British telephone sockets

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BS6312 431A plug; colloquially, a British Telecom plug.

British telephone sockets were introduced in their current plug and socket form on 19 November 1981 by British Telecom to allow subscribers to connect their own telephones. The connectors are specified in British Standard BS 6312.[1][2][3] Electrical characteristics of the telephone interface are specified by individual network operators,[4] e.g. in British Telecom's SIN 351.[5] Electrical characteristics required of British telephones used to be specified in BS 6305.[6]

They are similar to modular connectors (as used in RJ11), but feature a side-mounted hook, rather than a bottom-mounted one, and are physically incompatible.

History[edit]

GPO Plan 4 socket and plug.

Standard sockets were introduced, as part of the 'New Plan' wiring policy, to allow customers to easily purchase their own telephones, as required by Oftel, the phone regulator. Thus any phone whose plug conformed to BS 6312 and met certain other regulatory standards, such as BABT, could be connected to the network, rather than British Telecom controlling the market. The 'New Plan' was only new to the UK and was based extensively on systems which had been available elsewhere for many years, especially in the US.

The new system replaced the older hard-wired system, which came in many 'flavours', e.g. Plans 1, 1A, 1B, 1C, 2, 2A, 105, 107 etc., which could be very complicated and required the attendance at the premises of a GPO telephone-engineer, who needed a complete set of 'N' (wiring) Diagrams, which was very extensive and ran to over 15 volumes of little, black, ring binders. N diagrams also had their own numbering system e.g. a Plan 1A had an N diagram of N4502, and were frequently updated.

From the early years of the 1900s, the GPO (subsequently British Telecom) did have a plug and socket system available for rent (See Plate 3 in the 1909 edition of the Post Office Telegraphs 'Connections of Telephonic Apparatus and Circuits' 1909 edition). It was later called a "Plan 4" (N762 - first edition), and employed a heavy-duty, four-way jack plug 404, (circular in cross-section), on the end of the standard, plaited, cotton covered instrument cord. It also had to have a separate Bell-Set which was permanently in-circuit to provide ringing if there were no telephones plugged in. This system survived through various models of telephones from the 'candlestick', 200 and 300 type bakelite phones until the introduction of the 700 series in 1959 when a smaller 'Plug 420' was introduced. The separate bell-set, with its on-board capacitor and coils, also provided a testing circuit for remote engineers, by providing the mandatory 1000ohm, capacitive loop-back. Rental had to be paid on each telephone and on all the sockets, and hence the system was not that common.

Sockets[edit]

Master socket and NTE5 Line Box[edit]

A domestic single British telephone line installation will have a single master socket or line box in the premises, which is provided by BT or another service provider: this socket is the demarcation point between the customer-owned and maintained on-premises wiring, and the telephone network. For installations using the NTE5 line box (NTE = network termination equipment), the demarcation point is actually within the socket: the lower half of the front plate and associated wiring is the customer's, while the permanent wiring on the non-removable section behind this, remains the responsibility of the service provider. Customers are not permitted to access the wiring in a master socket without a removable lower section. Plug-in extension kits are available for customers with this type of installation. The two wires from the exchange are denoted the B leg at -48 V relative to ground when the line is not in use and the A leg which is near ground potential when the line is not in use. The A leg goes to pin 5 and the B leg to pin 2 in the master socket. (Although all equipment will work with a reversed line, so a reverse wired socket is not strictly a fault.) When current is flowing on the line, the B leg voltage collapses to nearer ground and the A leg voltage moves nearer to the B leg voltage. The exact voltage drop is a function of the distance to the exchange, and the network wiring type.

Until recently, this socket contained an enclosed spark gap that could safely flash over internally to provide high voltage surge protection (SP1) to limit high voltage spikes etc. This component is no longer used due to negative effects on VDSL speeds. The socket includes a 1.8 µF capacitor (Bell Circuit) to feed the AC ringing and a 470 kΩ resistor (R1 Out of Service Resistor) to permit remote testing when no telephones are plugged into any sockets. Additional internal extension (secondary) sockets are wired off the master socket (connected in parallel using the IDC system) and do not contain the surge protector, bell circuit capacitor and the out-of-service resistor.

