The story of Electricity Generation from pipelines to pylons
Part 5: Neutral and Ground; Delta and Star connections.
Let's now look at some three-phase basics. The red, yellow and blue colour code of the 3-phase system applies from the generator outputs of Killingholme "A" all the way through to the 415V transformers found near the end-user.
< Fig. 13a - Three separate windings need 6 wires to connect.
You might think that the use of three very large stator coils to generate three-phase power would demand six wires to conduct the current, as shown in Fig. 13a (left), noting the direction of each winding (or the start/ end of the coil) is denoted with a spot symbol.
After all, a single-phase load requires two supply wires to power it: normally known as live (or "hot) and neutral, though the live is more correctly called the line voltage.
< Fig. 13b A Delta connection uses three wires as shown.
However a three-phase system is able to transmit three times the power of a single phase design without the need for six wires, simply by arranging the windings as shown in Fig. 13b.
The start of one winding is connected to the end of another, and the three connections are brought out as shown. Because of its shape, this "triangular" configuration is known as a delta (or mesh) connection.
Three wires can be used to transmit a three phase supply in this way.
The alternative arrangement of windings shown in Fig. 14a below is called a star connection, (or Y connection) and the central connection is called the "star point" (or the neutral point). This is used universally by National Power on the output of all of its generators. The star point itself, to look at, is just an overhead box with three ducts entering:
By commoning the "starts" of each phase together as shown, a four wire-system (three "line" voltages or phases, and a neutral conductor) can be created. The net current flowing into or out of the star point is zero because each phase uses the other two for its return path. The net voltage is also zero at the star point.
|Fig. 14a. A star (or "Y") connection with the star being the neutral point. Compare with the Delta connection, Fig. 13b above.||Fig. 14b A 3-phase system has 415V between phases, and produces 240V across each "load" connected to the star point,. Since the net voltage is zero in the 3-phase system, the star point is neutral or zero volts. It is connected to the ground.|
The principles of delta and star circuits are relevant throughout the power transmission network, all the way down to the 230V supply delivered to a home. In the case of a three-phase 415V supply, Fig. 14b, a step-down transformer is used (not shown) which has a 11kV delta primary and a star-wired 415V secondary. Therefore 415V is supplied between any two lines as shown.
A voltage of 415V / √3 = 230V will be developed across a "load" placed between the star point and a single phase. This is precisely how a domestic 230V is derived - connected to the star/ neutral point and any of the 415V phases.
On Neutral Ground
Having outlined the overall process of power generation, let's explore the need for a neutral wire, and also the requirement for earthing.
In order to provide a 230V domestic supply from a 415V three phase supply, the "live" 230V wire is taken from one of the transformer's 415V phases and the neutral is taken from the star point. The star or neutral point is also physically connected to the earth at the transformer, as depicted by the earth symbol in Fig. 14b, for safety reasons which will become apparent later.
In a typical residential installation, the three 230V supplies which are provided by a 415V three-phase transformer are evenly distributed to balance the load on the transformer. This is achieved by, say, connecting every third house to the same phase. A whole street of small 2-3 bedroom houses may have one phase whilst a small development of much larger houses - which will demand more power - might use a different phase to try to balance the loading, and so on.
A significant side effect of this arrangement is that neighbouring premises, whilst each enjoying a 230V supply, may have a potential of 415V between their respective live supplies: it can be seen in Fig. 14b how 415V exists between any two phases or lines.
The voltage between separate 'live' wires on different phases is 415V (UK). Therefore the 230V "live" of one residence should never be used in neighbouring installations because they might not share the same phase.
^ Fig. 15 How a 415V 3-phase system is utilised to distribute 230V to the end user. Each house uses one phase in order to balance the loading on the sub-station (refer to Fig. 14b)
Note especially each house has a Live and Neutral, and see how the Star point (neutral) is also grounded or earthed.
< A typical 3 phase transformer distributing the mains supply to residences nearby. This one had previously burned out, setting fire to the wooden pole and then the field below!
In Fig. 15 each "house" is connected to one of the 3-phase "lines", and the diagram shows how all three houses have their neutral wires commoned together to the star/ neutral point of the 415V supply transformer. That same star point is connected to the earth.
Quite how an individual house will be connected in practice depends on several factors, but if an underground supply is taken to the property, then usually a 230V supply is routed there through an armoured cable, see photo. The steel armour is wired as a "protective earth" and connects the house earth system to the earthed star / neutral point of the local 415V transformer.
^ Armoured cable with protective earth and cable gland termination
At the consumer's property, the steel wires in the armoured braid are inherently in contact with a brass cable gland/ strain relief. As the brass component is earthed, then anything it's bolted will be earthed as well. This completes a good quality metal earthing connection between the domestic earth system and the star/ neutral point of the 415V transformer. All exposed domestic copper water pipes and other metalwork which could become live through an insulation or wiring fault, are hardwired together by "equipotential bonding". This ensures that no single route to earth will be more electrically resistant to a fault current than any other.
Incidentally, the consumer's neutral may also be directly connected to an earth stake at their incoming supply, but usually this only occurs if the existing earth connection path is found to be inadequate, or if no earth has been provided at all by the electricity supply authority. Oother variations of house wiring are defined in the UK's IEE Regulations that a competent electrician will be familiar with. In the case of overhead domestic supplies, the earth terminal of the property might only be connected to earth but not to neutral.
In the last part, electrical safety considerations are described.