![PoleMountedDistributionTransformerNZ](https://upload.wikimedia.org/wikipedia/commons/3/3c/PoleMountedDistributionTransformerNZ.jpg)
A low-voltage network or secondary network is a part of
electric power distribution
Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...
which carries electric energy from
distribution transformer
Single-phase distribution transformer in Canada
A distribution transformer or service transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the di ...
s to
electricity meter
North American domestic analog electricity meter.
Electricity meter with transparent plastic case (Israel)
North American domestic electronic electricity meter
An electricity meter, electric meter, electrical meter, energy meter, or kilowa ...
s of end customers. Secondary networks are operated at a
low voltage In electrical engineering, low voltage is a relative term, the definition varying by context. Different definitions are used in electric power transmission and distribution, compared with electronics design. electrical safety codes define "low vo ...
level, which is typically equal to the
mains voltage
Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to ...
of electric appliances.
Most modern secondary networks are operated at
AC rated voltage of 100–127 or 220–240
volts
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defini ...
, at the frequency of 50 or 60
hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...
(see
mains electricity by country
Mains electricity by country includes a list of countries and territories, with the plugs, voltages and frequencies they commonly use for providing electrical power to low voltage appliances, equipment, and lighting typically found in homes and ...
). Operating voltage, required number of phases (
three-phase
Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, Electric power transmission, transmission, and Electric power distribution, distribution. It is a type of polyphase system empl ...
or
single-phase
In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when load ...
) and required reliability dictate topology and configuration of the network. The simplest form are radial
service drop
In electric power distribution, a service drop is an overhead electrical line running from a utility pole, to a customer's building or other premises. It is the point where electric utilities provide power to their customers.Carson Dunlop "Elec ...
lines from the transformer to the customer premises. Low-voltage radial feeders supply multiple customers. For increased reliability, so-called ''spot networks'' and ''grid networks'' provide supply of customers from multiple distribution transformers and supply paths.
Electric wiring
Electrical wiring is an electrical installation of cabling and associated devices such as switches, distribution boards, sockets, and light fittings in a structure.
Wiring is subject to safety standards for design and installation. Allowable ...
can be realized by
overhead power line
An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-p ...
s,
aerial or underground
power cable
A power cable is an electrical cable, an assembly of one or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring wi ...
s, or their mixture.
Overview
Electric power distribution
Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...
systems are designed to serve their customers with reliable and high-quality power. The most common distribution system consists of simple radial circuits (feeders) that can be overhead, underground, or a combination. From the
distribution substation, feeders carry the power to the end customers, forming the ''medium-voltage'' or ''primary'' network, operated at a
medium voltage level, typically 5–35 kV. Feeders range in length from a few kilometers to several tens of kilometers. As they must supply all customers in the designated distribution area, they often curve and branch along the assigned corridors. A substation typically supplies 3–30 feeders.
Distribution transformer
Single-phase distribution transformer in Canada
A distribution transformer or service transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the di ...
s or ''secondary transformers'', placed along feeders, convert the voltage from the medium to a
low voltage In electrical engineering, low voltage is a relative term, the definition varying by context. Different definitions are used in electric power transmission and distribution, compared with electronics design. electrical safety codes define "low vo ...
level, suitable for direct consumption by end customers (
mains voltage
Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to ...
). Typically, a rural primary feeder supplies up to 50 distribution transformers, spread over a wide region, but the figure significantly varies depending on configuration. They are sited on pole tops, cellars or designated small plots. From these transformers, ''low-voltage'' or ''secondary'' network branches off to the customer connections at customer premises, equipped with
electricity meter
North American domestic analog electricity meter.
Electricity meter with transparent plastic case (Israel)
North American domestic electronic electricity meter
An electricity meter, electric meter, electrical meter, energy meter, or kilowa ...
s.
Design considerations
Most of differences in the layout and design of low-voltage networks are dictated by the
mains voltage
Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to ...
rating. In Europe and most of the world 220–240 V is the dominant choice, while in North America 120 V is the standard.
ANSI standard C84.1 recommends a +5%, −2.5% tolerance for the voltage range at a service point. North American LV networks feature much shorter secondary connections, up to , while in European design they can reach up to . North American distribution transformers must be therefore placed much closer to consumers, and are smaller (25–50 kVA), while European ones can cover larger areas and thus have higher ratings (300–1000 kVA); only the remote rural areas in European design are served by single-phase transformers.
As the low-voltage distribute the electric power to the widest class of end users, another main design concern is safety of consumers who use the electric appliances and their protection against electric shocks. An
earthing system
An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affec ...
