IT system (electrical engineering)

from Wikipedia, the free encyclopedia

An IT system ( French: Isolé Terre ) is a specific type of implementation of a low-voltage network in the electrical energy supply within an electrical installation. The most important feature is the type of earth connection at the power source and the electrical equipment . In this network, there is no galvanic connection between active conductors and earthed parts. A first error must not lead to the power supply being switched off.

Other forms of implementation of low-voltage networks are the TN system and the TT system .

Network construction

IT system without a neutral conductor

The IT network is a network that is only small in size. The small network size is necessary because finding a fault location in an extensive network can be very complex. The network must have a separate power supply. Power can be supplied either by a transformer , a generator or by batteries . It is thus possible to operate IT networks with alternating current or direct current. With three-phase networks it is possible to use the neutral conductor as the fourth conductor. In such four-wire networks, an overcurrent protection device for the neutral conductor may be required due to voltage increases in the event of a fault. The IT network has good EMC properties, and a simple earth fault in this network does not lead to an immediate shutdown of the network. The IT network is fail-safe and has a significantly higher level of reliability than other TN or TT networks . In terms of security of supply, IT systems offer most of the advantages of all network types.

In certain areas, such as B. in hospitals, there are IT networks in addition to the TN network. These are two-wire IT networks for supplying AC consumers in particularly sensitive areas. In these areas, the sockets of the IT network must be marked accordingly or equipped with unmistakable plug systems. The power of the transformers in these medical IT networks is limited to 8 kVA .

Grounding and equipotential bonding

In the IT system, protective grounding and operational grounding are carried out differently. The conductive housings of the equipment are earthed in the IT system as in a TT system or TN system. All conductive bodies that do not belong to the operating circuit must either be earthed individually or together, or they must be connected together with the system earth . It is also permissible to connect the conductive bodies to the protective conductor in groups. The grounding must be carried out in such a way that the following condition is met:

Where:

The operating voltage source is isolated from earth, i. H. open - symmetrical high-resistance earthing is common in DC networks. In normal operation there is no low-resistance connection between the active parts of the network and earth or protective conductor. If the network is fed from a transformer, the neutral point of the feeding transformer is therefore not earthed.

Due to stray capacitances, the resistance of the network to earth is not infinite, but rather reaches resistance values ​​in the kilohm range.

Personal protection

The following protective devices can be used for personal protection:

If insulation protection devices are used for personal protection, this protection corresponds to the previous protective measure, protective line system . Insulation monitoring devices are used to detect insulation faults, which permanently measure the resistance of the outer conductor and the neutral conductor to earth and immediately report an insulation fault, so that units that are not relevant for an emergency can be switched off in a controlled manner. If insulation protection devices are used in the IT network, local equipotential bonding is mandatory.

If overcurrent protection devices are used for automatic shutdown, the shutdown conditions according to DIN VDE 0100-410 must be observed. The disadvantage of using overcurrent protection devices is that they only respond at high currents . Overcurrent protection devices are not suitable for low fault currents.

When using RCD protective devices (outdated FI) it must be ensured that the protective device switches off at least one fault in the event of a double fault. If, when using backup power generators, circuits with rated currents greater than 32  A are operated in the IT network, each circuit must be protected by its own residual current circuit breaker. The switches must switch off in the event of fault currents of up to 30 mA.

The use of FI protective devices is restricted to special cases in the IT network. The same requirements apply to the use of FI protective devices in the IT network as for use in the TT network. However, there are hardly any practical applications that require the use of FI protective devices in the IT network.

Failure

A first insulation fault between an outer conductor and the earth is the earthing of this conductor. There is then neither a potential difference between conductive housings and earth nor a circuit to the transformer that is closed via earth . Since there are no potential differences, there is no contact voltage between the accessible housing parts and the earth. In this operating state, the IT network changes into the asymmetry of a TN or TT network. However, a significantly lower fault current flows. The fault current is distributed to all outlets and flows back into the other two outer conductors. The return flow occurs due to the isolated network via the conductor-earth capacities of the "healthy" outer conductor. The amount of this capacitive fault current depends on the size of the earth capacitance. The larger the network, the greater the earth capacitance and consequently the greater the fault current. The earth fault leads to an increase in voltage between the two "healthy" phases compared to earth. The external conductor voltages to earth increase to the value. The voltage to earth now equals the voltage between two outer conductors. The increase in voltage puts greater stress on the insulation of the lines. Since all active parts experience the same change in potential, the system can still be operated safely at first. However, because of the excessive voltage and the (albeit low) probability of a further error, the error should be eliminated quickly.

