Ring circuit
Encyclopedia
In electricity supply, a ring final circuit or ring circuit (often incorrectly called a ring main or informally a ring) is an electrical wiring
technique developed and primarily used in the United Kingdom
. This design enables the use of smaller-diameter wire than would be used in a radial circuit of equivalent total current. Appliances connected to sockets on a ring circuit are individually protected by a fused BS 1363
attachment plug.
Ideally, the ring circuit acts like two radial circuits proceeding in opposite directions around the ring, the dividing point between them dependent on the distribution of load in the ring. If the load is evenly split across the two directions, the current in each direction is half of the total, allowing the use of wire with half the current-carrying capacity. In practice, the load does not always split evenly, so thicker wire is used.
(also known as fuse
box or breaker
box), visits each socket
in turn, and then returns to the consumer unit. The ring is fed from a circuit breaker in the distribution board
.
Ring circuits are commonly used in British wiring with fused 13 A plug
s to BS 1363
. They are generally wired with 2.5 mm2 cable and protected by a 30 A fuse
, an older 30 A circuit breaker
, or a Europe
an harmonised 32 A circuit breaker. Sometimes 4 mm2 cable is used if very long cable runs (to help reduce volt-drop) or derating factors such as thermal insulation
are involved. 1.5 mm2 mineral-insulated copper-clad cable
(known as pyro) may also be used (as mineral insulated cable can withstand heat more effectively than normal PVC) though more care must be taken with regard to voltage drop on longer runs.
Many lay people in the UK refer to any circuit as a ring and the term lighting ring is often heard from novices. It is not unheard of to see lighting circuits wired as rings of cable (though usually still with a breaker below the cable rating) in DIY installations.
and the associated BS 1363
plug and socket system were developed in Britain during 1942–1947. They are commonly used in the United Kingdom
and to a lesser extent in the Republic of Ireland
. They are also found in the United Arab Emirates
.
The ring circuit came about because Britain had to embark on a massive rebuilding programme following World War II
. There was an acute shortage of copper, and it was necessary to devise a scheme that used less copper than would normally be the case. The scheme was specified to use 13 A fused socket outlets, and several designs for the plugs and sockets appeared. Only the square pin (BS 1363) system survives, but the round pin Dorman & Smith system was still in use in many locations well into the 1980s, and is still occasionally seen today. This latter plug had the distinctive feature that the fuse was also the live pin and unscrewed from the plug body.
The ring circuit was devised during a time of copper
shortage to allow two 3 kW
heaters
to be used in any two locations and to allow some power to small appliances, and to keep total copper use low. It has stayed the most common circuit configuration in the UK, although the 20 A radial (essentially breaking each ring in half and putting the halves on a separate breaker) is becoming more common. Splitting a ring into two 20 A radials can be a useful technique where one leg of the ring is damaged and cannot easily be replaced, but if the ring was wired with 1.5mm² wires, when it is split it could support only a 13 A current.
Another advantage of ring circuits was an economy of cable and labour, as one could connect a cable between two existing 15 A radially wired sockets to make one 30 A ring, then adding as many sockets as were desired. This was an important consideration in the austerity of the 1940s. This would leave the ring supplied by two 15 A fuses, which worked well enough in practice, even if unconventional.
Many pre-war (round pin) installations used double pole fusing. When two 15 A radials were converted to a ring on these systems, the ring would then be supplied by no fewer than 4 fuses. Such circuits are rare today.
ing of the cable can be considered minimal. In practice, however, it is extremely uncommon to encounter a ring with a protective device other than a 30 A fuse, 30 A breaker, or 32 A breaker, and a cable size other than those mentioned above.
The IEE
Wiring Regulations (BS 7671
) permit an unlimited number of socket outlets to be installed on a ring circuit, provided that the floor area served does not exceed 100 m2. In practice, most small and medium houses have one ring circuit per storey, with larger premises having more.
