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. [defined in the: IEC 60050-195-2021]

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

Main earthing terminal
Figure 1. Main earthing terminal

Requirements

IEC

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]:

NOTE 1. It is not intended to connect every individual protective conductor directly to the main earthing terminal where they are connected to this terminal by other protective conductors.

NOTE 2. The main earthing terminal of the building can generally be used for functional earthing purposes. For information technology purposes, it is then regarded as the connection point to the earth.

Where more than one earthing terminal is provided, they shall be interconnected.

Each electrical conductor connected to the main earthing terminal shall be able to be disconnected individually. This connection shall be reliable and such that it can only be disconnected by means of a tool.

NOTE 3. Disconnection means may conveniently be combined with the main earthing terminal, to permit measurement of the resistance of the earth electrode.

  1. The wiring of the concrete-embedded foundation earth electrode should have at least one terminal lug for connection to the electrical system of the building, either leaving the concrete inside the building to a suitable connection point (e.g. to the main earthing terminal) or ending at a special connection clamp embedded in concrete of a wall at its surface. At the point of connection, the terminal lug must be accessible for maintenance and measuring purposes.
  2. The main earthing terminal must provide an electrical connection between external conductive parts of the building and the exposed conductive parts of the electrical installation of the building. For this purpose, the switchgear protective busbar of the electrical installation of the building must be connected via a protective conductor to the main earthing terminal. The protective conductors of all distribution and final electrical circuits must be connected to the protective busbar of the switchgear.
  3. The main earthing terminal should normally be installed separately from electrical installation of building in the vicinity of the switchgear. It is permissible to install the main earthing bar inside the switchgear.
  4. In places accessible only to trained and qualified persons, the main earthing terminal can be installed openly. In places accessible to ordinary persons, the main earthing terminal must be equipped with a protective cover. The degree of protection of the enclosure is chosen according to the environmental conditions, but not less than IP2X according to IEC 60529.
  5. If the electrical installation in building has several separate inputs, a main earthing terminal must be provided for each input. If there are several isolated inputs, including inputs from different transformer substations, a main earthing terminal must be made for each switchgear. These busbars must be connected by a protective equipotential bonding conductor whose minimum cross-section must comply with 543.1 of IEC 60364-5-54-2011 for the largest cross-section input.
  6. If the main earthing terminal and the conductors to be connected to it are made of different metals, measures must be taken to ensure a reliable contact connection.
  7. It is recommended that the equivalent electrical conductivity of the cross-sectional area of the main earthing terminal installed near the switchgear is equal to half the conductivity of the protective busbar of the corresponding switchgear.
  8. There is no requirement in the IEC for the minimum cross-section of the main earthing terminal. However, logically, the minimum cross-section of the main earthing terminal cannot be smaller than the largest cross-section of the conductors to be connected to it.
  9. Color and alphanumeric identification of the main earthing terminal must comply with IEC 60445-2021.

BS 7671

The main earthing terminal shall be connected with Earth by one of the methods described in Regulations 542.1.2.1 to 3, as appropriate to the type of system of which the installation is to form a part and in compliance with Regulations 542.1.3.1 and 542.1.3.2 [3].

542.1.2.1. For a TN-S system, means shall be provided for the main earthing terminal of the installation to be connected to the earthed point of the source of energy. Part of the connection may be formed by the distributor’s lines and equipment.

[3]

542.1.2.2. For a TN-C-S system, where protective multiple earthing is provided, means shall be provided for the main earthing terminal of the installation to be connected by the distributor to the neutral of the source of energy.

[3]

542.1.2.3. For a TT or IT system, the main earthing terminal shall be connected via an earthing conductor to an earth electrode complying with Regulation 542.2.

[3]

In every installation a main earthing terminal shall be provided to connect the following to the earthing conductor:

  • (i) The circuit protective conductors;
  • (ii) The protective bonding conductors;
  • (iii) Functional earthing conductors (if required);
  • (iv) Lightning protection system bonding conductor, if any (see Regulation 411.3.1.2 [3]).

To facilitate measurement of the resistance of the earthing arrangements, means shall be provided in an accessible position for disconnecting the earthing conductor. Such means may conveniently be combined with the main earthing terminal or bar. Any joint shall be capable of disconnection only by means of a tool.

References

  1. IEC 60050-195-2021
  2. IEC 60364-5-54-2011
  3. BS 7671