Single-pole faults are the most common type of fault with a probability greater than ninety percent. Although the probability of a second fault occurring is less than ten percent, two-pole faults can occur to a minor extent. In this case, another earth fault occurs through another active conductor and thus creates a double fault. Due to this double fault, significantly higher fault currents occur. With two full earth faults, a short circuit occurs, which is only harmless if it is immediately switched off by the overcurrent protection devices. If a double error occurs, an automatic shutdown must take place in the IT system. Since double the loop resistance acts in the worst case , the overcurrent disconnection devices ( circuit breaker ) or the conductor cross-sections in IT networks must be dimensioned more critically than in earthed networks. In the case of backup power generators, there is no need to switch off if the voltage between the connection terminals falls below 50 volts.

Troubleshooting

An insulation fault location device is a device or a combination of devices for insulation fault location in IT systems and is used in addition to an insulation monitoring device . It impresses a test current between the live conductors and earth and localizes insulation faults. Insulation fault location devices (IFLS) can be used to localize insulation faults during operation or when the system is switched off. Devices for stationary installation and mobile devices are available for this purpose.

An insulation fault location system (IFLS, Insulation Fault Location System) usually consists of an insulation monitoring device (IMD, Insulation Monitoring Device) with an integrated test current generator (LCI, Locating Current Injector), an insulation fault locator (IFL, Insulation Fault Locator) and converters (LCS) , Locating Current Sensor).

commitment

This type of network is used in the operating theaters of hospitals , since an interruption in the power supply would endanger the life of the patient. IT networks are also used in intensive care units. Furthermore, IT networks are used in industrial plants when switching off the power supply would lead to an interruption of the production process and thereby cause economic damage. This is the case, for example, in glass production and in the chemical industry. But IT networks are also used for the energy supply in potentially explosive areas, for example in underground hard coal mining and in smelting works. Other areas of application are backup power supplies on ships and in fire services. Even with the power of pumping of groundwater conservation IT networks are used. The IT system is not used in the installation of living spaces .

Limits of the IT network

First of all, one would assume that due to the isolation of the star point, even in the first case of a fault, the person concerned would not have a current flowing through it, so a first fault is absolutely harmless. On closer inspection, however, it is noticeable that there is indeed a current flow. If the person touches z. B. outer conductor L1, then it creates a resistance of at least 1 kΩ to earth. On the other hand, there are also small capacitances between the outer conductors L2 and L3 and the earth, which act as a capacitive resistance . Thus one has a closed circuit from L1 via the person via the earth and back via the capacitance of the earth to L2 and L3. The greater the capacitance of L2 and L3 to earth, the greater the current; in this case this means: the longer the line, the greater the capacity and thus the flow of current. This is exactly where the limit of the IT network lies: if the network size is very small, then the capacities of the external conductors to earth are so small that a first fault is safe. However, if the network were to be expanded, the capacitance of the outer conductors to earth can become so great that the currents can occur in dangerous magnitudes.

Norms

  • DIN VDE DIN VDE 0100-100 (VDE 0100-100): 2009-06, "Setting up low-voltage systems - Part 1: General principles, provisions for general characteristics, terms", Appendix A.3 and Appendix A.6
  • DIN VDE DIN VDE 0100-410 (VDE 0100-410): 2007-06, "Setting up low-voltage systems - Part 410: Setting up low-voltage systems - Part 4-41: Protective measures - Protection against electric shock", Section 411.6.
  • DIN EN DIN EN 61557-8 (VDE 0413-8): 2015-12, "Electrical safety in low-voltage networks up to AC 1000 V and DC 1500 V devices for testing, measuring or monitoring protective measures - Part 8: Insulation monitoring devices for IT systems "
  • DIN EN DIN EN 61557-9 (VDE 0413-9): 2015-10, "Electrical safety in low-voltage networks up to AC 1000 V and DC 1500 V - devices for testing, measuring or monitoring protective measures - Part 9: Equipment for insulation fault location in IT Systems "
  • IEC 60364-3: 1993-03, section 312.2 ((withdrawn))
  • IEC 60364-3: 1993-03, Amendment 1: 1994-02 ((withdrawn))
  • IEC 60364-1 Ed 5.0: 2005-11, "Low-voltage electrical installations - Part 1: Fundamental principles, assessment of general characteristics, definitions". Figures 31G1, 31G2, 31M
  • IEC 60364-4-41 Ed 5.0: 2005-12, "Low-voltage electrical installations - Part 4-41: Protection for safety - Protection against electric shock", Section 411.6
  • IEC 60364-7-717 Ed 2.0: 2009-07, "Low-voltage electrical installations - Part 7-717: Requirements for special installations or locations - Mobile or transportable units", Section 717.411.6.
  • IEC 61557-8: 2014, "Electrical safety in low voltage distribution systems up to 1 000 V ac and 1 500 V dc - Equipment for testing, measuring or monitoring of protective measures - Part 8: Insulation monitoring devices for IT systems"
  • IEC 61557-9: 2014, "Electrical safety in low voltage distribution systems up to 1000 V ac and 1500 V dc - Equipment for testing, measuring or monitoring of protective measures - Part 9: Equipment for insulation fault location in IT systems"

Guidelines

  • VDV -Schrift 560: 2013-02 Electric point heating of direct current local railways

literature

  • Wolfgang Hofheinz: "Protection technology with insulation monitoring". 2nd edition, VDE series 114 standards understandable, VDE-Verlag GmbH Berlin / Offenbach, ISBN 978-380073026-1 .
  • Werner Hörmann, Bernd Schröder: Protection against electric shock in low-voltage systems - Comment from DIN VDE 0100-410 (VDE 0100-410): 2007-06. VDE publication series Volume 140, VDE-Verlag, Berlin 2010, ISBN 978-3-8007-3190-9 .

