Low voltage

Low voltage ( English Extra Low Voltage , ELV) is a voltage in the electrical engineering, the limit values for the voltage range I according to IEC does not exceed the 60,449th This is a low voltage range , the values of which do not exceed 50 V for alternating voltage (AC) and 120 V for direct voltage (DC). These values correspond to the limit for the continuously permissible contact voltage for adults and normal applications as non-life-threatening. Lower values are specified for special applications (see VDE 0100 group 700).

Voltage ranges		AC	DC
High voltage		> 1000 V	> 1500 V
Low voltage		≤ 1000 V	≤ 1500 V
Low voltage	Low voltage	≤ 50 V	≤ 120 V

The European low-voltage directive applies to alternating voltages over 50 V rms value and direct voltage over 75 V and the product safety directive applies to the low-voltage segment which falls below these limit values.

In the case of alternating voltages below 25 V or direct voltage below 60 V, protection against contact can be dispensed with entirely; these tensions are also considered harmless for animals and children. In areas of damp-proof installations , different conditions apply: insulation against contact cannot be dispensed with and alternating voltages are limited to 25 V or 12 V and direct voltages to 60 V.

A distinction is made between the following systems:

Safety extra-low voltage (SELV)
Functional extra-low voltage with electrically safe separation (PELV)
Functional extra-low voltage without electrically safe separation (FELV)

Safety extra-low voltage

Symbol protection class III (SELV)

Symbol for safe electrical separation

The safety extra-low voltage (Engl. Safety Extra Low Voltage , SELV) is a small electrical voltage that low because of their height and the insulation from circuits of higher voltage special protection against electric shock provides.

Devices operated with SELV that do not generate any higher voltages themselves are designated with protection class III in accordance with DIN EN 61140 ( VDE 0140-1) . The voltage is so small that electrical body currents are normally without consequences. The voltage source can either be a generator, for example a bicycle dynamo , or a battery . Otherwise, special requirements for insulation against live parts (e.g. primary winding of a transformer) must be observed, which are referred to as safe separation.

Mains transformers for generating SELV must, for example, be built in such a way that a short circuit between the primary winding and secondary winding and their connections is not possible. The windings can only lie on top of one another if there is double or reinforced insulation in between. Often the windings are placed one above the other or next to one another in separate insulating material chambers. Such transformers with safe electrical isolation are called safety transformers (EN 61558-2-6).

The test voltage (rated impulse voltage) depends on the overvoltage category and the nominal voltage of the power supply system (EN 60664-1, Table F.1; generic standard). In the case of overvoltage category II, this is 2.5 kV for a single-phase connection to a 230 V AC network (voltage conductor-neutral conductor between 150 and 300 V). With double insulation, both basic insulation must withstand this rated impulse voltage. Reinforced insulation is more often used for safe separation. The rated impulse voltage is set one class higher, which is 4 kV in the example mentioned (EN 60664-1, Table 1). With the rated impulse voltage and the degree of contamination , the required minimum clearance for safe disconnection (SELV) can be determined (EN 60664-1, Table 2). It is recommended to select the inhomogeneous field condition. In the above Example results in 3 mm (surge voltage 4 kV, inhomogeneous field, degree of pollution 2).

The minimum creepage distances for electrical isolation are determined using tables 3a, 3b and 4 of EN 60664-1. In addition to the knowledge of the above Working voltage (in the example class up to 250 V) and the above Degree of pollution also requires knowledge of the insulating material group (e.g. IG II for CTI values 400 ... 600). In the example, this is 1.8 mm (IG II, 250V, pollution degree 2). For safe separation (SELV), this value must be doubled (in the example now 3.6 mm).

If the nominal voltage is less than 25 V for alternating voltage or less than 60 V for direct voltage , protection against direct contact is not necessary with SELV. Examples are low-voltage halogen lamps (e.g. cable systems) or model train transformers. If the voltage is higher, protection against direct contact must be ensured, for example by insulation, covers or sheaths.

Well-known sources of SELV are batteries as well as bell transformers and transformers for model railways ( toy transformers ) as well as power packs in devices of protection class III (e.g. plug- in power packs or chargers).

According to EU Directive 2009/48 / EC, the nominal voltage for children's toys must not exceed 24 V direct voltage or the corresponding alternating voltage and the transformer for the protective low voltage must not form part of the toy.

When working in confined spaces and endangered areas, for example inside boilers and tanks, electrical handheld devices etc. a. a safety extra-low voltage of 42 V is usual.

Functional extra-low voltage with electrically safe separation

The functional extra-low voltage with electrically safe separation (English Protective Extra Low Voltage , PELV, formerly “ protective extra-low voltage”) also offers protection against electric shock.

With regard to the grounding of PELV circuits, the application has to be considered. For general installations (e.g. in buildings), DIN VDE 0100-410: 2018-10 in Section 414.4.1 stipulates: "PELV circuits and / or bodies of the equipment supplied by the PELV circuits may be earthed. "For the" electrical equipment of machines ", EN 60204-1 : 2006 (VDE 0113-1), section 6.4.1 specifies that one side of the circuit or one point of the energy source of the PELV circuit is connected to the protective conductor system must become. This statement therefore only relates to the electrical installation of machines.

