Home / Technical Articles / Electrical Lectures / Current Systems (AC/DC) And Voltage Levels Basics You Must Never Forget

Electrical Engineering Basics

There are many electrical engineering basics you really must know at any time, even in the middle of the night! The basics we will discuss here are current systems and voltage levels in transmission and distribution systems.

Current Systems (AC/DC) And Voltage Levels Basics You Must Never Forget
Current Systems (AC/DC) And Voltage Levels Basics You Must Never Forget


Current Systems

Electric currents are of three classes:

  • Direct (d.c.)
  • Alternating (a.c.), and
  • Pulsating

Distribution and Transmission electrical workers are mainly concerned with alternating currents. Pulsating currents will not be discussed in this article.

Direct currents (d.c.)

A direct current (d.c.) system is one in which current flows in one direction in the conductors of that system. An everyday example is the car battery, which has two terminals, one positive (+) and the other negative (-).

The accepted convention is that the current flows from the positive terminal to the external circuit and returns to the negative terminal.

High voltage transmission of electricity by direct current has been developed over recent years. In general, however, d.c. distribution is limited to use in:

  1. Tramway and traction systems with a voltage of usually 600V;
  2. Railway d.c. traction systems with a voltage of 1.5kV between rail and overhead collector wire;
  3. Lifts, printing presses and various machines where smooth speed control is desirable;
  4. Electroplating; and
  5. Battery charging.
Usually d.c. systems are of 2-wire or 3-wire types. In a 2-wire system one wire is positive and the other negative. The difference in potential for tramways is 500V with the rail negative and in the d.c. railway system the difference in potential is 1.5kV, again with the rail negative.

In a 3-wire system the standard voltages are 460 and 230V. There are three wires, one being at 230V positive (or + 230 volts potential), the second 230V negative (or – 230 volts potential), with the third called the “common” or neutral being at zero potential (see Figure 1).

Supply at 230V is taken from the “outer” (or positive) and the common conductors, or from “inner” (or negative) and the common conductors.

Potential in a 3-wire system
Figure 1 – Potential in a 3-wire system

Energy for motors at 480V is taken from the outer and the inner conductors.

Go back to contents ↑

Alternating current (a.c.)

An alternating current (a.c.) flows in an electrical circuit that is energized with an alternating voltage. This voltage is one that reverses its sense of direction in a regular manner, and this is caused by the method by which it is generated.

In simple terms, the generator is a copper coil, which is mounted on a shaft between opposite poles of a magnet. When the shaft spins, the copper cuts the magnetic field and a voltage appears at the ends of the coil.

The generator (or alternator) is shown in Figure 2 (left).

Figure 2 (left) - Simple a.c. generator; Figure 3 (right) - a.c. voltage wave form
Figure 2 (left) – Simple a.c. generator; Figure 3 (right) – a.c. voltage wave form

As the coil rotates one revolution the voltage follows the variation shown in Figure 3 (right). When the coil is at right angles to the magnetic field, it is not cutting the field and the voltage is zero. The maximum rate of cutting occurs when the coil is in line with the magnetic field and there is a maximum voltage output.

From zero to maximum and beyond maximum back to zero occurs in one half revolution and the voltage rises and falls. In the next half revolution, the generated voltage is opposite to the first half. One full revolution of the coil produces one “cycle” of variation.

The number of voltage cycles in one second of time is called the frequency of the supply, and is given the name Hertz (Hz). The standard frequency in Australia and most of the countries is 50Hz.

Go back to contents ↑

Advantage of a.c. for distribution

Alternating current has an important advantage over direct current in that the voltage can be changed by transformers to a high value for transmission over long distances and then reduced at the customer’s point of supply to a lower level suitable for operating lights, motors and other appliances.

As power = volts × amps, for the same power level to transmitted, a high voltage can be used so that the current can be kept to a low level thereby minimizing the voltage drop.

Transmission of high power levels therefore requires:

  1. Resistance of the transmission line to be as small as possible
  2. The transmission line current to be as low as possible

The first condition cannot always be met, as it needs conductors of large cross-sectional area. Large conductors are expensive and their great weight would require strong and costly supports.

On the other hand, the second condition can be met by raising the transmission line voltage so that high power levels can be transmitted with relatively small currents. The small currents in turn require relatively small cross-sectional area, lightweight conductors with correspondingly lighter supports.

Therefore, when high amounts of power levels are involved, it is general practice to use high transmission voltages and relatively small currents with correspondingly small voltage drops.

