Extra-low voltage (ELV) is an electricity supply voltage in a range
which carries a low risk of dangerous electrical shock.
There are various standards that define Extra-Low
1.1 Separated or safety extra-low voltage (SELV) 1.2 Protected extra-low voltage (PELV) 1.3 Functional extra-low voltage (FELV) 1.4 UK Reduced low voltage (RLV)
2 Stand-alone power systems 3 Regulations 4 See also 5 References 6 External links
Types Separated or safety extra-low voltage (SELV) IEC defines a SELV system as "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits". It is generally accepted that the acronym: SELV stands for separated extra-low voltage (separated from earth) as defined in installation standards (e.g. BS 7671), though BS EN 60335 refers to it as safety extra-low voltage. A SELV circuit must have:
Electrical protective-separation (i.e. double insulation, reinforced insulation or protective screening) from all circuits other than SELV and PELV (i.e. all circuits that might carry higher voltages) Simple separation from other SELV systems, from PELV systems and from earth (ground)
The safety of a SELV circuit is provided by
The extra-low voltage The low risk of accidental contact with a higher voltage The lack of a return path through earth (ground) that electric current could take in case of contact with a human body
The design of a SELV circuit typically involves an isolating
transformer, guaranteed minimum distances between conductors and
electrical insulation barriers. The electrical connectors of SELV
circuits should be designed such that they do not mate with connectors
commonly used for non-SELV circuits.
Typical examples for a SELV circuit: decorative out-door lighting,
Class III battery charger, fed from a Class II power supply. Modern
cordless hand tools are considered SELV equipment.
Protected extra-low voltage (PELV)
IEC 61140 defines a PELV system as "an electrical system in which the
voltage cannot exceed ELV under normal conditions, and under
single-fault conditions, except earth faults in other circuits".
A PELV circuit only requires protective-separation from all circuits
other than SELV and PELV (i.e., all circuits that might carry higher
voltages), but it may have connections to other PELV systems and earth
In contrast to a SELV circuit, a PELV circuit can have a protective
earth (ground) connection. A PELV circuit, just as with SELV, requires
a design that guarantees a low risk of accidental contact with a
higher voltage. For a transformer, this can mean that the primary and
secondary windings must be separated by an extra insulation barrier,
or by a conductive shield with a protective earth connection.
A typical example for a PELV circuit is a computer with a Class I
Functional extra-low voltage (FELV)
The term functional extra-low voltage (FELV) describes any other
extra-low-voltage circuit that does not fulfill the requirements for
an SELV or PELV circuit. Although the FELV part of a circuit uses an
extra-low voltage, it is not adequately protected from accidental
contact with higher voltages in other parts of the circuit. Therefore
the protection requirements for the higher voltage have to be applied
to the entire circuit.
Examples for FELV circuits include those that generate an extra low
voltage through a semiconductor device or a potentiometer or a
transformer. A typical example is an electronically controlled toaster
where the timer circuit runs off extra low voltage derived from a tap
on the heating element. Another is the old door-bell circuit fed from
UK Reduced low voltage (RLV)
The IET / BSI (BS 7671) also define Reduced low voltage (RLV) which
can be either single phase or three phase A.C. This system has been
used for many years on construction sites. Both single and three
phase. The single phase voltage is 110 V a.c. though having a
"centre tapped Earth" reducing the voltage to Earth to 55 V a.c.
The three phase is 110 V phase to phase, 63 V to Neutral /
Earth. This system is slightly outside ELV but still very commonly
used for cord powered hand tools and temporary lighting in hazardous
Stand-alone power systems
Cabling for extra-low voltage systems, such as in remote-area power
systems (RAPS), is designed to minimise energy losses while maximising
safety. Lower voltages require a higher current for the same power.
The higher current results in greater resistive losses in the cabling.
Cable sizing must therefore consider maximum demand, voltage drop over
the cable, and current-carrying capacity.
In Brazil, ELV (Extra-baixa tensão or EBT in Portuguese) is officially defined in Regulatory Standard no. 10 from the Brazilian Ministry of Labor and Employment as any voltage "not exceeding 50 volts a.c. or 120 volts d.c.". Although that standard defines safety rules for electricity, the Regulatory Standard no. 12 requires an even lower voltage for start and stop devices on machines and equipment made from March 2012 and on, stating it shall not exceed 25 volts a.c. or 60 volts d.c.
^ BS 7671 ^ DIN/VDE 0100-410 ^ IEC 60364-4-41: Low-voltage electrical installations – Part 4-41: Protection for safety – Protection against electric shock. ^ IEC 61140: Protection against electric shock – Common aspects for installation and equipment. ^ "NR-10 atualizada - Glossário" [updated NR-10 - Glossary] (PDF). Brazilian Ministry of Labor and Employment Website (in Portuguese). December 7, 2004. Retrieved September 9, 2015. ^ "NR-12 atualizada - item 12.36" [updated NR-12 - Item 12.36] (PDF). Brazilian Ministry of Labor and Employment Website (in Portuguese). June 25, 2015. Retrieved September 9, 2015.