What are neutral and phase
Lexicon> letter N> neutral conductor
Definition: a conductor intended for the transmission of electricity, the electrical potential of which is close to the earth potential
Counter-term: protective conductor, phase, outer conductor
English: neutral conductor, neutral terminal
Category: electrical energy
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: 08/14/2020; last change: 23.08.2020
Power systems, such as those used today in Germany for supplying households with electrical energy, typically offer the following connections:
The neutral conductor and protective conductor should both always have an electrical potential close to earth potential, but they have very different functions. In the case of single-phase alternating current, the neutral conductor carries an electrical current during operation that is normally the opposite of that in the phase line. The circuit used therefore leads through phase and neutral conductors and loads both equally with current. In contrast, the protective conductor should not carry any significant current during normal operation; it is only used for the safety grounding of electrically conductive parts.
Neutral conductor for three-phase current; Four-wire and three-wire systems
The situation is somewhat different with (typically three-phase) three-phase current. In the ideal case (without unbalanced load, harmonics, etc.) the neutral conductor is not loaded with current either; the currents flowing through the three phase lines add up to zero at any moment. So there are no electrical charges that would have to be dissipated through the neutral conductor. In practice, however, there are often certain unbalanced loads and / or harmonics, so that the neutral conductor is again loaded:The three phase currents do not always compensate each other in such a way that no current flows in the neutral conductor.
- If, as the simplest example of an extremely unbalanced load, current is drawn from only one of the three phases, this current must naturally flow back in full via the neutral conductor. In less extreme cases, the current strength is correspondingly lower there.
- For harmonics, the order of which is a number that can be divided by three (3, 6, 9, ...), the associated current in the neutral conductor is this sum of the currents in the phases and can therefore be quite high. Such harmonics can be generated, for example, by devices with power regulation through phase control.
A neutral conductor is not always carried with three-phase cables:
- Only in the area of low voltage is a so-called one always used Four-wire systemwhich contains the three phases and the neutral conductor.
- For medium-voltage networks, a is practically always used Three-wire systemwhere the neutral conductor is missing; this is practicable, since unbalanced loads are minimized by suitable measures, so the neutral conductor would hardly have to carry any current. A neutral point shift to a limited extent is also well tolerable; the neutral point shift is not transferred through the transformers to the low-voltage level. It is then sufficient to ground z. B. at the substation (where the medium voltage is obtained from high voltage with a transformer) and possibly at the low voltage transformer.
- As a rule, a three-wire system is also used for high-voltage lines, but with certain exceptions.
Neutral conductor with direct current
In the case of direct current lines, there are also neutral conductors, which in turn are not necessarily routed to high-voltage lines. Similar to the three-wire system for three-phase current, you often only have two outer conductors, which carry oppositely equal voltages to earth, and a neutral conductor with earth is only introduced again on the consumer side.
Household sockets and power cords of devices
In the case of simple earthed sockets in the household, the two recessed contacts are used for phase and neutral conductor - although, at least in Germany, it is not standardized which side the phase is on. However, when wiring in the house, a blue color of insulation is established for the neutral conductor; when you open a socket you can see which contact belongs to the neutral conductor. (Even without opening, this is possible with a phase tester: it lights up when there is contact with the phase, but not with the neutral conductor.)
In the supply lines of devices with a mains plug, the blue insulated cable is not necessarily used for the neutral conductor, as this depends on the orientation of the plug in the socket and how it is wired.
Interruption of the neutral conductor
In general, attempts are made as reliably as possible to prevent a consumer from ever being connected to one or more phases, but not to the neutral conductor at the same time. For example, a light switch must never be installed in the line of the neutral conductor; rather you have to switch the phase. This ensures that there is no dangerous voltage to earth when the light is switched off, e.g. B. is applied to lamp bases. Switching the neutral conductor is generally not necessary.
In some cases, however, the phase and neutral conductor must be switched together (“all-pole”), for example in the case of residual current circuit breakers. Then the switch should be designed in such a way that the neutral conductor is connected shortly before the phase and is only interrupted shortly after the phase when it is switched off.
If the neutral conductor is interrupted anyway, this can lead to the destruction of components, especially in three-phase systems. In the event of an unequal load on the three phases, the star center point can assume a potential far from the earth potential (zero point shift), and an overvoltage can then occur between this and a phase. However, endangering people should be avoided even then, at least as long as the protective conductor is not also interrupted.
Systems used to be with Zeroing common, where the function of neutral conductor and protective contact was fulfilled by a single line, which is known as Neutral conductor designated. If this line was interrupted in the event of a defect, it quickly became dangerous; You could then suffer an electric shock by touching metallic parts of a device, for example. Therefore, such systems are z. B. in the household today no longer allowed. However, one continues to use such TN-C systems z. B. between low-voltage transformer and house connection, then the conductor with almost earth potential as PEN head designated. After entering the house, this line is then branched into a neutral conductor (N) and a protective contact conductor (PE). This approach is called TN-C-S.
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See also: phase, protective conductor, PEN conductor, TN system, power grid, socket
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