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What Does Circuit Protective Conductor (CPC) Mean?

BS7671 defines a circuit protective conductor (CPC) as a protective conductor connecting exposed-conductive-parts of equipment to the main earthing terminal. The term is actively used in the United Kingdom.

Circuit Protective Conductor Requirements

- A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.
- Except where Regulation 412.1.3 [1] applies, a circuit supplying one or more items of Class II equipment shall have a circuit protective conductor run to and terminated at each point in wiring and at each accessory.
- In every installation a main earthing terminal shall be provided to connect to the earthing conductor the circuit protective conductors.

What Is an Earthing Conductor? Definition, Requirements, Size

Earthing conductor (UK) / Grounding conductor (US): conductor forming a conductive path between a conductive part and an earth electrode [definition: IEC 60050-195-2021]. Included as part of the earthing arrangement.

Example: conductor connected between a main earthing terminal or busbar and an earth electrode.

Earthing conductors shall comply with 543.1.1 or 543.1.2 [2]. Their cross-sectional area shall be not less than 6 mm2 for copper or 50 mm2 for steel. Where a bare earthing conductor is buried in the soil, its dimensions and characteristics shall also be in accordance with Table 54.1 [2].

Main Earthing Terminal (MET): Definition, Purpose, Requirements

Main earthing terminal (MET) (UK and IEC) or main grounding terminal (US): terminal or busbar that is part of the earthing arrangement of an installation and enabling the electric connection of a number of conductors used for earthing or bonding purposes. [defined in the: IEC 60050-195-2021]

Synonyms for term: main earthing busbar (UK and IEC), main grounding busbar (US).

Live Conductor: Definition, Meaning, Examples

Live conductor: a conductor intended to be energized in normal use, including a neutral conductor but, by convention, not a PEN conductor [defined in the BS7671].


The term “live conductor” is derived from the general term “live part” and refers to those live conductive parts intended to conduct certain electric currents.

Live conductors are primarily defined as phase conductors and pole conductors, which under normal conditions are energized in a way that poses a serious hazard to humans and animals. The neutral conductor and the mid-point conductor, which are under low voltage, are also live conductors. Phase and pole conductors in extra-low voltage electrical circuits have a voltage that are generally not hazardous to humans and animals.

PEL Conductor: Definition, Examples of Systems, Requirements

PEL Conductor: conductor combining the functions of both a protective earthing conductor and a line conductor [SOURCE: IEC 60050-195:2021, 195-02-14].

The designation “PEL” is formed from two short designations: protective conductor “PE” and line conductor “L”, indicating the conductors whose function is intended to be performed by a PEL conductor.

A PEL conductor, like other protective conductors, is not considered a live part. However, a PEL conductor is a current-carrying conductor that is counted in the total number of conductors of an electrical circuit, network or system.

Protective Bonding Conductor (PB): Definition, Sizes, Colour Identification

Protective bonding conductor: protective conductor provided for protective-equipotential-bonding [defined in the IEC 60050-195-2021].

The following metal parts are not permitted for use as protective bonding conductors:

- metallic water pipes;
- metallic pipes containing potentially flammable materials such as gases, liquids, powder;
- constructional parts subject to mechanical stress in normal service;
- flexible or pliable metal conduits, unless designed for that purpose;
- flexible metal parts;
- support wires; cable trays and cable ladders.

Where protective equipotential bonding conductors are installed, they should be parallel to and in closest contact as possible with d.c. cables and a.c. cables and accessories.

PEM Conductor: Definition, Examples of Systems, Requirements

PEM Conductor: conductor combining the functions of both a protective earthing conductor and a mid-point conductor [defined in the IEC 60050-195:2021].

Note. In the Russian Federation , the term “PEM conductor” is defined differently: PEM conductor – electrical conductor combining the functions of both a protective earthing conductor and a mid conductor [3].

A PEM conductor, like other protective conductors, is not classified as a live part. However, a PEM conductor is a current-carrying conductor that is counted in the total number of conductors of an electrical circuit, network or system. It does not refer to line conductors.

Mid-Point Conductor (M): Definition, Meaning, Examples of Use

In this article we will look at what a mid-point conductor is, examples of its use in various systems, and its purpose. We will focus separately on the color and alphanumeric identification of the mid-point conductor.

What Is a Mid-Point Conductor (M)?

Mid-point conductor: conductor electrically connected to the mid-point and capable of contributing to the distribution of electric energy [defined in the IEC 60050-195-2021].

