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, but
feature a side-mounted hook, rather than a bottom-mounted one, and are physically incompatible.
History
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' 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
did have a plug and socket system available for rent. It was later called a "Plan 4",
and employed a heavy-duty, four-way jack plug 404,, 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
Master socket and NTE5 Line Box 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, 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. 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 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 to feed the AC ringing and a 470 kΩ resistor to permit remote testing when no
telephones are plugged into any sockets. Additional internal extension sockets are wired off the
master socket and do not contain the surge protector, bell circuit capacitor and the
out-of-service resistor. Old style: Fixed plate
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. New style: Removable plate
In recent years NTE5 sockets 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 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 and the external telephone line/cabling fixed at the rear the physical disconnection
of the two wirings is crucial in identifying faults and allocating responsibility for their
rectification.
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 ZX Spectrum home computers 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 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 with pin 5 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 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 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, 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, or
it may be wired with pins 2 and 5, as a straight through cable from the BT plug. Thus cables
are not in general compatible between different phones, as the phone base may have a socket
with pins 2 and 5, or have an RJ11 socket. 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, 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 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, 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 As previously mentioned the actual connections
are made using Insulation-displacement connectors. 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.
Cabling arrangements 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.
Strictly speaking, a textbook installation will only actually use pins 2, 5 and 3. 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
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 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. 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 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 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. 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 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 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 Structured cabling systems are general-purpose
communications wires installed in offices and increasingly also homes, which can be
used for several different communication technologies. The most common type uses Category 5 cables
between 8P8C 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. 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 / 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.
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, although
earlier standards specified that the A wire be connected to 5 and the B wire to 2.
Socket and adapter gallery
See also RJ11, RJ14, RJ25
Technical standards in colonial Hong Kong GPO telephones
External links Wiring diagram
UK plug and socket telephones Using ADSL with extension telephone sockets
The RJ System of telephone plugs and sockets is gradually replacing the BT System in Hong
Kong, Office of the Telecommunications Authority, Hong Kong Special Administrative Region Government
Notes
"BT claims UK broadband boost breakthrough". The Register. 2008-10-01. Retrieved 2008-10-01.
This article is based on material taken from the Free On-line Dictionary of Computing prior
to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version
1.3 or later.