Functional ground

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Functional earth symbol
Functional grounding of various devices on one rail

The functional ground is for the correct operation of electrical systems and devices is important. The protective earthing can only fulfill this function with devices of protection class I. A distinction is made between two types of grounding : protective grounding and functional grounding . While the protective earth serves to protect people and animals from an electric shock and offers protection in the event of a fault, the functional earth is a functional part and essential for the regular operation of the electrical system.

Basics

The functional earth is often used to install an electrical system in an electromagnetically compatible manner. The functional earth connection is marked with the corresponding circuit symbol or with the capital letters " FE ". The protective earth (PE) used for personal protection is often not suitable for guaranteeing the electromagnetic compatibility (EMC) of an electrical system. For this it is often necessary to connect an additional functional earth.

In devices of protection class II or III, there is often a separation of functional earth and protective earth. The protective earthing of a consumer must not be established by connecting it to the functional earth - the functional earth usually does not meet the requirements for a protective conductor . A connection of the protective conductor to the functional earth connection cannot guarantee personal safety. Conversely, however, the protective conductor can possibly meet the requirements for a functional earth (e.g. laptop power supply unit with protective contact plug). A conductive connection between protective earth and functional earth is permitted, but it is often just a loose coupling with a capacitor and resistor to prevent hum loops (e.g. audio devices).

The design of the electrical low-voltage supply network is also important for the connection of functional earths. The TN-S system with central star point earthing is in many cases more suitable than other systems for meeting EMC requirements. In the ground who has personal security priority. The safety of the functional earth of electrical equipment is only decisive in the second place. The marking of the functional earth is therefore not uniform and must not be the same as that of the protective earth.

Tasks and areas of application of functional earthing

The main aim of functional earthing is to improve electromagnetic compatibility :

  • Discharge of interference currents and thus limitation of their voltage drops on ground lines, which otherwise lead to interference voltages
  • Definition of a common reference potential for signals
  • Connection of shields to this potential in order to avoid electrical interference
  • Earthing of antennas (counterweight), radio equipment or the shielding of interference sources in order to improve or reduce the generation or reception of radio waves

Metal housings or mounting plates have no defined potential in protection class II. They therefore do not protect against electrical interference and are therefore often connected to functional earth.

Transformers in electronic devices are often provided with a shield winding, which is connected to functional earth, in order to divert mains-side interference.

In electronic control systems, in order to achieve voltage equalization, the masses and the housing are often connected to one another to form functional earth. With a symmetrical power supply this is the midpoint with 0 V and with an asymmetrical power supply this is often the negative pole of the voltage source.

Portable music systems that comply with VDE regulations are often connected using isolating transformers . To ensure trouble-free operation here, it is often necessary to connect the connected housings, system parts and shielding braids of the low-frequency signal paths (this is usually the shielding ) either directly or indirectly to functional earth . An indirect connection via an RC element can be useful to avoid earth loops .

In the case of telecommunications systems, the functional earth enables the system to function properly. Business premises with a consistent earthing concept, e.g. B. control rooms provide a separate functional earth.

With other electronic devices, a functional earth connection is often necessary despite protection class II or III. This ensures radio interference suppression . In addition, the functional earth in fluorescent lamps with electronic ballast often improves the starting properties of the lamp . For lights with digitally dimmable ballasts, connection of a functional earth is sometimes required.

With electronic measuring and testing devices, it is often necessary to connect the shield to earth. In addition to the connection for the measuring device, high-voltage measuring tips also have an earth connection as a second measuring point.

Equipotential bonding

Equipotential bonding for the purpose of reducing interference voltage must be suitable for high frequencies and with low resistance. This is often made possible by flat connections (bands, sheet metal strips) of metallic system parts. A large surface is less important because of the current carrying capacity, but rather to keep the inductance low.

Equipotential bonding bars serve as a ground reference point in electronic controls. There all ground lines and shields are brought together in a star shape. This is particularly important with mixed digital and analog signals. In order to avoid ground loops , ground lines must be routed separately from each other in front of the ground reference point. With some devices, the device ground is not the same as the earth potential. Their reference potentials are floating (undefined) and must not be connected directly to the ground reference point.

