Extra-low voltage (ELV) is a voltage not exceeding the maximum value of the prospective touch voltage that is acceptable to be maintained indefinitely under specified conditions of external influences [this term is defined in the IEC 60050-195-2021].
The International Electrotechnical Commission (IEC) and the UK IET (BS 7671:2008) define an ELV device or circuit as one in which the electrical potential between conductor or electrical conductor and earth (ground) does not exceed 50 V AC or 120 V DC (ripple free).
In international and national regulations, extra-low voltage refers to any AC voltage up to and including 50 V and DC up to and including 120 V. Some IEC standards and national standards based on these standards set lower extra-low voltage upper limits for specific electrical equipment, e.g. household equipment, or conditions of use such as high or particularly high risk of electric shock.
The term “extra-low voltage” describes voltages in AC and DC electrical circuits which, under certain conditions, are not hazardous to humans. A person who touches a live part or an exposed-conductive-part that is under extra-low voltage is not in most cases at risk of electric shock.
The IEC and IET go on to define actual types of extra-low voltage systems, for example safety extra-low voltage (SELV), protective extra-low voltage (PELV), and functional extra-low voltage (FELV).
SELV system is an electric system in which the voltage cannot exceed the value of extra-low voltage:
Note 1 to entry: SELV is the abbreviation for safety extra-low voltage.
PELV system is an electric system in which the voltage cannot exceed the value of extra-low voltage:
- under normal conditions and
- under single fault conditions, except earth faults in other electric circuits.
Note 1 to entry: PELV is the abbreviation for protective extra-low voltage.
Functional extra-low voltage (FELV) is an extra-low voltage system in which not all of the protective measures required for SELV or PELV have been applied.
Protection by extra-low-voltage is a protective measure which consists of either of two different extra-low-voltage systems:
This protective measure requires:
- limitation of voltage in the SELV or PELV system to the upper limit of voltage Band I, 50 V a.c. or 120 V d.c. (see IEC 60449), and
- protective separation of the SELV or PELV system from all circuits other than SELV and PELV circuits, and basic insulation between the SELV or PELV system and other SELV or PELV systems, and
- for SELV systems only, basic insulation between the SELV system and earth.
The use of SELV or PELV is considered as a protective measure in all situations.
NOTE. In certain cases the standards of the IEC 60364-7 series limit the value of the extra-low voltage to a value lower than 50 V a.c. or 120 V d.c.
Depending on the conditions in which the electrical equipment is operated, the value of extra-low voltage may be set by regulations significantly less than 50 VAC and 120 VDC. For parts of the electrical installation of a building, placed in rooms with an increased risk of electric shock, extra-low voltage is a voltage not exceeding 25 V AC and 60 V DC, and in particularly hazardous conditions – a voltage that does not exceed 12 V AC and 30 V DC.
The use of extra-low voltage (or more precisely, safety extra-low voltage) in individual parts of the electrical installation of a building serves to protect humans and animals against electric shocks if they come into contact with live parts as well as exposed-conductive-parts that have been energized. To this end, class III electrical equipment is used in the electrical installations of buildings, live parts of which are under voltage not exceeding the extra-low voltage. Safe isolating transformers are usually used as power supply sources for class III electrical equipment.
Basic protection and fault protection is deemed to be provided when:
- the nominal voltage cannot exceed the upper limit of voltage Band I,
- the supply is from one of the sources listed in subclause “Sources for SELV and PELV”, and
- the conditions of subclause “Requirements for SELV and PELV Circuits” are fulfilled.
NOTE 1. If the system is supplied from a higher voltage system by equipment which provides at least simple separation between that system and the extra-low-voltage system, but which does not meet the requirements for SELV and PELV sources in subclause “Sources for SELV and PELV”, the requirements for FELV may be applicable, see article https://www.asutpp.com/automatic-disconnection-of-supply.html#functional-extra-low-voltage-felv.
NOTE 2. DC voltages for ELV circuits generated by a semiconductor convertor (see IEC 60146-2) require an internal a.c. voltage circuit to supply the rectifier stack. This internal a.c. voltage exceeds the d.c. voltage for physical reasons. This internal a.c. circuit is not to be considered as a higher voltage circuit within the meaning of this clause. Between internal circuits and external higher voltage circuits, protective separation is required.
NOTE 3. In d.c. systems with batteries, the battery charging and floating voltages exceed the battery nominal voltage, depending on the type of battery. This does not require any protective provisions in addition to those specified in this clause. The charging voltage should not exceed a maximum value of 75 V a.c. or 150 V d.c. as appropriate according to the environmental situation as given in Table 1 of IEC 61201:1992.