Examples of Use

Based on the definition, it follows that mid-point conductors can occur in 3-wire DC systems. Figure 1 shows an example of the TN-S system, Figure 2 shows the TN-C-S system, Figure 3 shows the TT system, and Figure 4 shows the IT system, which uses mid-point conductors.

What Is Conductive Parts? Definition & Examples

Conductive part: a part that can carry electric current [this term is defined in the IEC 60050-195-2021].

What Are the Conductive Parts?

The term ‘conductive part’ defines a qualitative characteristic of any part of the electrical equipment, the electrical installation of a building or a building, namely its ability to conduct an electric current. Conductive parts of electrical equipment or the electrical installation of a building include cores of wires and cables, busbars and other electrical conductors, as well as other conductive elements of electrical equipment – metallic and conductive non-metallic fittings, enclosures, means of fastening, etc.

What Is a Live Parts (in Electrical Installations and Equipment)?

Live parts: a conductive part intended to be energized under normal operating conditions, including the neutral conductor and mid-point conductor, but excluding the PEN conductor, PEM conductor and PEL conductor [this term is defined in the IEC 60050-195-2021]. This term does not necessarily imply a risk of electric shock.

The term under consideration defines those conductive parts of electrical equipment and electrical installations which are energized during their operation under normal conditions. A live part, as a special case of a conductive part, has one distinguishing feature – it can be live under normal conditions. Conductive parts which are not energized under normal conditions, but which may become energized under fault conditions, are not live parts.

Examples of Live Parts

The live parts include phase and neutral conductors of AC electrical circuits, pole and mid conductors of DC electrical circuits, and other conductive parts of a low-voltage electrical installation that have electrical connections to these electrical conductors and are under normal conditions at an electrical potential different from the earth potential.

Electrical Distribution System Explained

Electrical distribution system: low-voltage electrical system consisting of a distribution network and an electrical installation [this term is defined in the IEC TS 62257-5-2015].

The structure of the distribution system and its examples.

Note 1 to entry: The distribution system usually includes an electrical installation of building which is connected to the low-voltage distribution network consisting of a step-down transformer substation and an overhead line or an underground cable (see Figure B.1).

Note 2 to entry: The smallest distribution system includes a power source and one item of a current-using equipment (see Figure B.2).

Power sources can also be: a local power plant, a separate small power generator driven by an internal combustion engine, and even an isolation transformer, on the basis of which the IT system is implemented in a part of the building’s electrical installation. However, the listed power sources are exceptions to the general rule. In the vast majority of cases in the low-voltage distribution networks to which the building installation is connected, the power sources are transformers installed in step-down transformer substations.

Electrical Installation of Building: Definition, Meaning, Structure

Electrical installation of building: assembly of associated electric equipment located in a building and having co-ordinated characteristics to fulfil specific purposes [SOURCE: IEC 60050-826:2004, 826-01-01, modified: addition of “located in a building and”].

Electrical Installation of the Apartment

Currently, there is a new concept – “apartment electrical installation”, which characterizes a separate part of the electrical installation of an apartment building, located in one apartment. The electrical installation of the apartment is a set of interconnected electrical equipment installed in the apartment. Usually it functions independently of the electrical installations of other apartments.

The electrical installation of an apartment consists of several parts-electrical circuits. Until the nineties of the last century, the electrical installation of an apartment was usually a single-phase electrical installation and consisted of two final electric circuits for lighting and plug sockets. In residential buildings where electric stoves were installed, a third electric stove final electric circuit was added to the two mentioned circuits. These circuits were connected to the floor distribution board.

What Is an Electrical Equipment? Definition, Meaning, Examples, Classification

Electrical equipment: item used for generation, conversion, transmission, distribution, or utilization of electric energy (source: IEC 60445:2021).

NOTE. In the Russian Federation, the term “electrical equipment” is defined differently: item intended for generation, transmission, and variation of characteristics of electric energy, change its characteristics, and also for convert electric energy into another form of energy.

Examples of such items are electric machines, transformers, switchgear and controlgear, measuring instruments, protective devices, wiring systems, and current-using equipment.

Electrical Distribution Network: What it is, Definition, Diagram

Electrical distribution network: low-voltage electrical network consisting of the power source and a distribution line (overhead line or underground cable) and intended for supplying of the electric power to electrical installations of buildings and other low-voltage electrical installations [this term is defined in the IEC TS 62257-5:2015, B.1.2].

