|IEC voltage range||AC||DC||defining risk|
|High voltage (supply system)||> 1000 Vrms||> 1500 V||electrical arcing|
|Low voltage (supply system)||50–1000 Vrms||120–1500 V||electrical shock|
|Extra-low voltage (supply system)||< 50 Vrms||< 120 V||low risk|
Extra-low voltage (ELV), 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 (ground) is not more than either 25 volts RMS (35 volts peak) for alternating current[inconsistent], 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.[further explanation needed]
The IEC defines three types of extra-low-voltage systems (FELV, PELV, and SELV), which are distinguished by their successively more restrictive safety properties.
This section is ordered from most restrictive (safest), to least restrictive.
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".
There exists some confusion regarding the origin of the acronym: "SELV" stands for "separated extra-low voltage" in installation standards (e.g., BS 7671) and for "safety extra-low voltage" in appliance standards (e.g., BS EN 60335).
A SELV circuit must have:
- 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.
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 (ground).
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.
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.
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. 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.
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 126.96.36.199 Extra-Low Voltage (ELV)states: "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 (TNV) limits ... i.e. 120 V d.c. or 70.7 V a.c. peak (50 V a.c. r.m.s.)"
TNV is the traditional line limit to accommodate telephone ringing machine voltage (ripple)from 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 (but not all) 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 (TPS), and fine stranded multi-strand cable (like automotive cable, although this may only be rated to 32 V DC, and not the full ELV range).
State regulations override the Australian Standards, and there are some differences.
- 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.
- Stand-alone Power Systems Components. Edition 1 (Dec 2002) Resource Book. Renewable Energy Centre. ISBN 1-876880-31-7