Old style: Fixed plate[edit]

A fixed BT Master Socket front LJ 2/1A that does not allow disconnection of internal wiring with previous 1980-1991 BT "T" logo - surface mounted

The 'old style' fixed master socket (see image at right) had only one set of terminals on the back and customers were supposed to use extension kits plugged into the front socket, however many customers hard-wired their own extensions anyway for neatness and robustness reasons which was a poor arrangement since it provided no way to isolate the customer's internal extension wiring from BT's wiring.

New style: Removable plate (NTE5)[edit]

BT NTE 5 Linebox Socket allows easy disconnection of the internal wiring shown with cable attached (internal test jack visible) - surface mounted. Note the previous 1980-1991 BT "T" logo, used from the NTE5's introduction to 1991

In recent years NTE5 sockets (see image at left) have been fitted in place of master sockets. These have a front plate where the lower half is removable so allowing customer's access to the terminals required for connecting internal extension sockets; it also provides access to a test jack, to determine if line faults are due to the customer's wiring or BT's. The removable panel also allows the external telephone line to be easily disconnected from the internal wiring, provided the wiring of the premises has been correctly carried out. The terminals on the back part are large screw terminals allowing direct connection to many types of external phone cable; cable companies use a similar socket with insulation-displacement connectors (IDCs) instead of screw terminals on the back piece, known as a CTE5.

Now that BT does not have a monopoly of internal wiring, they make a substantial charge if a fault reported to them turns out to be in the customer's internal/domestic wiring. It is therefore important for the customer to have the facility to check whether any problem or fault is in their internal wiring/equipment or externally in BT's cabling or systems. Since the NTE5 socket represents the official demarcation point between the internal/domestic wiring (at the removable front of the socket which is the customer's responsibility) and the external telephone line/cabling fixed at the rear (which is BT's responsibility) the physical disconnection of the two wirings (made possible by the NTE5's removable front plate) is crucial in identifying faults and allocating responsibility for their rectification.

Plugs[edit]

British telephone plug with only two pins present (2 and 5), from a modem cable

There are two types of modern British Telecom plugs – 431A and 631A.

431A is 4-way and 631A 6-way. They fit a standard "Type 600" telephone socket. There are also plugs with only two contacts commonly seen on modem leads. These are a recent introduction and do not seem to be easily available as separate parts.

The Sinclair QL and 128k ZX Spectrum home computers used type 630W connectors for their serial ports (on the Spectrum 128 and +2, MIDI signals could also be sent from the port). These closely resemble standard 631A connectors but the keying slots on the back of the plug have a different size and position.

The 430A and 630A plugs had the latch on the opposite side of the plug, and were used as headset plugs on some switchboards and as handset connectors on some telephones, e.g. Ambassador.

The 631A and 630A plugs are also used for connecting sensors to interfaces for computer based measurements in educational environments, the former for connecting analogue sensors and the latter for digital sensors. Companies using these plugs include Vernier, TI and Casio, for interfaces connecting to their graphical calculators, and in the Netherlands CMA.

For some reason the pins on the BT431A plug are numbered in the opposite direction to those on the corresponding BT431A socket. Thus pin 1 on the socket mates with pin 6 on the plug, pin 2 (socket) with pin 5 (plug) and so on. Although the pins are not actually marked on any plug likely to be encountered, on the illustrations to the right, pin 1 (actually missing) is the bottom most pin reading upwards for 2, 3 etc. Although the generally active pins remain 2 and 5 in both cases, the ringer line is pin 3 on the socket but pin 4 on the plug.

Connector on phone[edit]

The connector on the phone is not standardized: the connector at the wall is standardized by regulation, to allow individuals to use their own phones (interconnection), but the wire from the phone to the wall may be hard-wired to the phone, or use various connectors.

Typically it will have a 6P4C or 6P2C modular connector at the telephone end: this latter may be wired as per the RJ11 standard (with pins 3 and 4), or it may be wired with pins 2 and 5, as a straight through cable from the BT plug (which uses pins 2 and 5 for the line, unlike RJ11, which uses pins 3 and 4). Thus cables are not in general compatible between different phones, as the phone base may have a socket with pins 2 and 5 (requiring a straight through cable), or have an RJ11 socket (requiring a crossover cable).

