British telephone sockets
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On November 19th 1981, British Telecom introduced the current plug and socket system to allow subscribers to connect their own telephones. The new system replaced the older hard-wired system, which was very complicated, required an engineer to attend the premises and also required a complete set of N Diagrams, which were frequently changed. The real reason for the change though 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.
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[edit] Sockets
A domestic single British telephone line installation will have one master socket in the premises, which is provided by BT or another service provider. The 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 also together with a 470 kΩ resistor (R1 Out of Service Resistor) to provide remote testing when no telephones are plugged into to any sockets. Additional secondary sockets are wired off the master socket and do not contain any components. The wires are connected to the sockets in parallel using the IDC system.
The old style master socket had only one set of terminals on the back and customers were supposed to use plug in extension kits, however many customers hardwired their own extensions anyway for neatness and robustness reasons. This was a poor arrangement as it provided no way to isolate the customers internal extension wiring.
In recent years NTE5 sockets have been fitted in place of master sockets, these have a removable (lower half) panel, which houses the terminals to connect wiring to the secondary sockets. The removable panel 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 domestic wiring. It is therefore important for the customer to have the ability to check whether the perceived fault is in the domestic wiring/equipment or beyond the premises in BT's cabling or equipment. This is where the disconnection made possible by the removal plate becomes useful.
[edit] Plugs
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.
[edit] 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 (incorrectly called an 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, India, Israel Jordan, Kenya, Kuwait, Lesotho, Malawi, Malta, Myanmar (Burma), Nigeria, Oman, Qatar, St. Vincent, Saudi Arabia, Sri Lanka, Swaziland, Tanzania, the United Arab Emirates, Zambia, and Zimbabwe. The jack is still found in Hong Kong, but since 1997, it has been phased out in favour of RJ11.
[edit] 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 |
[edit] 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.
[edit] Broadband
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.
[edit] 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 |
[edit] See also
[edit] External links
- Wiring diagram
- UK plug and socket telephones
- The RJ System of telephone plugs and sockets is gradually replacing the BT System in Hong Kong, Office of the Telecommunications Authority (OFTA), Hong Kong Special Administrative Region Government
- BS 6312 Specification BSI's official nomenclature and description