Influence of cable length and cross section

In the case of high-frequency currents, the electrons do not flow over the entire cross-section of the line, but increasingly on the conductor surface ( skin effect ). For this reason, it is not the conductor cross-section but the conductor surface that is decisive for the discharge of high-frequency currents. Round conductors have a smaller surface area than flat conductors with a rectangular cross-section. For this reason, round conductors are less suitable for deriving high-frequency signals.

Since every line has a certain inductance per meter of conductor length , its impedance increases with increasing frequency and increasing line length. Short lines are therefore often advantageous for diverting high-frequency interference currents. In mass bands , the inductance is substantially lower than in round wires of equal cross-section. However, this only comes into play at frequencies above 10 MHz.

The ground straps should be as short as possible, have a large contact area and have a large length to width ratio. In order to achieve an effective, low-impedance earthing connection between the individual earthing connections and the earth electrode, there are three options: flat conductors (earth straps) with large contact surfaces, many individual conductors isolated from one another (earth wire, earth wire, several connections) and short connections.

Earthing system

An earthing system ( earth electrode , earth connection lines) should have a low impedance. In order to safely earth currents , the earth electrode must have extensive contact with the ground. The requirements for low impedance in the high frequency range are met by normal earth rods, as they are, for. B. are used for protective earths, usually not met.

The introduction of additional earth electrodes, in the form of an earth electrode system, reduces the impedance. In addition to the already installed foundation earth or deep earth, additional ring earths are laid in the ground and connected to the earthing system. A ring earth electrode made of copper tape with a minimum cross-section of 50 mm², which is laid in the ground around a building at a distance of approx. 1 m and a minimum depth of 50 cm, is particularly suitable. The separation of protective earth, functional earth or lightning protective earth is not permitted due to possible differences in potential - earths must be connected to one another in a well-conducting manner in order to compensate for any potential differences.

When using different earthing materials, not only the corrosion resistance of the materials but also their potential differences due to the electrochemical series must be observed.

Combined earthing

In order to be able to use an earthing as protective earthing and functional earthing at the same time, this earthing must meet the following criteria:

  1. In order to be able to safely discharge dangerous body currents into the earth, it must have a low ohmic resistance and be able to carry current (see also loop impedance ).
  2. In order to be able to divert interference signals, it must have a low inductance.
  3. In order to provide a reference potential free of interference voltage, the functional earth must be designed as flat or mesh-shaped as possible.

In the low frequency range

  • the cross-section of a line is decisive for its line resistance (or line impedance).
  • the line must be capable of carrying current, the impedance is only important for thermal considerations.
  • the equipotential bonding must have current carrying capacity.
  • the earth electrode must have a low earthing resistance and also be able to carry current.

In the high frequency range

  • Short cable length is crucial.
  • the line cross-section has a small influence on the line impedance.
  • a large conductor surface has a great influence.
  • the low inductance is decisive.
  • the contact must be extensive.
  • the earth electrode must have a low impedance (large surface and extensive contact with the ground).

In the meshed grounding system, conductive metal parts, e.g. B. water pipes , cable trays , cable trays and steel reinforcement are included. The metal structures are connected to one another, but also to the metal housings of the electrical equipment, at several points in the form of a mesh. The individual grounding meshes or "grounding islands" are connected to each other as multiple as possible. In multi-storey buildings, the earthing networks should be vertically connected to one another at several points. The mesh-like structure of the earthing network creates a large surface area and short cable lengths and thus a low earthing impedance, which is beneficial for a good discharge capacity for interference signals. At the same time, the meshed system creates a ground potential that is the same everywhere and has only slight potential differences compared to the reference point.