Note. The word “electrical” is often not used when talking about distribution network.

Figure 1 below illustrates the connection of a building’s electrical installation to the low-voltage distribution network.

Current-Using Equipment: What it is, Definition, Examples

Current-using equipment: electrical equipment intended to convert electrical energy into another form of energy [SOURCE: IEC 60050-826:2004, 826-16-02, modified].

Let’s refer to the book [2], in which Y.V. Kharechko gives examples of current-using equipments:

« Current-using equipments is the predominant part of electrical equipment, which is used to convert electrical energy into mechanical, thermal, light and other types of energy. As an example, current-using equipments include such electrical equipment as electric motors, electric heaters, electric lamps, the vast majority of household electrical equipment: electric stoves, hair dryers, irons, washing machines, vacuum cleaners, refrigerators, etc. »

Earthing Arrangements: What Is it? (Definition, Requirements, Example)

Earthing arrangement (UK and IEC) or grounding arrangement (US): all electrical means involved in the earthing of a system, installation or equipment [definition from IEC 60050-195:2021].

Note 1 to entry: Electric connection and devices used for earthing are examples of electrical means.

Note 2 to entry: If a protective earthing and a functional earthing are needed both in the electrical installation, a common erthing arrangement should be provided satisfying both purposes or both erthing arrangements should be connected together alternatively according to IEC 60364-4-44

General Requirements
The earthing arrangements may be used jointly or separately for protective and functional purposes according to the requirements of the electrical installation. The requirements for protective purposes shall always take precedence.

Where provided, earth electrodes within an installation shall be connected to the main earthing terminal using an earthing conductor.

NOTE. An installation does not need to have its own earth electrode.

Where the supply to an installation is at high voltage, requirements concerning the earthing arrangements of the high voltage supply and of the low-voltage installation shall also comply with Clause 442 of IEC 60364-4-44:2007.

What Is an Overcurrent? Definition, Causes, Protection

What is meant by overcurrent?

Overcurrent: electric current exceeding the rated electric current [SOURCE: IEC 60050-826].

NOTE. For conductors, the rated current is considered as equal to the current-carrying capacity.

An overcurrent is any electrical current which exceeds the rated current of any part of the building installation or the rated current of the electrical equipment used in it, for example: the rated current of a circuit, the continuous current-carrying capacity of a conductor, the rated current of a circuit breaker, a fuse, a residual current device, etc.

An overcurrent in any part of the electrical installation of a building can lead to overheating, combustion and, consequently, fire in the building. For this reason, overcurrent protection is provided in electrical installations of buildings.

Inductor Symbols for Diagrams

This article provides overview of common and less common inductor symbols.

An inductor, also called a coil, choke, winding, inductance or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a coil.

What Is Voltage? Definition & Meaning, Formula

One of the most fundamental terms in electrical engineering is the term “electrical voltage”. In this article, we will explain what it is and how to calculate it.

Definition and Meaning

Voltage U is the very cause that “makes” electric current I flow. Voltage always occurs when the charges are separated from each other, that is, all the negative charges are on one side and all the positive charges are on the other side. If you connect these two sides with an electrically conductive material, electric current will flow.

The generally accepted definition of the term “electrical voltage”.

Electrical voltage (or simply voltage) is the potential difference between two points in an electric field. It is the driving force for an electric charge.

A potential in an electric field is the energy of a charged body, independent of its electric charge. For clarification, you can look at the comparison to the water circuit just below in the article.

Main Earthing Terminal (MET): Definition, Purpose, Requirements

Definition and Purpose

Main earthing terminal (MET) (UK and IEC) or main grounding terminal (US): terminal or busbar that is part of the earthing arrangement of an installation and enabling the electric connection of a number of conductors used for earthing or bonding purposes. [this term is defined in the: IEC 60050-195-2021]

Synonyms for term: main earthing busbar (UK and IEC), main grounding busbar (US).


In every installation where protective equipotential bonding is used, a main earthing terminal shall be provided and the following shall be connected to it [2]:

- protective bonding conductors;
- the earthing conductor by which the main earthing terminal is connected to the earth electrode;
- is the protective conductor that connects the protective busbar of the switchgear to the main earthing terminal;
- functional earthing conductors and functional equipotential bonding conductors used to carry out functional earthing and functional equipotential bonding.