An installation designer may determine by experience and calculation whether additional circuits are required for areas of high demand; for example, it is common practice to put kitchen
s on their own ring circuit or sometimes a ring circuit shared with a utility room
to avoid putting a heavy load at one point on the main downstairs ring circuit. A heavy concentration of load close together on a ring circuit can cause minor overloading of one of the cables if near the end of the ring, so kitchens should not be wired at one end of a ring circuit.
Unfused spurs from a ring wired in the same cable as the ring are allowed to run one single or double socket from each of the sockets on the ring (the use of two singles was previously allowed but was banned because of people replacing them with doubles) or one fused connection unit (FCU). Spurs may either start from a socket or be joined to the ring cable with a junction box or other approved method of joining cables. Triple and larger sockets are generally fused and therefore can also be placed on a spur.
It is not permitted to have more spurs than sockets on the ring, and it is considered bad practice by most electrician
s to have spurs in a new installation (some think they are bad practice in all cases).
Where loads other than BS 1363 sockets are connected to a ring circuit or it is desired to place more than one socket for low power equipment on a spur, a BS 1363 fused connection unit (FCU) is used. In the case of fixed appliances this will be a switched fused connection unit (SFCU) to provide a point of isolation for the appliance, but in other cases such as feeding multiple lighting points (putting lighting on a ring through is generally considered bad practice in new installation but is often done when adding lights to an existing property) or multiple sockets, an unswitched one is often preferable.
Fixed appliances with a power rating over 3 kW (for example, water heaters and some electric cookers) or with a non-trivial power demand for long periods (for example, immersion heaters) are no longer recommended to be connected to a ring circuit, but instead are connected to their own dedicated circuit. There are however plenty of older installations with such loads on a ring circuit.
The only way to see the pros and cons of ring circuits is to compare them to the other option: radials.
It is also becoming very apparent that a majority of UK electricians are unfamiliar with the test requirements and, as a result, most ring circuits are not adequately tested, either at first installation or subsequently during the infrequent periodic inspections.
requires that the installed load is distributed around the ring such that no part of the cable exceeds its capacity. This requirement is difficult to fulfil and may be largely ignored in practice, as loads are often co-located (washing machine, tumble dryer, dish washer all next to kitchen sink) and not necessarily near the centre of the ring.
. This prevents the circuit forming a large magnetic coil (loop antenna
), which would otherwise induce a magnetic field at the AC frequency (50 or 60 Hz).
In a ring circuit, on the other hand, it is possible that the live and neutral currents are not equal on each side of the ring. Mains-frequency currents follow the path of least resistance, and it is possible, especially with ageing oxidised contacts, that from a socket, the lowest-resistance live connection is along the left-hand side of the ring, and the lowest-resistance neutral connection is along the right-hand side. As a result, current is flowing around the ring and will therefore induce a magnetic field
. In the extreme case of a defect ring circuit, the live connection could become completely interrupted on one side of the ring and the neutral connection on the other, and then the full current would supply the magnetic field. This can lead to substantial electromagnetic interference
, such as mains hum
in audio devices, accidental triggering of alarm and protection devices (burglar alarms, RCDs, etc.), malfunctions of consumer electronics and medical devices, ground loop
s, etc.
s, particularly, as is often the case, if there is an undetected fault, AND the circuit conductors are not sized to match the Overcurrent Protective Device (OPD) as a radial run as opposed to a ring. The purpose of ring circuits is to supply a large number of sockets; therefore, they are protected only with high-rated overcurrent circuit breakers (typically 32 A). In comparison, the radial circuits used in other countries typically supply only a small number of sockets and are therefore protected with lower-rated circuit breakers (typically 10–20 A). As a result, countries using ring circuits find it necessary to add additional lower-rated fuses into the plugs of each appliance. This does create a possible improvement in safety in that an appliance with blown plug fuse will not be live when plugged in again (unless the fuse is first replaced), whereas with fuseless plugs a faulty appliance remains potentially dangerous to plug in, though in most cases it would trip a lower-rated circuit breaker if plugged in again.