This condition is much more efficient than if an equivalent power level were transmitted at low voltage and high current with a relatively high voltage drop.

Transformers are used to provide the high voltages necessary for the transmission of high power levels over long distances. In keeping with the value of the transmission line voltage employed, it is necessary to insulate the conductors against leakage to earth.

Go back to contents ↑

Voltage Values

In the following, “voltage” means the voltage between the conductors. The standard voltage values used are:

  1. Extra low voltage (ELV) – means any voltage not exceeding 50V a.c. or 120V ripple free d.c.
  2. Low voltage – means any voltage exceeding 50V a.c. or 120V ripple free d.c. but not exceeding 1kV a.c. or 1.5kV d.c.
    Thus the normal voltages of 240V and 415V delivered to most customers are “low voltage”.
  3. High voltage (HV) – means and voltage exceeding 1kV a.c. or 1.5kV d.c.
  4. Extra high voltage (EHV) means any voltage exceeding 220kV.

Go back to contents ↑

Standard line voltages

The standard line voltages in use are:

Line VoltagesUsage
240/415V (3 phase)Used to supply customers installations
240/480V (1 phase)
6.6kVUsed for urban and rural HV distribution
12.7kV (SWER)
33kVUsed for sub-transmission of larger power levels in distribution over middle distances
110kVUsed for transmission of large power levels over long distance

Go back to contents ↑

Voltage between live conductors and voltage to neutral

The voltage between any two live conductors is often referred to as the “line voltage”. The voltage to neutral, often referred to as the “phase voltage”, is the voltage between any live conductor and the neutral point or earth of the system.

Three-phase system with earthed neutral
Figure 4 – Three-phase system with earthed neutral

Figure 4 shows the line and phase voltages in a three-phase system. The neutral point is usually earthed at the supply end (for protection and safety reasons) and each live conductor is then at a definite potential to earth.

For instance, in an 11kV three-phase system, the voltage between any two live conductors gives a line voltage of 11kV while the voltage between any live conductor and neutral (or earth) gives a phase voltage of 6.35kV.

Go back to contents ↑

Voltage Systems

High voltage overhead systems

The two systems most commonly used for transmission and distribution are:

  1. Single-phase
  2. Three-phase

Go back to contents ↑

High voltage single-phase system

This system is generally associated with the distribution of low power levels over relatively short distances. Single-phase systems are generally fed from a three-phase line.

The single-phase line consists of two conductors, neither directly earthed to the general mass of earth. In this system there is no neutral conductor (see Figure 5).

It is usual to have the three-phase system earthed (at the neutral point of the transformer or generator supplying the system) either solidly or through some current limiting resistance (for safety and protection purposes). As the single-phase HV system is part of the three-phase HV system, each phase of the single-phase system has a definite voltage to earth.

For safety reasons alone, it is important to remember that each phase is alive to earth and that a definite voltage exists between each phase and the equipment connected to the ground.

Three-phase high voltage system with single-phase spur
Figure 5 – Three-phase high voltage system with single-phase spur

Go back to contents ↑

High voltage three-phase system

This system is widely used for the transmission of high power levels and is also the standard system used in distribution and reticulation.

It consists of three conductors, each called a “phase”. To standardize the identification of the phases, they are known as A, B and C phases or red, white and blue phases respectively.

The voltage in each phase alternates, in a similar manner to the alternating voltage shown in Figure 3 but one follows the other in regular order (see Figure 6).

Representation of the three sine waves in a three-phase system
Figure 6 – Representation of the three sine waves in a three-phase system

Brie y, phase A reaches its maximum positive value first, then is followed by phase B, then by phase C and so on. The order in which the phases reach their peak is called the phase sequence.

Go back to contents ↑

Phase sequence

It is essential that the order of phase sequences and the identity of the A, B and C be known. In the case just cited, the order of phase sequence was from A to B to C because the voltage in phase B reached its maximum value after that in phase A and the voltage in phase C reached its maximum value after that in phase B.

Phase sequence has an important bearing on the direction of rotation of three-phase a.c. motors, which depend on the phase sequence and the relative position of the three-phases connected to the motor terminals.

A reversal in the order of the phase sequence (eg. by interchanging any two of the three wires connected to its main terminals) will cause the motor to run in the reverse direction of rotation.

For this reason alone, it is important that electrical workers know what happens if there is an inadvertent change in the position of the phases supplying a factory in which motors are installed.