As with SELV voltage sources, safe separation means that the primary circuit of mains transformers must be separated from the secondary circuit by double or reinforced insulation. For the determination of the required rated impulse voltages as well as the minimum creepage and clearance distances, what is stated in the "SELV" chapter applies.

PELV is used when, for operational reasons, active conductors of the extra-low voltage or the body of the equipment must be earthed. This is the case, for example, when equipotential bonding has to be implemented to avoid sparks in containers and areas at risk of explosion.

Another example is audio equipment and amplifiers, where the housings must be grounded to shield against interference. The grounding is not used here as a protective measure or protective grounding, but as a function. Accordingly, it is called functional grounding .

Another advantage is the avoidance of malfunctions in the control. If damage to a live wire causes a short-circuit to the body, the fuse element trips. Without PELV, another body shock would lead to malfunctions.

Due to the grounding of the housing, dangerous leakage currents can flow through the body, regardless of the extra-low voltage, if faults occur in other devices or facilities where their accessible, conductive parts assume mains voltage.

Possible power sources for protective extra-low voltage are:

Accumulators or other electrochemical power sources (DIN 57510 / VDE 0510) or galvanic elements
Safety transformers according to VDE 0551 or, under certain conditions, transformers with safe electrical isolation (e.g. DIN 57804 / VDE 0804)
Power sources with the same level of safety as the safety transformers as above. E.g. generators with separate windings (VDE 0530)
Electronic devices, if it is ensured that in the event of a fault the output voltage (also to earth) does not rise higher than permitted within the scope of the safety extra-low voltage or that a higher voltage is reduced immediately and in less than 0.2 seconds when touched.

Functional extra-low voltage without electrically safe separation

The functional extra-low voltage without electrically safe separation ( FELV) is a small electrical voltage which, in terms of its level, does not pose a risk when touched. This system is not permitted as a measure to protect against electric shock.

Double or reinforced insulation is not provided here. Earthing and connections of the circuits with protective conductors are permitted. However, the housing and body must be connected to the protective conductor on the primary side.

In order to be able to guarantee protection against direct contact, the insulation must be selected according to the nominal voltage of the primary circuit of the power source or, alternatively, by covers or sheaths (covers or sheaths are designed to prevent contact with live parts). The bodies of the equipment in the FELV circuit must be connected to the protective conductor of the primary circuit of the power source. (DIN VDE 0100-410)

The earthing of the secondary circuit can lead to self- switching of contactors in the event of earth faults or earth faults , if these are switched via earth / minus. A frame fault or ground fault but Live conductor leads with grounded secondary circuit to a shutdown of the overcurrent protective devices, so that a single fault can be detected. In non-earthed secondary circuits (SELV), however, double body or earth faults are necessary (for example in front of and behind the switching devices) in order to cause self-switching.

Typical applications are control of machines. When designing, one must therefore weigh up between these cases.

literature

DIN VDE 0100-410 (VDE 0100-410): 2018-10 Construction of low-voltage systems - Part 4-41: Protective measures - Protection against electric shock (IEC 60364-4-41: 2005, modified + A1: 2017); German adoption HD 60364-4-41: 2017. VDE publishing house, Berlin.
Werner Hörmann, Bernd Schröder: Protection against electric shock in low-voltage systems - Comment from DIN VDE 0100-410 (VDE 0100-410): 2007-06. VDE series of publications Volume 140, VDE-Verlag, Berlin, ISBN 978-3-8007-3190-9 .
DIN EN 50178 VDE 0160: 1998-04 Equipment of high-voltage systems with electronic equipment

Individual evidence

↑ http://www.elektro-wissen.de/Elektroinstallation/ Fehlerarten.html .
↑ Damp rooms such as bathrooms are divided into areas, see e.g. B. Boy / Dunkhase: Electrical installation technology, Vogel-Buchverlag, ISBN 978-3-8343-3079-6 .
↑ Directive 2009/48 / EC of the European Parliament and of the Council of June 18, 2009 on the safety of toys , accessed on January 10, 2013 (Annex II, Section VI. Electrical properties).
↑ Basics of electrical energy technology : supply, equipment, network operation, overvoltages and insulation, safety - Gerhard Hosemann, Wolfram Boeck; 4th edition, Springer-Verlag

[1] ttp://www.elektro-wissen.de/Elektroinstallation/ Fehlerarten.html .

[2] Damp rooms such as bathrooms are divided into areas, see e.g. B. Boy / Dunkhase: Electrical installation technology, Vogel-Buchverlag, ISBN 978-3-8343-3079-6 .

[3] Directive 2009/48 / EC of the European Parliament and of the Council of June 18, 2009 on the safety of toys , accessed on January 10, 2013 (Annex II, Section VI. Electrical properties).

[4] Basics of electrical energy technology : supply, equipment, network operation, overvoltages and insulation, safety - Gerhard Hosemann, Wolfram Boeck; 4th edition, Springer-Verlag