This incompatibility in the overcurrent protection of appliance leads between countries using ring and radial circuits has been a major stumbling block on the road to worldwide standardisation of domestic AC power plugs and sockets
. Although plug-fuses can, in principle, be better matched to the maximum current required by an appliance, in practice, some plugs in the UK are necessarily fitted with a fuse of the maximum permitted rating of 13 A, because a lower-rated device may well operate intermittently due to "surges" (e.g. fit a 3A BS1362 fuse in the plug-top of a fridge, and it will often blow). This is not a problem since all appliances are required to be safe with a 13 A fuse (and in any case, in other EU countries, the appliance concerned is often protected by a 16 A or 20 A OPD for the circuit concerned), but it does mean the potential safety advantage is only partially realised and that the fused plug offers little advantage over an unfused plug used on radial circuit with a 13 A or lower fuse, or B16 or lower circuit breaker. The introduction of regulations in the UK - the Plugs and Sockets (Safety) Regulations - requiring new appliances to be sold with correctly fused pre-fitted plugs improves this situation further.
One theoretical advantage of individually-fused plugs is that a faulty appliance or flexible cord has a high likelihood of blowing only its plug-top fuse, leaving other appliances on the same ring circuit operating. However, with the introduction of EN60898 mcb's and the increased use of RCD protection for general purpose socket outlets in the UK (under BS7671: 2008 and earlier editions of the same standard) means that it is now more likely that the circuit protective device will operate before the plug-top fuse.
Electrical wiring
Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring. This article is intended to...
technique developed and primarily used in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
. This design enables the use of smaller-diameter wire than would be used in a radial circuit of equivalent total current. Appliances connected to sockets on a ring circuit are individually protected by a fused BS 1363
BS 1363
BS 1363 is a British Standard which specifies the most common type of single-phase AC power plugs and sockets that are used in the United Kingdom. Distinctive characteristics of the system are shutters on the line and neutral socket holes, and a fuse in the plug...
attachment plug.
Ideally, the ring circuit acts like two radial circuits proceeding in opposite directions around the ring, the dividing point between them dependent on the distribution of load in the ring. If the load is evenly split across the two directions, the current in each direction is half of the total, allowing the use of wire with half the current-carrying capacity. In practice, the load does not always split evenly, so thicker wire is used.
Description
The ring starts at the consumer unitConsumer unit
A consumer unit is a box of fuses or breakers, usually arranged in a single row. This is unlike a distribution board which has multiple rows of fuses or breakers and usually serves two or more locations, which may be split phase, two phase, two phases taken from three phase, or three phases.A...
(also known as fuse
Fuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
box or 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...
box), visits each socket
Domestic AC power plugs and sockets
AC power plugs and sockets are devices for removably connecting electrically operated devices to the power supply. Electrical plugs and sockets differ by country in rating, shape, size and type of connectors...
in turn, and then returns to the consumer unit. The ring is fed from a circuit breaker in the distribution board
Distribution board
A distribution board is a component of an electricity supply system which divides an electrical power feed into subsidiary circuits, while providing a protective fuse or circuit breaker for each circuit, in a common enclosure...
.
Ring circuits are commonly used in British wiring with fused 13 A plug
Plug
Plug may refer to:* Plug for the plughole in the bathtub, washbasin and sink* Plug , a short lived British comic that ran from 1977 until 1979, when it merged with The Beezer, which featured a character from The Bash Street Kids called Plug as it's main star.* Plug , a family of fishing lures*...
s to BS 1363
BS 1363
BS 1363 is a British Standard which specifies the most common type of single-phase AC power plugs and sockets that are used in the United Kingdom. Distinctive characteristics of the system are shutters on the line and neutral socket holes, and a fuse in the plug...
. They are generally wired with 2.5 mm2 cable and protected by a 30 A fuse
Fuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
, an older 30 A 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...
, or a Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
an harmonised 32 A circuit breaker. Sometimes 4 mm2 cable is used if very long cable runs (to help reduce volt-drop) or derating factors such as thermal insulation
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
are involved. 1.5 mm2 mineral-insulated copper-clad cable
Mineral-insulated copper-clad cable
Mineral-insulated copper-clad cable is a variety of electrical cable made from copper conductors inside a copper sheath, insulated by inorganic magnesium oxide powder. The name is often abbreviated to MICC or MI cable, and colloquially known as pyro...