Use in other countries[edit]

The BS 6312 jack has been used in New Zealand since the 1980s, replacing a number of other connectors and hard-wired connections, and was subsequently replaced by a "2-wire" version suited to daisy chain wiring that eliminated the 3rd ringing voltage wire. The "BT Jack" is still the most common phone jack in use, although many installations in business use structured cabling with "RJ45" 8p8c modular connectors for telephone as well as data services. Since 2010 the TCF Premises Wiring Code of Practice[7] has deprecated BT jacks in favour of "RJ45" modular jacks for all new residential and SOHO phone/data networks, although not yet a mandatory standard in 2011.

It is also used in Bahrain, Bangladesh, Belize, Botswana, Brunei, Cyprus, Eritrea, the Falkland Islands, Ghana, Gibraltar, Israel, Jordan, Kenya, Kuwait, Lesotho, Malawi, Malta, Myanmar (Burma), Nigeria, Oman, Qatar, Saudi Arabia, Swaziland, Tanzania, the United Arab Emirates, Zambia, and Zimbabwe. The jack is still found in Hong Kong, where new installations ceased in 1998, while in Saint Vincent and the Grenadines, new installations ceased in 2001, with RJ11 now used instead.

Making the connections[edit]

As previously mentioned the actual connections are made using Insulation-displacement connectors (IDC). A punch down tool is required to do this and two sorts are available. One is of plastic construction and only intended for occasional use. The other is a tool manufactured by Krone and is of more robust construction : an example is shown in the photograph. It also comes with a tool for removing wires from sockets. The outer sheath of the cable is removed but the insulation from each wire is left and just placed in the connector. The Krone tool both inserts the wire into the connector and cuts off the excess wire in one action. The action of pushing the wire into the connector cuts into the insulation and makes contact with the wire. A maximum of three wires can be attached to each connector but it is best to stick to two if possible as the third is not usually a good connection. The Line Jack Sockets should have their cables secured with a cable tie through the securing lug and also enough cable needs to be left to enable it to be reterminated.


LJ 2/1A - 4 wire cable connected to a master socket - surface mounted
LJ 2/1A - 6 wire cable connected to a master socket - surface mounted


Cabling arrangements[edit]

Shown below are the cabling arrangements for both 4-wire and 6-wire cable. Initially 4-wire was used and many older installations still use it, then the 6-wire became the new standard, but the 4-wire has latterly been reissued to all Openreach engineers as part of cost savings. Modern 4-wire however is the same diameter as 6-wire to allow engineers to retain existing tacking guns and cable clips. Note that the wires in the 6-wire cable are coloured with two colours in a ratio of four to one in length, with the first colour mentioned being the predominant one, e.g. if the indicated colour of the wire is W-B then the wire will be coloured white for 12 mm, then blue for 3 mm and so on. (In other words it looks like a white wire with blue patches on it.)

Strictly speaking, a textbook installation will only actually use pins 2, 5 (for the voice) and 3 (for the ringer). Having said this, most modern telephones contain their own ringing capacitor, to cater for the Irish telephone system which does not have a capacitor built into the socket, and 2-wire extensions, which means that extension wiring can usually be run with only pins 2 and 5. Often where multi-core cable is used, the remaining cables are used for wiring extensions on additional incoming telephone lines.

Broadband[edit]

DSL Filter
Replacement face plate for an NTE5 containing a built-in ADSL filter

In order to use Broadband internet services simultaneously with voice telephony, it is necessary to use a DSL filter. This is basically two filters, which divide the single line into two discrete channels. Enough bandwidth is retained for voice telephony and the majority is used for high speed data. All phones must be connected via a filter (either a separate filter for each phone or one filter covering multiple phones) to avoid interference between the phones and the DSL signal. Where the data transmission is still unduly audible, using two DSL filters, daisychained in tandem, will eliminate the problem. The DSL modem connects directly to the phone line (most DSL filters have a socket marked DSL that just connects directly through to the incoming phone line). The ringer wire is unnecessary in unfiltered parts of the wiring and its removal can often improve performance and reliability of the broadband service.

When ADSL was first introduced in the UK it was installed by an engineer who replaced the front part of the NTE5 (if the property still had an old style master socket it would be replaced with an NTE5) with one containing a filter. Any hardwired phone extensions were disconnected from the original front part and connected to filtered terminals on the back of the filter. The DSL modem (which at the time was also BT supplied) and, if present, a phone or plug-in extension, could then be plugged into the front. If it was desired to locate the DSL modem away from the master socket a plug-in ADSL extension kit could be purchased.