, in combination with protective devices such as
fuses
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to protec ...
and
residual current device
A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device that quickly breaks an electrical circuit with leakage current to ground. It is to protect equip ...
s, must ultimately ensure that a person must not come into touch with a metallic object whose potential relative to the person's potential (which is, in turn, equal to the ground potential unless
insulating mat
Insulation may refer to:
Thermal
* Thermal insulation, use of materials to reduce rates of heat transfer
** List of insulation materials
** Building insulation, thermal insulation added to buildings for comfort and energy efficiency
*** Insulated ...
s are used) exceeds a "safe" threshold, typically set at about 50 V.
Topology
Radial networks
![Radial Networks](https://upload.wikimedia.org/wikipedia/commons/4/49/Radial_Networks.svg)
Radial operation is the most widespread and most economic design of both MV and LV networks. It provides a sufficiently high degree of reliability and service continuity for most customers. In American (120 V) systems, the customers are commonly supplied directly from the distribution transformers via relatively short
service drop
In electric power distribution, a service drop is an overhead electrical line running from a utility pole, to a customer's building or other premises. It is the point where electric utilities provide power to their customers.Carson Dunlop "Elec ...
lines, in star-like topology. In 240 V systems, the customers are served by several low-voltage feeders, realized by
overhead power line
An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-p ...
s,
aerial or underground
power cable
A power cable is an electrical cable, an assembly of one or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring wi ...
s, or their mixture; in an overhead network, service drops are drawn from pole tops to roof connections. In a cable network, all necessary connections and protection devices are typically placed in pad-mounted cabinets or, occasionally, manholes (buried T-joint connections are prone to failures).
Power-system protection
Power system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the disconnection of faulted parts from the rest of the electrical network. The objective of a prot ...
in radial networks is simple to design and implement, since short-circuit currents have only one possible path that needs to be interrupted.
Fuses
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to protec ...
are most commonly used for both short-circuit and overload protection, while low-voltage
circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
s may be used in special circumstances.
Spot networks
![Spot Network](https://upload.wikimedia.org/wikipedia/commons/7/74/Spot_Network.svg)
Spot networks are used when increased reliability of supply is required for important customers. The low-voltage network is supplied from two or more distribution transformers at a single site, each fed from a different MV feeder (which may originate from the same or different substations). The transformers are connected together with a bus or a cable on secondary side, termed ''paralleling bus'' or ''collector bus''. The paralleling bus typically does not have connecting cables (''reaches'') to other network units, in which case such networks are termed ''isolating spot networks''; when they have, they are referred to as ''spot networks with reach''. In some cases, fast-acting secondary bus tie breakers may be applied between bus sections to isolate faults in the secondary switchgear and limit loss of service.
Spot systems are commonly applied in high load-density areas such as business districts, large hospitals, small industry and important facilities such as water supply systems. In normal operation, the energy supply is provided by both primary feeders in parallel. In case of an outage of either primary feeder,
network protector
A network protector is a type of electric protective device used in electricity distribution systems. The network protector automatically disconnect its associated distribution transformer from the secondary network when the power starts flowing i ...
device at the corresponding spot transformer secondary automatically opens; the remaining transformers continue to provide supply through their respective primary feeders. Only in cases when the
short circuit
A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...
is located at the paralleling bus, or a total loss of primary supply occurs, the customer will remain out of service. Faults on the low-voltage network are handled by fuses or local circuit breakers, resulting in loss of service only for the affected loads.
Grid networks
![Grid Network](https://upload.wikimedia.org/wikipedia/commons/a/ae/Grid_Network.svg)
A grid networks consist of an interconnected grid of circuits, energized from several primary feeders through distribution transformers at multiple locations. Grid networks are typically featured in downtowns of large cities, with connecting cables laid out in underground conduits along the streets. Numerous cables allow for multiple current paths from every transformer to every load within the grid.
As with spot networks, network protectors are used to protect against primary feeder faults, and prevent fault current to propagate from the grid to the primary feeder. Individual cable sections may be protected by
cable limiter
In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows thr ...
s on both ends, special fuses providing very fast short-circuit protection. Cable limiters do not have an ampere rating, and cannot be used to provide overload protection; their sole purpose is to isolate the fault. Under high short-circuit conditions, limiters blow and cut off the faulted cable, while the unaffected cables take over its load and continue to provide service. Primary feeder outages, as well as limiters and network protectors cleared because of previous faults, cause changes in load flow that are not readily detected, so their statuses may require a periodic inspection. The inherent system redundancy generally prevents any customer from experiencing outage.
See also
*
Earthing system
An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affec ...
*
Split-phase electric power
A split-phase or single-phase three-wire system is a type of single-phase electric power distribution. It is the alternating current (AC) equivalent of the original Edison Machine Works three-wire direct-current system. Its primary advantage is th ...
Footnotes
References
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* {{cite book, last1=Warne , first1=D.F. , title=Newnes Electrical Power Engineer's Handbook, url=https://books.google.com/books?id=j8Exol1R9nsC, date=2 June 2005, publisher=Elsevier, isbn=978-0-08-047969-9
Electric power distribution