(known as pyro) may also be used (as mineral insulated cable can withstand heat more effectively than normal PVC) though more care must be taken with regard to voltage drop on longer runs.
Many lay people in the UK refer to any circuit as a ring and the term lighting ring is often heard from novices. It is not unheard of to see lighting circuits wired as rings of cable (though usually still with a breaker below the cable rating) in DIY installations.
History and use
The ring circuitElectrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
and the associated BS 1363
BS 1363
BS 1363 is a British Standard which specifies the most common type of single-phase AC power plugs and sockets that are used in the United Kingdom. Distinctive characteristics of the system are shutters on the line and neutral socket holes, and a fuse in the plug...
plug and socket system were developed in Britain during 1942–1947. They are commonly used in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
and to a lesser extent in the Republic of Ireland
Republic of Ireland
Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...
. They are also found in the United Arab Emirates
United Arab Emirates
The United Arab Emirates, abbreviated as the UAE, or shortened to "the Emirates", is a state situated in the southeast of the Arabian Peninsula in Western Asia on the Persian Gulf, bordering Oman, and Saudi Arabia, and sharing sea borders with Iraq, Kuwait, Bahrain, Qatar, and Iran.The UAE is a...
.
The ring circuit came about because Britain had to embark on a massive rebuilding programme following World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
. There was an acute shortage of copper, and it was necessary to devise a scheme that used less copper than would normally be the case. The scheme was specified to use 13 A fused socket outlets, and several designs for the plugs and sockets appeared. Only the square pin (BS 1363) system survives, but the round pin Dorman & Smith system was still in use in many locations well into the 1980s, and is still occasionally seen today. This latter plug had the distinctive feature that the fuse was also the live pin and unscrewed from the plug body.
The ring circuit was devised during a time of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
shortage to allow two 3 kW
Kw
kw or KW may refer to:* Kuwait, ISO 3166-1 country code** .kw, the country code top level domain for Kuwait* Kilowatt* Self-ionization of water Kw* Cornish language's ISO 639 code* Kitchener–Waterloo, Ontario, Canada...
heaters
Convector heater
A convection heater is a heater which operates by air convection currents circulating through the body of the appliance, and across its heating element. This heats up the air, causing it to increase in volume and so become buoyant and rise...
to be used in any two locations and to allow some power to small appliances, and to keep total copper use low. It has stayed the most common circuit configuration in the UK, although the 20 A radial (essentially breaking each ring in half and putting the halves on a separate breaker) is becoming more common. Splitting a ring into two 20 A radials can be a useful technique where one leg of the ring is damaged and cannot easily be replaced, but if the ring was wired with 1.5mm² wires, when it is split it could support only a 13 A current.
Another advantage of ring circuits was an economy of cable and labour, as one could connect a cable between two existing 15 A radially wired sockets to make one 30 A ring, then adding as many sockets as were desired. This was an important consideration in the austerity of the 1940s. This would leave the ring supplied by two 15 A fuses, which worked well enough in practice, even if unconventional.
Many pre-war (round pin) installations used double pole fusing. When two 15 A radials were converted to a ring on these systems, the ring would then be supplied by no fewer than 4 fuses. Such circuits are rare today.
Installation rules
Rules for ring circuits say that the cable rating must be no less than two thirds of the rating of the protective device. This means that the risk of sustained overloadOverload
-Bands:* Overload * Overload * Overload -Albums:*Overload *Overload *Overload -Songs:*Overload , a dance song by Voodoo and Serano...
ing of the cable can be considered minimal. In practice, however, it is extremely uncommon to encounter a ring with a protective device other than a 30 A fuse, 30 A breaker, or 32 A breaker, and a cable size other than those mentioned above.
The IEE
Institution of Electrical Engineers
The Institution of Electrical Engineers was a British professional organisation of electronics, electrical, manufacturing, and Information Technology professionals, especially electrical engineers. The I.E.E...