BT also offered "wires only" ADSL service and promoted the technique of using a separate plug-in filter on every socket.[8] While both technically inferior and far less tidy than the solution BT engineers had used, it was usually adequate and was simple enough for a non technical householder to understand. The more discerning customer can purchase a variety of hardwired filtering products, including replacement front plates for the NTE5, some of which have unfiltered as well as filtered terminals on the back to avoid the need to plug in the extension wiring that leads to the DSL modem.

NTE5 showing it being fitted with a BT IPlate. The IPlate is the part in the middle of the sandwich

In 2008 BT trialled and launched their 'IPlate'; the "I" is for interstitial, as it is fitted between the socket and the front panel. This plate is fitted by the consumer inside the NTE 5 and reduces interference carried by the 3rd (bell) wire. The reduced interference allows faster broadband speeds - BT claim a speed improvement of up to 1.5 Mbit/s with a theoretical 4 Mbit/s. By November 2009 BT were calling the I-plate a "BT Broadband Accelerator".

Structured cabling[edit]

Structured cabling systems are general-purpose communications wires installed in offices and increasingly also homes, which can be used for several different communication technologies (analog phone, ADSL, ISDN, Ethernet, video, etc.). The most common type uses Category 5 cables (four twisted pairs with 100 ohm impedance) between 8P8C (sometimes incorrectly called RJ45) room sockets and a central patch panel.

The A and B wires of an analogue phone line appear in a structured cabling system usually on the centre pins of the 8P8C connector (pins 4 and 5; the blue/white TIA/EIA-568 pair 1 in Cat5 cables). In most other countries, those two wires are all that is needed to connect an analogue telephone. In the UK, however, many telephones expect the 25 Hz ring signal on a third wire. But such a 3-wire interface is not the symmetric interface needed for balanced twisted-pair transmission lines, and therefore prone to electromagnetic interference and crosstalk with nearby other wiring. Therefore, the capacitor that separates the bell signal from the A/B wires should be located close to the telephone. Any bell wire provided by the master socket is therefore not connected to the structured cabling, and the telephone is plugged at the other end into the structured cabling via a line adapter unit (LAU) / Mod-Tap / Molex that contains the capacitor needed to create the separate bell signal. Different types of LAUs are on the market:

  • PSTN secondary/slave – for phones that need no bell wire
  • PABX master – contains ring capacitor, to provide from the 8P8C 2-wire interface the 3rd "bell wire" required by UK phones
  • PSTN master – implements all the circuitry found in a BT master socket, including surge protection, ring capacitor, test resistor

There exists no well established standard in which polarity pins 2 and 5 of the phone socket are connected to pins 4 and 5 of the 8P8C connector; both alternatives are common.[9] This polarity is not important; e.g. BT also no longer specifies in which polarity the A and B wires are connected to pins 2 and 5 of their master telephone socket,[5] although earlier standards specified that the A wire be connected to 5 and the B wire to 2.

Socket and adapter gallery[edit]

See also[edit]

External links[edit]

Notes[edit]

  1. ^ BS 6312-1:1994. Connectors for analogue telecommunication interfaces. Specification for plugs.
  2. ^ BS 6312-2.1:1994. Connectors for analogue telecommunication interfaces. Sockets for use with plugs specified in BS 6312:Part 1. Specification for sockets. General requirements.
  3. ^ BS 6312-2.2:1997. Connectors for analogue telecommunication interfaces. Sockets for use with plugs specified in BS 6312-1. Particular requirements for fixed socket-outlets used in permanent wiring installations.
  4. ^ Publication of Customer Interface Specifications, Ofcom.
  5. ^ a b BT Public Switched Telephone Network (PSTN): technical characteristics of the single analogue line interface, British Telecom, Suppliers Information Note SIN 351, Issue 4.5, May 2006.
  6. ^ BS 6305:1992. Specification for general requirements for apparatus for connection to public switched telephone networks run by certain public telecommunication operators. (obsolescent)
  7. ^ "Premises Wiring Code of Practice". Telecommunications Carriers Forum. Retrieved 5 June 2011. 
  8. ^ Initial provisions were usually NTE5, with a new faceplate, but previous Home Highway (ISDN-like) installations received microfilters.
  9. ^ Colour chart for telephone wiring over CAT5 cables (UK / US)