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". The acronym may variously stand for
safety extra-low voltage or
separated extra-low voltage. It is generally accepted that the acronym: BS EN 60335 and the IEC refer to it as
safety extra-low voltage, while
separated extra-low voltage (separated from earth) is used and defined in installation standards (e.g.
BS 7671). 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 (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 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 cased
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 a 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. FELV is defined in IEC 60364-4-41 clause 411.7 as a circuit that: • does not meet the requirements for PELV or SELV and where PEV or SELV is not necessary, and • must have Basic protection against direct contact, and • must have fault protection against indirect contact if accessible Basic protection against direct contact (clause 411.7.2) can be provided either by Basic insulation rated to the nominal voltage of the primary circuit (e.g. AC mains), or by a barrier or enclosure complying with the various requirements in Annex A.2. Fault protection against indirect contact (clause 411.7.3) requires that any "exposed-conductive-parts" of the FELV circuit be connected to the Protective Conductor (Earth/Ground, similar to
Class I construction), and where the primary circuit (e.g. AC mains) is protected by "automatic disconnection of supply" through an
RCD. Any circuit connected to a "live" part that is considered Accessible to the user needs two levels of insulation safeguards between the Accessible circuit and the voltage source. Unless the inclusion of an RCD-like device is guaranteed by such a device being built in to the equipment then FELV circuits should remain not user accessible. Therefore, the protection requirements for the higher voltage have to be applied to the entire circuit. Examples of a FELV circuit include
DALI where each end of the link is specified as a FELV circuit with at least Basic insulation to AC mains. Circuits that generate an extra low voltage from a "live" part through a
semiconductor device,
resistor,
potentiometer or an
autotransformer do not provide an insulation safeguard and cannot be considered FELV. Instead they are considered to be part of the "live" circuit. An example of this type of circuit is an electronically controlled toaster where the electronic timer circuit runs off an extra low voltage derived from a tap on the
heating element. Another might be ELV signalling between mains powered smoke alarms, with the signalling voltage referred to supply neutral. In such cases these circuits must be enclosed or insulated as to the standard of the mains voltage.
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 AC This system has been used for many years on construction sites, in both single- and three-phase configurations. The single-phase voltage is 110V a.c. though having a "centre tapped Earth" reducing the voltage to earth to 55V AC. The three-phase system is 110V phase-to-phase and 63V 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. ==Stand-alone power systems==