Wiring Regulations (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....
) permit an unlimited number of socket outlets to be installed on a ring circuit, provided that the floor area served does not exceed 100 m2. In practice, most small and medium houses have one ring circuit per storey, with larger premises having more.
An installation designer may determine by experience and calculation whether additional circuits are required for areas of high demand; for example, it is common practice to put kitchen
Kitchen
A kitchen is a room or part of a room used for cooking and food preparation.In the West, a modern residential kitchen is typically equipped with a stove, a sink with hot and cold running water, a refrigerator and kitchen cabinets arranged according to a modular design. Many households have a...
s on their own ring circuit or sometimes a ring circuit shared with a utility room
Utility room
A utility room is a room within a house where equipment that are not used in day to day activities are kept. Utility refers to an item which is designed for usefulness or practical use, so in turn most of the items kept in this room have functional attributes...
to avoid putting a heavy load at one point on the main downstairs ring circuit. A heavy concentration of load close together on a ring circuit can cause minor overloading of one of the cables if near the end of the ring, so kitchens should not be wired at one end of a ring circuit.
Unfused spurs from a ring wired in the same cable as the ring are allowed to run one single or double socket from each of the sockets on the ring (the use of two singles was previously allowed but was banned because of people replacing them with doubles) or one fused connection unit (FCU). Spurs may either start from a socket or be joined to the ring cable with a junction box or other approved method of joining cables. Triple and larger sockets are generally fused and therefore can also be placed on a spur.
It is not permitted to have more spurs than sockets on the ring, and it is considered bad practice by most electrician
Electrician
An electrician is a tradesman specializing in electrical wiring of buildings, stationary machines and related equipment. Electricians may be employed in the installation of new electrical components or the maintenance and repair of existing electrical infrastructure. Electricians may also...
s to have spurs in a new installation (some think they are bad practice in all cases).
Where loads other than BS 1363 sockets are connected to a ring circuit or it is desired to place more than one socket for low power equipment on a spur, a BS 1363 fused connection unit (FCU) is used. In the case of fixed appliances this will be a switched fused connection unit (SFCU) to provide a point of isolation for the appliance, but in other cases such as feeding multiple lighting points (putting lighting on a ring through is generally considered bad practice in new installation but is often done when adding lights to an existing property) or multiple sockets, an unswitched one is often preferable.
Fixed appliances with a power rating over 3 kW (for example, water heaters and some electric cookers) or with a non-trivial power demand for long periods (for example, immersion heaters) are no longer recommended to be connected to a ring circuit, but instead are connected to their own dedicated circuit. There are however plenty of older installations with such loads on a ring circuit.
Criticism
The final ring-circuit concept has been criticized in a number of ways, and some of these disadvantages could explain the lack of widespread adoption outside the United Kingdom.The only way to see the pros and cons of ring circuits is to compare them to the other option: radials.
Fault conditions are not apparent when in use
Ring circuits continue to operate without the user being aware of any problem if there are fault conditions or installation errors that make the circuit unsafe:| Fault condition | Observations |
|---|---|
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Complexity of safety tests
Testing ring circuits may take 5–6 times longer than testing radial circuits. The installation tests required for the safe operation of a ring circuit are substantially more time consuming than those for a radial circuit, and DIY installers or electricians qualified in other countries may not be familiar with them.It is also becoming very apparent that a majority of UK electricians are unfamiliar with the test requirements and, as a result, most ring circuits are not adequately tested, either at first installation or subsequently during the infrequent periodic inspections.
Balancing requirement
Regulation 433-02-04 of BS 7671BS 7671
British Standard BS 7671 "Requirements for electrical installations" is the national standard in the United Kingdom for low voltage electrical installations....
requires that the installed load is distributed around the ring such that no part of the cable exceeds its capacity. This requirement is difficult to fulfil and may be largely ignored in practice, as loads are often co-located (washing machine, tumble dryer, dish washer all next to kitchen sink) and not necessarily near the centre of the ring.
