Extra-low voltage, in electricity supply, is one of several means to
protect against electrical shock. The International Electrotechnical
Commission and its member organizations define an ELV circuit as one in which
the electrical potential of any conductor against earth is not more than
either 25 volts RMS for alternating current, or ripple-free 60 volts for
direct current under dry conditions. Lower numbers apply in wet conditions,
or when large contact areas are exposed to contact with the human body.
The IEC defines three types of extra-low-voltage systems, which are
distinguished by their successively more restrictive safety properties.
Some types of landscape lighting use extra-low voltage.
Types This section is ordered from most
restrictive, to least restrictive. = Separated or safety extra-low voltage=
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".
There exists some confusion regarding the origin of the acronym: "SELV" stands
for "separated extra-low voltage" in installation standards and for "safety
extra-low voltage" in appliance standards.
A SELV circuit must have: protective-separation from all circuits
other than SELV and PELV simple separation from other SELV
systems, from PELV systems and from earth.
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
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.
A typical example for a SELV circuit is a Class III battery charger, fed from a
Class II power supply. = Protected extra-low voltage=
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, 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 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 power supply.
= Functional extra-low voltage= The term functional extra-low voltage
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. Stand-alone power systems
Cabling for extra-low voltage systems, such as in remote-area power systems, 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.
Voltage drop is usually the main factor considered, but current-carrying
capacity is as important when considering short, high-current runs
such as between a battery bank and inverter.
Arcing is a risk in DC ELV systems, and some fuse types which can cause
undesired arcing include semi-enclosed, rewireable and automotive fuse types.
Instead high rupturing capacity fuses and appropriately rated circuit breakers
are the recommended type for RAPS. Cable termination and connections must be done
properly to avoid arcing also, and soldering is not recommended.
Regulations = Australia and New Zealand=
ELV is defined in AS/NZS 3000 Wiring Rules as "Not exceeding 50 V a.c or 120
V ripple-free d.c." However, AS/ACIF S009 Clause 3.1.78.1 Extra-Low
Voltagestates: "a voltage not exceeding 42.4 V peak or 60 V d.c.[AS/NZS
60950.1:2003]" and adds a note: "This definition differs from the ELV
definition contained in AS/NZS 3000:2000" which is more closely aligned
to the Telecommunications Network Voltage limits ... i.e. 120 V d.c. or
70.7 V a.c. peak" TNV is the traditional line limit to
accommodate telephone ringing machine voltagefrom a current limiting source on
top of the nominally -48Vdc battery supply which could be encountered on a
telephone line and was not considered hazardous, whereas 120Vac without
current limiting at its source can inject 115mA into individuals leading to
fibrillation of the heart. In most Australian states there are no formal
constraints as to who can work on ELV systems. AS 4509.1 Stand-alone Power
Systems: Safety requires that work be performed by a "competent person" that
is "a person who has acquired through training, qualifications, experience, or
a combination of these, knowledge and skill enabling that person to correctly
perform the task required". ELV wiring in domestic premises must be
installed at a minimum distance of 50 mm from low voltage wiring or have a
separate insulating barrier such a conduit. ELV cable and wire types
include PVC insulated building wire, double insulated Thermo-Plastic Sheath,
and fine stranded multi-strand cable. State regulations override the
Australian Standards, and there are some differences.
= Brazil= ELV is officially defined into the
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. References