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| IEC 61140voltage bands | ACRMS | DC | Defining risk |
|---|---|---|---|
| High voltage | > 1,000 | > 1,500 | Electrical arcing |
| Low voltage | ≤ 1,000 | ≤ 1,500 | Electrical shock |
| Extra-low voltage | < 50 | < 120 | Electrical fire |
Extra-low voltage (ELV) is anelectricity supplyvoltage and is a part of the low-voltage band[1] in a range which carries a low risk of dangerouselectrical shock.[2][3][4][5] There are various standards that define extra-low voltage. TheInternational Electrotechnical Commission (IEC) inIEC 61140 define an ELV device or circuit as one in which theelectrical potential between twoconductors or between an electrical conductor andEarth (ground)does not exceed 120 volts (V) for ripple-freedirect current (DC) or 50 VRMS (root mean square volts) foralternating current (AC).
The IEC and IET go on to define actual types of extra-low voltage systems, for example separated extra-low voltage (SELV), protected extra-low voltage (PELV), functional extra-low voltage (FELV). These can be supplied using sources including motor / fossil fuel generator sets, transformers, switched PSU's or rechargeable battery. SELV, PELV, FELV, are distinguished by varioussafety properties, supply characteristics and design voltages.
Some types oflandscape lighting use SELV / PELV (extra-low voltage) systems. Modern battery operated hand tools fall in the SELV category. In more arduous conditions, 25 VRMS alternating current or 60 V (ripple-free) DC can be specified to further reduce hazard. Lower voltage can apply in wet or conductive conditions where there is even greater potential for electric shock. These systems should still fall under the SELV / PELV (ELV) safety specifications.
IEC defines a SELV system as "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions,includingearth faults in other circuits". The acronym may variously stand forsafety extra-low voltage orseparated extra-low voltage. It is generally accepted that the acronym: BS EN 60335 and the IEC refer to it assafety extra-low voltage, whileseparated extra-low voltage (separated from earth) is used and defined in installation standards (e.g.BS 7671).[6]
A SELV circuit must have:
The safety of a SELV circuit is provided by
The design of a SELV circuit typically involves an isolatingtransformer, guaranteed minimum distances between conductors andelectrical insulation barriers. Theelectrical 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 IIIbattery charger, fed from a Class IIpower supply. Modern cordless hand tools are considered SELV equipment.
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 aprotective 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 reinforced insulation, or by a conductive shield with a protective earth connection, or that the secondary winding itself has an earthed terminal, so that any primary to secondary fault will cause automatic disconnection.(The principle of double fault to danger requires either basic and additional insulation to fail or basic insulation and the connection to the protective earth to fail simultaneously before danger arises.)
A typical example for a PELV circuit is a metal casedcomputer with aClass Ipower supply.
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 asemiconductor device or apotentiometer or anautotransformer. A typical example is an electronically controlled toaster where the electronic timer circuit runs off an extra low voltage derived from a tap on theheating element. Another might be ELV signalling between mains powered smoke alarms, with the signalling voltage referred to supply neutral.In such cases the extra low voltage parts must be enclosed or insulated as to the standard of the mains voltage.
The IET / BSI (BS 7671) also define Reduced Low Voltage (RLV) which can be either single-phase or three-phase AC This system has been used for many years on construction sites, in both single- and three-phase configurations. The single-phase voltage is 110 V a.c. though having a "centre tapped Earth" reducing the voltage to earth to 55 V AC. The three-phase system is 110 V phase-to-phase and 63 V to neutral / earth. This system voltage is slightly above the ELV limit, but is still very commonly used for cord-powered hand tools and temporary lighting in hazardous areas. As it is transformer-derived, the exposed voltage during an earth fault is depressed below the ELV level.
Cabling for ELV systems, such as inremote-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, andcurrent-carrying capacity. Voltage drop is usually the main factor considered, butcurrent-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 andautomotive fuse types. Instead,high rupturing capacity fuses and appropriately ratedcircuit breakers are the recommended type for RAPS. Cable termination and connections must be done properly to avoid arcing also, andsoldering is not recommended.
Precise definitions of "extra low voltage" are given in applicable wiring regulations in a region.
According to DIN EN 61140:2016 chapter 4.2 Table 1 (German version of EU standard EN 61140), Extra Low Voltage (≤ 50 V a.c. or ≤ 120 V d.c.) is defined as a sub-category of Low voltage (≤ 1000 V a.c. or ≤ 1500 V d.c.).This is similar to the definition provided in IEC 61140:2016.[7]
The European Union directives on the other hand do not define extra-low voltage. The nearest they get to this concept is in theLow Voltage Directive (2014/35/EU),[8] which applies to the range between 50 V AC / 75 V DC and 1,000 V AC / 1,500 V DC. TheGeneral Product Safety Directive (2001/95/EC) covers consumer goods with a voltage below 50 V for alternating current, or below 75 V for direct current.[8] The directive only covers electrical equipment and not voltages appearing inside equipment or voltages in electrical components.
IEC 61140: 2016, Basic safety publication defines ELV as ≤ 50 VRMS AC and ≤ 120 V DC ripple-free.
IEC 60364-4-41: 2017, Group safety publication defines ELV as ≤ 50 VRMS AC and ≤ 120 V DC ripple-free.
AS/NZS 3000Wiring Rules define "extra low voltage" as "Not exceeding 50 V AC or 120 V ripple-free DC" However, AS/ACIF S009 Clause 3.1.78.1 Extra-Low Voltage (ELV)states: "a voltage not exceeding 42.4 V peak or 60 V DC [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 DC or 70.7 V AC peak (50 VRMS AC)" which accommodates telephone ringing voltage on the nominally −48 V DC battery supply which could be encountered on a telephone line and was not considered hazardous, whereas 120 V AC without current limiting at its source can inject 115mA into individuals leading to fibrillation of the heart.
In Brazil, ELV (Extra-baixa tensão orEBT inPortuguese) is officially defined in Regulatory Standard no. 10 from theBrazilian Ministry of Labor and Employment as any voltage "not exceeding 50 volts a.c. or 120 volts d.c.".[9] 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 AC or 60 volts DC[10]