What Is a Functional Earthing? Definition, Meaning, Example

Definition

Functional earthing or functional grounding (US): earthing for functional purposes other than electrical safety [defined in the IEC 60050-195-2021].

Functional earth: earthing of a point or points in a system or in an installation or in equipment, for purposes other than electrical safety, such as for proper functioning of electrical equipment [defined in the BS 7671:2018+A2:2022].

To define a clear borderline between functional earthing and protective earthing the following explanations are given:

  • Functional earthing: If any connection of the functional earthing is interrupted, it does not impair any kind of protection or any kind of protective measure or protective provision provided for electrical safety. Therefore, its application mainly relates to communication, measurement and electromagnetic compatibility (EMC).
  • Protective earthing: If any connection of the protective earthing is interrupted, it impairs the protection or the function of a protective measure or protective provision provided for electrical safety.

Functional Earthing Explained

The most common type of earthing we see everywhere in electrical installations is protective earthing. Its main function is to protect people from electric shock and equipment from failure in an emergency.

But there is another task for which earthing is necessary: to protect equipment from parasitic interference. It is a low current disturbance, not capable, in general, of damaging equipment or affecting the health of people. However, the absence of such earthing, referred to as functional earthing leads to the occurrence of unpleasant background noise in loudspeaker equipment, malfunctions in communication systems, etc.

Functional earthing must be placed as close as possible to the object to be protected. This is because this earthing is designed to protect against high-frequency interference, among other things.

Functional earthing is basically equipotential bonding; this is the discharge of coupled transient currents that build up on screens, or a FE function in device parts for people with non-dangerous voltages. The same applies for couplings on the ground leads inside equipment. These, too, must be safely discharged.

If a functional earth is required for certain equipment, for example measuring and control equipment, supplementary protective equipotential bonding shall be provided between all exposed-conductive-parts and extraneous-conductive-parts inside the location and the functional earth.

Functional Earthing and Functional-Equipotential-Bonding for Information and Communication Technology Equipment and Systems (ICT)

A functional-equipotential-bonding system may comprise:

Where the functional-equipotential-bonding system is not locally connected to the protective-equipotential-bonding system, the functional bonding conductors shall be:

  • insulated, and
  • installed separately from the protective conductor, and
  • connected to the main earthing terminal only once.

NOTE: The functional-bonding-conductors are insulated because those conductors could under certain circumstances achieve a dangerous potential.

If there are multiple functional bonding conductors present in the electrical installation, a separate main functional earthing terminal (MFET) shall be installed for ease of connection for these conductors. The main functional earthing terminal shall be connected to the main earthing terminal only once.

The cross-sectional area of every functional bonding conductor or functional earthing conductor shall be capable of withstanding all mechanical and thermal stresses caused by the expected operational current. This current shall be determined in accordance with the manufacturer’s instructions or by measurement taking into account the ICT equipment or system.

Minimum cross-sectional area.

In the absence of requirements, for example stated by the equipment manufacturer, the following minimum cross-sectional area shall be applied for functional earthing conductors and functional bonding conductors:

  • 2,5 mm2 Cu or 16 mm2 Al, if protection against mechanical damage is provided,
  • 4 mm2 Cu or 16 mm2 Al, if protection against mechanical damage is not provided.

NOTE: Larger cross-sectional areas can be specified for EMC reasons.

If part of an item of equipment can be removed, the functional bonding conductor for the remaining part of the electrical installation shall not be disconnected.

Main functional earthing terminal (MFET).

The following conductors shall be connected to the main functional earthing terminal (MFET), if any:

  • functional earthing conductors;
  • functional bonding conductors.

The main functional earthing terminal (MFET) and the main earthing terminal (MET) may be combined, as stated in 542.4 [2].

Functional Earthing Example

Where there are problems in existing building installations due to electromagnetic influences, the following measures may improve the situation; see Figure 1:

Illustration of measures in an existing building
Figure 1. Illustration of measures in an existing building. The figure shows the functional earthing conductor
ReferenceDescription of the illustrated measures
1)Cables and metal pipes enter the building at the same place
2)Common route with adequate separations and avoidance of loops
3)Bonding leads as short as possible, and use of earthed conductor parallel to a cable
4)Signal cables screened and/or conductors twisted pairs
5)Avoidance of TN-C beyond the incoming supply point
6)Use of transformers with separate windings
7)Local horizontal bonding system
8)Use of class II equipment

References

  1. IEC 60050-195-2021
  2. IEC 60364-5-54:2011+AMD1:2021
  3. IEC 60364-4-44:2007+AMD1:2015+AMD2:2018