Protective earth conductors must be dimensioned in accordance with DIN VDE regulations. A correctly dimensioned protective conductor is only suitable to a limited extent for discharging EM interference. Combined protective and functional earthing conductors must have a large surface in addition to the prescribed cross-section. Braided grounding straps have proven themselves for this application. These grounding straps usually have a cross-section of 10 mm² to 25 mm². If the discharge tasks are to be carried out by round conductors, these must occasionally have a larger cross-section than is required for the protective function. Since the protective conductor is not dimensioned larger than the outer conductor in the case of multi-core connection cables, it is also possible to lay a functional earth.

Bare mounting plates as equipotential bonding with a large area of ​​contact to cables and shields in the control cabinet are helpful.

Regulations and rules

  • Law on the electromagnetic compatibility of equipment (EMVG)
  • DIN * VDE 0100-540 Installation of low-voltage systems, Part 5-54: Selection and installation of electrical equipment - earthing systems, protective conductors and protective equipotential bonding conductors
  • DIN EN 50310 VDE 0800-2-310 "Application of measures for earthing and equipotential bonding in buildings with information technology equipment"
  • DIN VDE 0618-1: 1989-08 "Equipment for equipotential bonding, equipotential bonding rail (PAS) for main equipotential bonding"
  • Accident prevention regulation BGI 811 "Occupational safety in broadcast vehicles"
  • Leaflet of the statutory accident insurance GUV-I 810 "Occupational safety in production facilities for scenic representation"

literature

Individual evidence

  1. ↑ Symbol for the functional earth [1]
  2. ^ A b Carl Donath, Christian Orgel, Rainer Rottmann: Handbook of testing stationary electrical systems and equipment. Examination procedures - limits and guidelines, Forum Verlag Herkert GmbH, Mering 2016, ISBN 978-3-86586-703-2 .
  3. ↑ a b Dieter Anke, H.-D. Brüns, B. Deserno, Heyno Garbe, P. Hansen, J. Luiken ter Haseborg, S. Keim, S. Kohling, K. Rippl, V. Schmidt, H. Singer: Electromagnetic compatibility. Basics - analyzes - measures, BG Teubner Verlag, Stuttgart 1992, ISBN 978-3-322-82992-4 , p. 181.
  4. ↑ General earthing recommendation. Brüel & Kjær Vibro GmbH Online (accessed July 17, 2017; PDF; 908 kB).
  5. ↑ TÜV Süddeutschland: Protection against electromagnetic interference through low-interference neutral point earthing. Online (PDF; 360 kB) (accessed on July 21, 2016).
  6. ↑ Low frequency line transformer NFLUE 1 OHP automation systems. Online ( Memento of the original from July 21, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (accessed on July 21, 2016). @1@ 2Template: Webachiv / IABot / www.ohp.de
  7. ↑ Energy supply and protective measures for telecommunication systems Dipl.-Ing. Walter Schlothauer, Dipl.-Ing. Klaus Schwarz
  8. ↑ Protective and functional earth of electronic ballasts, especially in luminaires of protection class 2 Tridonic.Atco.
  9. ↑ Rainer Thüringer, University of Applied Sciences Gießen, Department of Electrical and Information Technology: Impedance of electrical lines. Online  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 102 kB) (accessed on July 21, 2016).@1@ 2Template: Toter Link / wiki.fed.de  
  10. ↑ Telematic Limited: Manual TAN 1003 Grounding for Surge Protection - a guide. Online (PDF; 437 kB) (accessed on July 21, 2016).
  11. ↑ EMV easily reached Pocket Guide; Zentralverband Elektrotechnik und Elektroindustrie e. V.
  12. ↑ Handbook excerpt from drive technology practice . Online (accessed September 10, 2012; PDF; 9.2 MB).
  13. ↑ Committee for lightning protection and lightning research: Lightning protection in practice . Online (accessed December 30, 2011; PDF; 789 kB).
  14. ^ Frank Schneider Masses on Earth. Online (PDF; 1.1 MB) (accessed on July 21, 2016).
  15. ↑ EMC-compliant switch cabinet installation Jetter AG. Online (accessed July 21, 2016).
  16. ↑ SEW-Eurodrive (Ed.): Drive Technology Practice, Volume 9 EMC in Drive Technology . Online (PDF; 1.5 MB) (accessed on July 21, 2016).

See also