British telephone socket
[1][2][3] Electrical characteristics of the telephone interface are specified by individual network operators,[4] e.g. in British Telecom's SIN 351.
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.
From the early years of the 1900s, the GPO (subsequently British Telecom) had a plug and socket system available for rent.
[8] 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.
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.
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 for 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.
this socket contained an enclosed spark gap, SP1, that could safely flash over internally to provide high voltage surge protection.
The lower part of the front plate can be removed after unscrewing two screws, allowing users access to the terminals required for connecting internal extension sockets.
Companies using these plugs include Vernier, TI and Casio, for interfaces connecting to their graphical calculators, and in the Netherlands CMA.
The pins of the 631A plug are numbered in ascending order from left to right with the contacts facing upwards and the latch on the right-hand side.
It is also used in Bahrain, Bangladesh, Belize, Botswana, Brunei, Cyprus, Eritrea, Eswatini, the Falkland Islands, Ghana, Gibraltar, Israel (but wired differently from the British Standard), Jordan, Kenya, Kuwait, Lesotho, Malawi, Malta, Myanmar (Burma), Nigeria, Oman, Qatar, Saudi Arabia, Tanzania, the United Arab Emirates, Zambia, and Zimbabwe.
While BS 6312 was not adopted in the Republic of Ireland, Malaysia or Singapore, Irish, Malaysian and Singaporean phone jacks bear some varying levels of similarity to their UK cousins due to GPO influences in the old Irish, Malaysian and Singaporean phone wiring systems.
Irish, Malaysian and Singaporean phone sockets are normally wired, as per the international standard, with the line carried on the centre pair (pin 3 and 4).
However, although rarely connected in practice, Irish phone jacks also contain a ringing capacitor circuit very similar to their UK counterparts.
This arrangement was introduced for the same reason as the capacitor in BS 6313; to allow backwards compatibility with older GPO style type 3 wire phones that lacked an anti-tinkle circuit, which were common in the 1970s and into the 1980s.
Having said this, most modern telephones no longer require a ringing capacitor may have 2-wire connections, which means that extension wiring can usually be run with only pins 2 and 5.
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.
[13] 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 most common type uses Category 5 cables (four twisted pairs with 100 ohm impedance) between 8P8C (colloquially and incorrectly called RJ45) room sockets and a central patch panel.
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 connect the bell signal.
