consumer - has its input on one of the three phases and thus receives the "line voltage" of 230 volts. Since the individual circuits are loaded differently depending on the number and power of the devices that are currently connected, asymmetries arise between the three phases, which are balanced via the "neutral conductor" N.

Safety through protective contact

In the past, dangerous defects in electrical devices would occur again and again if the phase line came into contact with the metal casing of the device, thereby giving the unsuspecting user an electric shock. In order to eliminate the risk of such electric shocks, the "protective contact" (Schuko for short) was introduced: It consists of a third contact that is attached on both sides next to the current-carrying contacts of the socket and is directly earthed. This protective contact connects all conductive materials on electrical devices with "earth".

This ensures that a defective phase conductor cannot voltage the housing of an electrical device unnoticed, because such a defect immediately causes a short circuit.

Motors for single-phase alternating current

The single-phase alternating current from the socket naturally cannot generate a rotating field. It can only be used to operate so-called universal motors, which in principle are constructed like DC motors. The universal motors admittedly have the disadvantage that they require a commutator. This disadvantage increases with the performance of the motor or generator, because with the voltages and currents required for this, the formation of sparks on the commutator also increases. This problem particularly troubled the railway engineers when they built the first electric locomotives for single-phase alternating current. In order to keep sparks at the commutator within tolerable limits, they reduced the frequency of the traction current to a third of the network frequency. To this day, the Bundesbahn does not run at 50 hertz, but 16 2/3 hertz.

Nevertheless, even with single-phase alternating current, there is no need to forego the advantages of three-phase current and the asynchronous motor. Today, electric locomotives can not only convert the single-phase alternating current from the contact wire into three-phase current, but also change the frequency of this three-phase current as required. Electronic power converters make it possible. In washing machines, lawnmowers and other household appliances, even a relatively simple circuit, the main component of which is a capacitor, is sufficient: This means that an "auxiliary phase" is obtained from the single-phase alternating current, which is offset by 90 degrees from the initial phase and thus also enables a rotating field to be generated.