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British telephone socket
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British telephone socket
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. Electrical characteristics of the telephone interface are specified by individual network operators, e.g. in British Telecom's SIN 351. Electrical characteristics required of British telephones used to be specified in BS 6305.
They are similar to modular connectors (as used in RJ11), but have a side-mounted hook, rather than a bottom-mounted one, and are physically incompatible.
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,[better source needed] 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) had a plug and socket system available for rent. 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 1000 ohm capacitive loop-back. Rental had to be paid on each telephone and on all the sockets, and hence the system was not that common.
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. 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. According to SIN 352 the average DC current in the loop and voltage across the phone will be up to 42 mA at 12.5 V (short line), up to 33.5 mA at 10 V, and will be not less than 25 mA at 9 V. (long line limit) Line polarity reverses during calls if caller ID is in use. Once in a call the audio and tone levels superimposed on the DC voltage are expressed as dBm in 600 ohms, although the line impedances are permitted to be some way off 600 ohms (as per SIN351) -9 dBm (275 mV RMS) [0 dBm = 1 mW ( 0.775V RMS) in 600 ohms.
Until recently,[when?] this socket contained an enclosed spark gap, SP1, that could safely flash over internally to provide high voltage surge protection. 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) but not containing the surge protector, bell circuit capacitor, and the out-of-service resistor.
The 'old style' fixed master socket 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.
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British telephone socket
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. Electrical characteristics of the telephone interface are specified by individual network operators, e.g. in British Telecom's SIN 351. Electrical characteristics required of British telephones used to be specified in BS 6305.
They are similar to modular connectors (as used in RJ11), but have a side-mounted hook, rather than a bottom-mounted one, and are physically incompatible.
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,[better source needed] 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) had a plug and socket system available for rent. 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 1000 ohm capacitive loop-back. Rental had to be paid on each telephone and on all the sockets, and hence the system was not that common.
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. 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. According to SIN 352 the average DC current in the loop and voltage across the phone will be up to 42 mA at 12.5 V (short line), up to 33.5 mA at 10 V, and will be not less than 25 mA at 9 V. (long line limit) Line polarity reverses during calls if caller ID is in use. Once in a call the audio and tone levels superimposed on the DC voltage are expressed as dBm in 600 ohms, although the line impedances are permitted to be some way off 600 ohms (as per SIN351) -9 dBm (275 mV RMS) [0 dBm = 1 mW ( 0.775V RMS) in 600 ohms.
Until recently,[when?] this socket contained an enclosed spark gap, SP1, that could safely flash over internally to provide high voltage surge protection. 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) but not containing the surge protector, bell circuit capacitor, and the out-of-service resistor.
The 'old style' fixed master socket 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.
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