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 50 V for alternating current;
- not more than 120 V for direct current.
(In IEC terminology, low voltage ranges up to 1000 V AC or 1500 V DC, everything above is high voltage.)
Three types of extra low voltage sources are distinguished by their safety properties.
Separated Extra Low Voltage (SELV)
A Separated Extra Low Voltage (SELV) circuit must be
- safely separated from other circuits that carry higher voltages;
- isolated from earth (ground) and from the protective earth conductors of other circuits.
The safety of an 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 a current could take in case of contact with a human body.
The design of an SELV circuit typically involves an insulation transformer, guaranteed minimum distances between conductors, and insulation barriers. The 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.
(The term Safety Extra Low Voltage can still be encountered in some older publications. The modern IEE/IEC term Separated Extra Low Voltage clarifies the distinction between SELV and PELV circuits better.)
Protected Extra Low Voltage (PELV)
In contrast to an SELV circuit, a Protected Extra Low Voltage (PELV) circuit has 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.