Extra low voltage
Encyclopedia
In electricity supply, the use of extra-low voltage (ELV) 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 volt
s RMS
(35 volts peak) for alternating current
, 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.
The IEC defines three types of extra-low-voltage systems (FELV, PELV,and SELV), which are distinguished by their successively more restrictive safety
properties.
s 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:
The safety of a SELV circuit is provided by
The design of a SELV circuit typically involves an isolating transformer
, guaranteed minimum distances between conductors and electrical insulation
barriers. The electrical connector
s 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 III
battery charger
, fed from a Class II
power supply
.
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
.
Examples for FELV circuits include those that generate an extra low voltage through a semiconductor
device or a potentiometer
.
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 breaker
s are the recommended type for RAPS. Cable termination and connections must be done properly to avoid arcing also, and soldering
is not recommended.
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 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 , and not the full ELV range).
State regulations override the Australian Standards, and there are some differences.
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
and its member organizations define an ELV circuit as one in which the electrical potential of any conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
against earth (ground) is not more than either 25 volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s RMS
Root mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
(35 volts peak) for alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
, or ripple-free 60 volts for direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
under dry conditions. Lower numbers apply in wet conditions, or when large contact areas are exposed to contact with the human body.
The IEC defines three types of extra-low-voltage systems (FELV, PELV,and SELV), which are distinguished by their successively more restrictive safety
Safety
Safety is the state of being "safe" , the condition of being protected against physical, social, spiritual, financial, political, emotional, occupational, psychological, educational or other types or consequences of failure, damage, error, accidents, harm or any other event which could be...
properties.
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 faultGround (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
s 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
BS 7671
British Standard BS 7671 "Requirements for electrical installations" is the national standard in the United Kingdom for low voltage electrical installations....
) 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 currentElectric currentElectric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
could take in case of contact with a human body.
The design of a SELV circuit typically involves an isolating transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
, guaranteed minimum distances between conductors and electrical insulation
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
barriers. The electrical connector
Electrical connector
An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...
s 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 III
Appliance classes
In the electrical appliance manufacturing industry, the following IEC protection classes are used to differentiate between the protective-earth connection requirements of devices.-Class 0:...
battery charger
Battery charger
A battery charger is a device used to put energy into a secondary cell or rechargeable battery by forcing an electric current through it.The charge current depends upon the technology and capacity of the battery being charged...
, fed from a Class II
Appliance classes
In the electrical appliance manufacturing industry, the following IEC protection classes are used to differentiate between the protective-earth connection requirements of devices.-Class 0:...
power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
.
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
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
with a Class I
Appliance classes
In the electrical appliance manufacturing industry, the following IEC protection classes are used to differentiate between the protective-earth connection requirements of devices.-Class 0:...
power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
.
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
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
device or a potentiometer
Potentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
.
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 an important when considering short, high-current runs such as between a battery bank and inverter.Arcing
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
is a risk in DC ELV systems, and some fuse types which can cause undesired arcing include semi-enclosed, rewireable and automotive fuse
Fuse (automotive)
Automotive fuses are a class of fuses used to protect the wiring and electrical equipment for vehicles. They are generally rated for circuits no higher than 24 volts direct current, but some types are rated for 42-volt electrical systems...
types. Instead high rupturing capacity fuses and appropriately rated circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s are the recommended type for RAPS. Cable termination and connections must be done properly to avoid arcing also, and soldering
Soldering
Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
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."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 from low voltage
Low voltage
Low voltage when used as an electrical engineering term concerning an electricity supply grid or industrial use, broadly identifies safety considerations of the system based on the voltage used. The meaning of the term "low voltage" is somewhat different when used with regard to a more typical end...
wiring or have a separate insulating barrier such a conduit. ELV cable and wire types include PVC
Polyvinyl chloride
Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...
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 , and not the full ELV range).
State regulations override the Australian Standards, and there are some differences.