Every electrician or industrial electronics engineer knows about three-phase power and wiring. They may be aware of this because they often have to connect a three-phase motor or check the inputs and outputs of the contactor terminal set.
In any case, there is an intriguing contrast between 1-phase and 3-phase wiring. In a 1-phase connection, there is always a neutral conductor (often white or blue). But in a three-phase system, sometimes there is a neutral wire, and sometimes not. Even if it exists, the load devices do not have a neutral connection. Why?
Neutral in a 3-phase system
In an AC system, a neutral conductor is a de-energized conductor through which current flows. It is not energized because it is not connected to any active power source from the mains input. This is the job of a "linear" conductor. It transfers current back to the back of the main service panel, which must be connected directly to earth ground.
Due to grounding, the neutral wire will not supply energy if it touches a grounded object - a spark will not occur and no current will flow. When considering a three-phase power system, some power supplies will include three-phase lines, neutral and earth. Others may omit the neutral, leaving only three lines plus ground. If neutral is necessary for current to flow, why might some systems exclude this wire from connections?
Single phase system
When one separate phase line is used to supply power to the load device, there must be a return path to complete the circuit. The neutral and earth conductors provide this return path and a backup return path in case of faults. In this sense, you can use a three-phase power system, then access one line plus neutral, and you have a single-phase power supply. Thus, most power distribution systems provide single-phase power, for example for a home or shop. The system was originally a three-phase system, but it can be easily converted into three separate single-phase supplies, each of which will be directed to specific consumers.
Since there is only one line with power, the second wire must provide a return path. This is the neutral wire. If you were connecting a single phase motor or any common lighting or home appliance, you would connect both line input and neutral. Grounding only provides a back-up return path in the event of a problem.
Due to the structure of single-phase power, as in conventional sockets and lighting circuits, there will always be the same current in the line and neutral wires. One more note: the 240V single phase power supply consists of 2 line wires. If used, neutral is not required for the same reason as for the three-phase circuit described below.
Three-phase loads
In a 3-phase power system, things are different. Since the voltage of all lines alternates at the same frequency, there will always be at least one wire with forward current and one with reverse current with respect to the load. No two wires will ever have the same current.
At any time, if you add up the current in all three phase conductors, the total current to or from the load is 0 amps. This does not mean that there is no current. It simply means that there is no additional current going to or from the load between the total current load of all three lines. Even if a neutral wire were connected to the load, it would never be used. This is why three-phase load devices only have three line inputs. The ground wire must still be connected to provide a backup safe connection in the event of a failure. If the line touches the outer case, there is no longer a total zero sum of current between the three line inputs, so a ground must exist for the excess current to safely disconnect the device with the maximum current.
Three-phase sources
Now it becomes clearer why the load devices only use line inputs, but another question arises with 3-phase sources. Sometimes they have a neutral wire. In other cases, they do not have such a wire. If the three-phase power panel provides a neutral in addition to the three lines and earth, then this indicates either 4-wire wye or 4-wire delta. Most likely a star topology system if it is a modern service panel.
The purpose of the neutral wire is that it allows both three-phase loads to be connected as usual, but also allows the electrician to use any of the three lines, as well as the neutral, to form a single-phase power supply. There can be three identical single phase power supplies. It is a normal power supply for industrial lighting circuits and outlets for offices and single phase equipment. The delta topology model is slightly less common, but allows three-phase loads and multiple single-phase voltages, all with one power supply and no additional transformers. This is useful in some cases, but sometimes it is difficult to balance the three lines properly.
Conclusion
Neutral wires are always required to close single-phase circuits, for example, in a residential or socket / lighting network. In the case of 3-phase circuits, a neutral conductor is not required if the sum of the currents on the live lines adds up to 0 amperes.
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