Earthing connections

In a building, the connection of all metal parts of the building and all exposed conductive parts of electrical equipment to an earth electrode prevents the appearance of dangerously high voltages between any two simultaneously accessible metal parts

Definitions

National and international standards (IEC 60364) clearly define the various elements of earthing connections. The following terms are commonly used in industry and in the literature. Bracketed numbers refer to Figure E1 :

Fig. E1: An example of a block of flats in which the main earthing terminal (6) provides the main equipotential connection; the removable link (7) allows an earth-electrode-resistance check

• Earth electrode (1): A conductor or group of conductors in intimate contact with, and providing an electrical connection with Earth (cf details in section 1.6 of Chapter E.)
• Earth: The conductive mass of the Earth, whose electric potential at any point is conventionally taken as zero
• Electrically independent earth electrodes: Earth electrodes located at such a distance from one another that the maximum current likely to flow through one of them does not significantly affect the potential of the other(s)
• Earth electrode resistance: The contact resistance of an earth electrode with the Earth
• Earthing conductor (2): A protective conductor connecting the main earthing terminal (6) of an installation to an earth electrode (1) or to other means of earthing (e.g. TN systems);
• Exposed-conductive-part: A conductive part of equipment which can be touched and which is not a live part, but which may become live under fault conditions
• Protective conductor (3): A conductor used for some measures of protection against electric shock and intended for connecting together any of the following parts:

  - Exposed-conductive-parts
  - Extraneous-conductive-parts
  - The main earthing terminal
  - Earth electrode(s)
  - The earthed point of the source or an artificial neutral

• Extraneous-conductive-part: A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation (4).

For example:
  - Non-insulated floors or walls, metal framework of buildings
  - Metal conduits and pipework (not part of the electrical installation) for water, gas, heating, compressed-air, etc. and metal materials
    associated with them 

• Bonding conductor (5): A protective conductor providing equipotential bonding
• Main earthing terminal (6): The terminal or bar provided for the connection of protective conductors, including equipotential bonding conductors, and conductors for functional earthing, if any, to the means of earthing.
  

Connections

The main equipotential bonding system
The bonding is carried out by protective conductors and the aim is to ensure that, in the event of an incoming extraneous conductor (such as a gas pipe, etc.) being raised to some potential due to a fault external to the building, no difference of potential can occur between extraneous-conductive-parts within the installation.
The bonding must be effected as close as possible to the point(s) of entry into the building, and be connected to the main earthing terminal (6).
However, connections to earth of metallic sheaths of communications cables require the authorisation of the owners of the cables.

Supplementary equipotential connections
These connections are intended to connect all exposed-conductive-parts and all extraneous-conductive-parts simultaneously accessible, when correct conditions for protection have not been met, i.e. the original bonding conductors present an unacceptably high resistance.

Connection of exposed-conductive-parts to the earth electrode(s)
The connection is made by protective conductors with the object of providing a low-resistance path for fault currents flowing to earth.

Components

(see Fig. E2) Effective connection of all accessible metal fixtures and all exposed-conductive-parts of electrical appliances and equipment, is essential for effective protection against electric shocks.

Component parts to consider: as exposed-conductive-parts	as extraneous-conductive-parts
Cableways Conduits Impregnated-paper-insulated lead-covered cable, armoured or unarmoured Mineral insulated metal-sheathed cable (pyrotenax, etc.)	Elements used in building construction Metal or reinforced concrete (RC):   - Steel-framed structure   - Reinforcement rods   - Prefabricated RC panels Surface finishes:   - Floors and walls in reinforced concrete without further      surface treatment   - Tiled surface Metallic covering:   - Metallic wall covering
Switchgear cradle of withdrawable switchgear
Appliances Exposed metal parts of class 1 insulated appliances
Non-electrical elements metallic fittings associated with cableways (cable trays, cable ladders, etc.) Metal objects:   - Close to aerial conductors or to busbars   - In contact with electrical equipment.	Building services elements other than electrical Metal pipes, conduits, trunking, etc. for gas,water and heating systems, etc. Related metal components (furnaces, tanks,reservoirs, radiators) Metallic fittings in wash rooms, bathrooms, toilets, etc. Metallised papers

Component parts not to be considered: as exposed-conductive-parts	as extraneous-conductive-parts
Diverse service channels, ducts, etc. Conduits made of insulating material Mouldings in wood or other insulating material Conductors and cables without metallic sheaths	Wooden-block floors Rubber-covered or linoleum-covered floors Dry plaster-block partition Brick walls Carpets and wall-to-wall carpeting
Switchgear Enclosures made of insulating material
Appliances All appliances having class II insulation regardless of the type of exterior envelope

Fig. E2: List of exposed-conductive-parts and extraneous-conductive-parts