Electromagnetic interference
Ring circuits can generate strong unwanted magnetic fields. In a normal (non-ring, radial) circuit, the current flowing in the circuit must return through (almost exactly) the same path through which it came, especially if the live and neutral conductors are kept in close proximity of each other and form a twisted pairTwisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
. This prevents the circuit forming a large magnetic coil (loop antenna
Loop antenna
A loop antenna is a radio antenna consisting of a loop of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line...
), which would otherwise induce a magnetic field at the AC frequency (50 or 60 Hz).
In a ring circuit, on the other hand, it is possible that the live and neutral currents are not equal on each side of the ring. Mains-frequency currents follow the path of least resistance, and it is possible, especially with ageing oxidised contacts, that from a socket, the lowest-resistance live connection is along the left-hand side of the ring, and the lowest-resistance neutral connection is along the right-hand side. As a result, current is flowing around the ring and will therefore induce a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. In the extreme case of a defect ring circuit, the live connection could become completely interrupted on one side of the ring and the neutral connection on the other, and then the full current would supply the magnetic field. This can lead to substantial electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
, such as mains hum
Mains hum
Mains hum, electric hum, or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity, which is usually 50 Hz or 60 Hz, depending on the local power line frequency...
in audio devices, accidental triggering of alarm and protection devices (burglar alarms, RCDs, etc.), malfunctions of consumer electronics and medical devices, ground loop
Ground loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
s, etc.
Overcurrent protection
Ring circuits may not always be adqeuately protected against overcurrentOvercurrent
In electricity supply, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible causes for overcurrent include short circuits, excessive load,...
s, particularly, as is often the case, if there is an undetected fault, AND the circuit conductors are not sized to match the Overcurrent Protective Device (OPD) as a radial run as opposed to a ring. The purpose of ring circuits is to supply a large number of sockets; therefore, they are protected only with high-rated overcurrent circuit breakers (typically 32 A). In comparison, the radial circuits used in other countries typically supply only a small number of sockets and are therefore protected with lower-rated circuit breakers (typically 10–20 A). As a result, countries using ring circuits find it necessary to add additional lower-rated fuses into the plugs of each appliance. This does create a possible improvement in safety in that an appliance with blown plug fuse will not be live when plugged in again (unless the fuse is first replaced), whereas with fuseless plugs a faulty appliance remains potentially dangerous to plug in, though in most cases it would trip a lower-rated circuit breaker if plugged in again.
This incompatibility in the overcurrent protection of appliance leads between countries using ring and radial circuits has been a major stumbling block on the road to worldwide standardisation of domestic AC power plugs and sockets
Domestic AC power plugs and sockets
AC power plugs and sockets are devices for removably connecting electrically operated devices to the power supply. Electrical plugs and sockets differ by country in rating, shape, size and type of connectors...
. Although plug-fuses can, in principle, be better matched to the maximum current required by an appliance, in practice, some plugs in the UK are necessarily fitted with a fuse of the maximum permitted rating of 13 A, because a lower-rated device may well operate intermittently due to "surges" (e.g. fit a 3A BS1362 fuse in the plug-top of a fridge, and it will often blow). This is not a problem since all appliances are required to be safe with a 13 A fuse (and in any case, in other EU countries, the appliance concerned is often protected by a 16 A or 20 A OPD for the circuit concerned), but it does mean the potential safety advantage is only partially realised and that the fused plug offers little advantage over an unfused plug used on radial circuit with a 13 A or lower fuse, or B16 or lower circuit breaker. The introduction of regulations in the UK - the Plugs and Sockets (Safety) Regulations - requiring new appliances to be sold with correctly fused pre-fitted plugs improves this situation further.
One theoretical advantage of individually-fused plugs is that a faulty appliance or flexible cord has a high likelihood of blowing only its plug-top fuse, leaving other appliances on the same ring circuit operating. However, with the introduction of EN60898 mcb's and the increased use of RCD protection for general purpose socket outlets in the UK (under BS7671: 2008 and earlier editions of the same standard) means that it is now more likely that the circuit protective device will operate before the plug-top fuse.