Individual evidence

  1. ↑ a b c ÖVE / ÖNORM E 8001-1: Construction of electrical systems with nominal voltages of up to 1000 V AC and 1500 V DC. Part 1: Terms and protection against electric shock (protective measures) Online (accessed March 30, 2012; PDF; 582 kB).
  2. ↑ Dieter Anke, H.-D. Brüns, B. Deserno, H. Garbe, K.-H. Gonschorek, P. Hansen, J. Luiken ter Haseborg, S. Keim, S. Kohling, K. Rippl, V. Schmidt, H. Singer: Electromagnetic compatibility. Basics - Analyzes - Measures, BG Teubner, Stuttgart 1992, ISBN 978-3-322-82992-4 , p. 185.
  3. ↑ a b c d e Friedhelm Noack: Introduction to electrical energy technology . Carl Hanser Verlag, Munich As 2003, ISBN 3-446-21527-1 .
  4. ↑ a b c d e TÜV Süddeutschland: Protection against electromagnetic interference through low-interference neutral-point earthing online (accessed on March 30, 2012; PDF; 360 kB).
  5. ↑ a b c d Wilfried Knies, Klaus Schierack: Electrical systems technology; Power plants, networks, switchgear, protective devices. 5th edition, Hanser Fachbuchverlag. 2006 ISBN 978-3446405745 .
  6. ↑ a b c d e f g Hannes-Christian Blume, Hartmut Karsten: Risk assessments. WEKA Verlag 2000, - ISBN 3-8111-4401-4 .
  7. ↑ a b c d Technical Information No. 01; IT systems the basis for a reliable power supply. Online (accessed March 30, 2012; PDF; 292 kB).
  8. ^ Günter Springer: Electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
  9. ^ A b Rüdiger Kamme: Medical technology . 4th edition, Springer-Verlag, Berlin Heidelberg New York 2011, ISBN 978-3-642-16186-5 .
  10. ↑ a b c d e f g Gerhard Kiefer: VDE 0100 and the practice. 1st edition, VDE-Verlag GmbH, Berlin and Offenbach, 1984, ISBN 3-8007-1359-4 .
  11. ↑ Technical article Earthing Unearthed Power Supply Online (accessed on January 14, 2018).
  12. ↑ Rolf Fischer, Hermann Linse: Electrical engineering for mechanical engineers. 13th edition, Vieweg + Teubner Fachverlage GmbH, Wiesbaden 2009, ISBN 978-3-8348-0799-1 .
  13. ↑ Schneider Electric: Technical Bulletin No. 177; Malfunctions in electronic systems - the correct earthing online (accessed on March 30, 2012; PDF; 281 kB).
  14. ↑ Zentralverband Elektrotechnik- und Elektronikindustrie eV (Ed.): Guidelines for residual current protective devices and electric drives online (accessed on January 16, 2018).
  15. ↑ a b c BGI 867 professional association information: Instructions for selecting and operating replacement power generators on construction and assembly sites online (accessed on April 2, 2012; PDF; 1.4 MB).
  16. ↑ a b Wolfgang Neuwirth: Basic investigations into the medium-resistance inductive neutral point treatment. Diploma thesis online (accessed April 2, 2012; PDF; 1.8 MB).
  17. ^ Wolfgang Hofheinz: Electrical safety in medically used areas 3rd edition. VDE publication series 117 standards understandable, VDE-Verlag GmbH Berlin / Offenbach, ISBN 978-3-8007-35884 .
  18. ↑ Bender: IT system expertise; Advantages of the IT system online (accessed on August 6, 2016).
  19. ↑ MONITOR: The current magazine for electrical safety online (accessed on August 6, 2016).
  20. ^ A b Klaus Tkotz, Peter Bastian, Horst Bumiller: Electrical engineering. 27th revised and expanded edition, Verlag Europa-Lehrmittel Nourney Vollmer GmbH & Co. KG, Haan Gruiten 2009, ISBN 978-3-8085-3188-4 .
  21. ^ Karl Volger, Erhard Laasch: House technology. Basics - planning - execution, 9th revised and expanded edition, BG Teubner, Stuttgart 1994, ISBN 978-3-322-94746-8 , p. 414.