The following sources may be used for safety extra-low-voltage and protective extra-low-voltage systems:
- A safety isolating transformer in accordance with IEC 61558-2-6.
- A source of current providing a degree of safety equivalent to that of the safety isolating transformer (e.g. motor generator with windings providing equivalent isolation).
- An electrochemical source (e.g. a battery) or another source independent of a higher voltage circuit (e.g. a diesel-driven generator).
- Certain electronic devices complying with appropriate standards where provisions have been taken in order to ensure that, even in the case of an internal fault, the voltage at the outgoing terminals cannot exceed the values specified in IEC 60364-4-41:2005 and IEC 60364-4-41:2005 / AMD1:2017, 414.1.1. Higher voltages at the outgoing terminals are, however, permitted if it is ensured that, in case of contact with a live part or in the event of a fault between a live part and an exposed-conductive-part, the voltage at the output terminals is immediately reduced to those values or less.
NOTE 1. Examples of such devices include insulation testing equipment and monitoring devices.
NOTE 2. Where higher voltages exist at the outgoing terminals, compliance with 531.5 can be assumed if the voltage at the outgoing terminals is within the limits specified in IEC 60364-4-41:2005 and IEC 60364-4-41:2005/AMD1:2017, 414.1.1 when measured with a voltmeter having an internal resistance of at least 3 000 Ω.
- Mobile sources supplied at low voltage, for example safety isolating transformers or motor generators, shall be selected or erected in accordance with the requirements for protection by the use of double or reinforced insulation.
SELV and PELV circuits shall have:
- basic insulation between live parts and other SELV or PELV circuits, and
- protective separation from live parts of circuits not being SELV or PELV, provided by double or reinforced insulation or by basic insulation and protective screening for the highest voltage present.
SELV circuits shall have basic insulation between live parts and earth.
The PELV circuits and/or exposed-conductive-parts of equipment supplied by the PELV circuits may be earthed.
NOTE 1. In particular, protective separation is necessary between the live parts of electrical equipment such as relays, contactors, auxiliary switches, and any part of a higher voltage circuit or a FELV circuit.
NOTE 2. The earthing of PELV circuits may be achieved by a connection to earth or to an earthed protective conductor within the source itself.
Protective separation of wiring systems of SELV and PELV circuits from the live parts of other circuits, which have at least basic insulation, may be achieved by one of the following arrangements:
- SELV and PELV circuit conductors shall be enclosed in a non-metallic sheath or insulating enclosure in addition to basic insulation;
- SELV and PELV circuit conductors shall be separated from conductors of circuits at voltages higher than Band I by an earthed metallic sheath or earthed metallic screen;
- circuit conductors at voltages higher than Band I may be contained in a multi-conductor cable or other grouping of conductors if the SELV and PELV conductors are insulated for the highest voltage present;
- the wiring systems of other circuits are in compliance with 412.2.4.1 [IEC 60364-4-41-2017];
- physical separation.
Exposed-conductive-parts of SELV circuits shall not be connected to earth, or to protective conductors or exposed-conductive-parts of another circuit.
NOTE. If the exposed-conductive-parts of SELV circuits are liable to come into contact, either fortuitously or intentionally, with the exposed-conductive parts of other circuits, protection against electric shock no longer depends solely on protection by SELV, but also on the protective provisions to which the latter exposed-conductive-parts are subject.
Basic protection is generally unnecessary in normal dry conditions for:
- SELV circuits where the nominal voltage does not exceed 25 V a.c. or 60 V d.c.;
- PELV circuits where the nominal voltage does not exceed 25 V a.c. or 60 V d.c. and exposed-conductive-parts and/or the live parts are connected by a protective conductor to the main earthing terminal.
In all other cases, basic protection is not required if the nominal voltage of the SELV or PELV system does not exceed 12 V a.c. or 30 V d.c.
Plugs and socket-outlets in SELV or PELV systems shall comply with the following requirements:
- plugs shall not be able to enter socket-outlets of other voltage systems;
- socket-outlets shall not admit plugs of other voltage systems;
- plugs and socket-outlets in SELV systems shall not have a protective contact.
NOTE: For socket-outlets according to IEC 60884-1 the protective contact is referred to as earthing contact.
- IEC 60050-195-2021
- BS 7671:2018+A2:2022.
- IEC 60364-4-41-2017
