Extra-Low Voltage (ELV): Definition, Range, Requirements

Extra-low voltage (ELV): 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 [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), functional extra-low voltage (FELV).

SELV system: an electric system in which the voltage cannot exceed the value of extra-low voltage:

  • under normal conditions and
  • under single fault conditions, including earth faults in other electric circuits.

Note 1 to entry: SELV is the abbreviation for safety extra-low voltage.

PELV system: 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): an extra-low voltage system in which not all of the protective measures required for SELV or PELV have been applied.

Range

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.

Requirements for SELV and PELV Circuits

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.

Plugs and socket-outlets in SELV and 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 conductor contact.

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.

References

  1. IEC 60050-195-2021
  2. BS 7671:2018+A2:2022.
  3. IEC 60364-4-41-2017