British telephone sockets

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British telephone sockets were introduced in their current plug and socket form on November 19th 1981 by British Telecom to allow subscribers to connect their own telephones. The new system replaced the older hard-wired system which came in many 'flavours', e.g. Plan 1A, 1B,1C,2,2A,105,107 etc, which could be very complicated and required 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 which was frequently updated. N diagrams also had their own numbering system e.g. a Plan 1A had an N diagram of N4502. From at least 1920, the GPO (subsequently British Telecom) did have a plug and socket system available for rent. It was called a Plan 4 system (N4306) and employed a much heavier-duty plug (circular in cross-section) than the present-day ones. It also had to have a separate Bell-Set No. 26 which was permanently in-circuit to provide ringing if there were no telephones plugged in; in these circumstances, the bell-set also provided a testing circuit for remote engineers. Rental had to be paid on the phones and on all the sockets, so the system was hardly a fore-runner of the present system. The real reason for the introduction of the new sockets in 1981 was the requirement from government/Oftel that customers should be able to buy their phones anywhere (provided they met certain standards, such as BABT approval) and fit them themselves on a simple plug and socket basis.

BS6312 is the British Standard governing telephone plugs and sockets.

1 Sockets
2 Plugs
3 Use in other countries
4 Making the Connections
5 Cabling Arrangements
6 Broadband
7 Socket and Adapter Gallery
8 See also
9 External links
10 Notes

Sockets

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

BT NTE 5 Linebox Socket which allows easy disconnection of the internal wiring shown with cable attached - surface mounted

A domestic single British telephone line installation will have a single master socket in the premises, which is provided by BT or another service provider. This master socket contains a high voltage surge protection (SP1) to suppress high voltage spikes etc, a 1.8 µF capacitor (Bell Circuit) to feed the AC ringing and a 470 kΩ resistor (R1 Out of Service Resistor) to provide remote testing when no telephones are plugged into to 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.

The 'old style' fixed master socket (see upper image 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.

In recent years NTE5 sockets (see lower image right) 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. 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 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

A British telephone plug.

British telephone plug with only two pins present, 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 Spectrums used type 630W connectors for their serial ports. These closely resemble standard 631A connectors but the keying slots on the back of the plug have a different size and position.

The 431B and 631B 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, eg Ambassador.

Use in other countries

The BS 6312 jack has been used in New Zealand since the 1980s, replacing a number of other connectors and hard-wired connections. The "BT Connector" is still the most common phone jack in use, although many installations in business use structured cabling with 8P8C ("RJ45") connectors for telephone as well as data services.

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 1997, while in Saint Vincent and the Grenadines, new installations ceased in 2001, with RJ11 now used instead.

Making the Connections

As previously mentioned the actual connections are made using the Insulation Displacement Connectors IDC method. A 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.

Krone IDC Tool, showing the wire removal tool open officially known as Inserter Wire No. 2a

Plastic IDC Tool, for occasional use only

4 wire cable the old sort

6 wire cable the current type

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

Shown below are the cabling arrangements for both 4-wire and 6-wire cable. The 6-wire is now standard but the 4-wire, if it can be obtained, does have the advantage of being thinner and it is therefore easier to get it into small spaces. 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 colour, e.g. if the colour of the wire is W-B then the wire will be coloured White for 12mm, then Blue for 3mm and so on. In other words it looks like a white wire with blue patches on it.

Four wire cabling

Six wire cabling

Phone Socket - The principle of Bells in Parallel and Opt Out of Service

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 badly wired extensions, which means you can usually run your extension wiring 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

DSL Filter

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

In order to provide Internet Broadband services over the telephone line it is necessary to use a DSL filter. 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. 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 had to be installed by a BT engineer who would replace 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.

Later BT started offering "wires only" ADSL service and promoted the technique of using a separate plug-in filter on every socket. 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.

In 2008 BT trialed and launched their 'IPlate'. 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 1.5Mb/s with a theoretical 4Mb/s.

Socket and Adapter Gallery

LJ 2/1A - BT Master socket showing components - surface mounted	LJ 2/3A - BT Secondary socket - surface mounted	LJ 1/1A - Compact Master socket for use where space is at a premium - surface mounted	LJ 4/1A - Two Exchange Line Master socket - flush fitting. Standard BS fitting
LJ 3/3A - Secondary - flush fitting. Standard BS fitting	LJ 5/1A - Double Master - flush fitting- note straps paralleling the second socket into a master as well. Standard BS fitting	LJ 5/3A - Double secondary - flush fitting. Standard BS fitting	LJ 2/1A - BT Master socket converted to a secondary by removing components
Master with ADSL Filter - surface mounted	Adapter to provide two sockets off a single socket	Adapter to provide three sockets off a single socket	Adapter to provide four sockets off a single socket
ADSL Filter Plug	Adapter to allow USA-type telephone plugs to be used