The use of extra-low voltage (ELV) in an electrical circuit is one of several means to provide protection 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 50 volts RMS (70 volts peak) for alternating current, or 120 volts for direct current. In IEC terminology, "low voltage" ranges up to 1000 V AC or 1500 V DC, everything above is high voltage. Importantly, in household and residential uses (including doorbells, thermostats, intercoms, home theater, home automation, sprinkler systems, and especially in lighting), "low voltage" always refers to ELV. What the IEC calls "low" voltage (which is still enough to kill a person) is in turn called mains or line voltage in common use. Three types of extra-low-voltage systems are distinguished by their safety properties.
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Separated or safety extra-low voltage (SELV)
IEC 61140 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". SELV is described as Safety extra-low voltage in appliance standards e.g. BS EN 60335, and separated extra-low voltage in installation standards e.g. BS 7671 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 electrical 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 II battery charger.
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. A typical example for a PELV circuit is a computer with a Class I power supply.
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.