Go back to contents ↑

Low voltage single-phase 2-wire overhead system

In this system there are two conductors, one generally solidly earthed at the transformer and known as the “neutral”, while the other is known as the “live”, “active” or “phase” conductor.

The voltage between phase and neutral is nominally 240V and the voltage of the phase or active conductor to earth is therefore also 240V (see Figure 7).

Single-phase 2-wire system
Figure 7 – Single-phase 2-wire system

Go back to contents ↑

Low voltage single-phase 3-wire system

In certain rural areas, it is often more economical to install a single-phase high voltage line, saving the cost of the third high voltage phase and to supply the load by stepping down through a transformer to a 3-wire system. One conductor is earthed and known as the neutral while the other conductors are both “actives”. (see Figure 8).

Single-phase 3-wire system
Figure 8 – Single-phase 3-wire system

The voltage between either of the actives and the neutral is 240V while the voltage between the two active conductors is 480V. It is the a.c. equivalent of the three-wire d.c. system. It facilitates the supply of larger loads or loads at greater distances from the transformer than the single-phase 2-wire system.

Half of a domestic 240V load is connected between one active and the neutral and the other half between the other active and the neutral. This balances the load on each phase and reduces, if not eliminates, the residual current in the neutral.

Go back to contents ↑

Low voltage three-phase 4-wire system

This system employs four conductors and is widely used in all areas where it is considered economical to supply large amounts of energy for industrial and domestic purposes.

The system is shown in Figure 9 – a, b and c are the active conductors and n is the neutral which is connected to the “star point” of the transformer. It is usual for the “star point” to be earthed as shown.

Three-phase system with earthed neutral
Figure 9 – Three-phase system with earthed neutral

The standard voltage between actives is 415V, while the voltage between any one of the actives, (a, b and c respectively) and the neutral is 240V.

The same phase relationship of “phase sequence” exists on the LV as on the HV side of the transformer, so care must be taken when renewing mains to avoid upsetting the phase sequence to the supply of motor loads.

Go back to contents ↑

High voltage single-wire earth return (SWER) system

The power system known as the SWER system uses only one HV conductor with the earth being used as the return conductor, (see Figure 10).

This system was first developed in New Zealand and is now used in Australia, South Africa and many other countries. It can have great economic advantages in hilly areas where the loading is relatively light, where long distances are involved and where the line can be strung from ridge top to ridge top.

Because of the generally lower impedance of the line to earth circuit, it usually has better voltage regulation than a conventional single-phase 2-wire circuit.

To restrict noise interference in telecommunications systems, the amount of earth current allowed to ow in the earth return circuit is limited. Furthermore, there must be a minimum separation between SWER lines and any telecommunication lines.

A special transformer is used to isolate the SWER line from the main distribution line. The SWER line voltage is 12.7kV to earth. The distribution transformers tted to the SWER line can be either single-phase 2-wire 240V supply or single-phase 3-wire 240/480V supply.

Particular attention must be paid to the good earthing of the transformers on a single-wire line and to the protection of these earth wires from physical damage.

Single wire earth return system
Figure 10 – Single wire earth return system

Go back to contents ↑

Reference // VESI Fieldworker Handbook

SEARCH: Articles, software & guides

Premium Membership

Premium membership gives you an access to specialized technical articles and extra premium content (electrical guides and software).
Get Premium Now ⚡

About Author


Edvard Csanyi

Edvard -

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on


  1. Oluchukwu Okoye
    Dec 01, 2017

    As an Electrical Engineer, I must commend the author of this. It was well explained. Thanks for sharing!

  2. scott davis
    Nov 29, 2017

    I have read several of the articles graciously made available without Premium membership. I have been tempted to sign up for that subscription. I suspect there are many others who have done the same.
    Perhaps it would encourage Premium participation if you made available a snapshot glimpse of what the Premium features contain. A list of monthly article titles would be good. Better would be the titles with a one or two sentence overview for each. Best would be the title, overview, and a count of associated diagrams or charts within the article. It is always easier to spend money when one feels assured of value.
    Thank you for the effort and for all the content you already offer. -Scott Davis | Electrical Plans Examiner

  3. Gerzan Argueta
    Nov 29, 2017

    This is an excellent web site, sometimes, you cannot down load pdf, to review it. However, excellent topics……..

Leave a Comment

Tell us what you're thinking... we care about your opinion!