There is some confusion about the requirement for the bell wire in a UK telephone installation, and a number of myths have arisen - especially around DSL filtering. This page aims to explain why the bell wire exists, and why you might still need it.

History

When the 'new' BT Phone Socket was introduced in the early 1980s the telephone system had not yet been deregulated - British Telecom had a virtual monopoly, subscribers rented phones from them, and pulse dialling (aka Loop Disconnect) was the order of the day. The BT Phone Socket was a step forward, to allow subscribers to buy their own phones and simply plug them in. Before the BT Phone Socket, an engineer would have to install a new phone because the wiring was more complicated.

UK Telephone Wiring

A UK telephone service is (traditionally) delivered to the premises on a twisted pair of insulated copper wires - in the future this may change to fibre-optic cable. The two wires are referred to as 'A' and 'B' wires. In the past BT stated that the 'B' wire would always have a more negative voltage on it that the 'A' wire, but this seems to have now been dropped. The 'A' and 'B' wires carry voice signals (including tones such as dial tone and ringing tone), and ringing current to drive telephone bells. With pulse dialling, the wires also carry dial pulses - whereas tone dialling sends tones on the voice circuit instead.

When a telephone is 'on hook' (a term dating back to candlestick phones) - i.e. the handset is on the base, then there is no current drawn from the 'A' and 'B' wires, and the voice circuit is disconnected from the line. When a telephone is taken 'off hook' - i.e. the handset lifted off the base, current is drawn on the line, connecting the voice circuit, and the current drawn (looping the line) signals the exchange to issue dial tone.

With the telephone back 'on hook', incoming calls are signalled by ringing current being applied to the line by the exchange. This is a large AC voltage - about 80V. In order to separate the ringing current from the rest of the telephone, a bell capacitor is used to route the ringing current to the bell. The bell capacitor is connected at one end to the 'B' wire, and to the bell at the other, the other end of the bell connects to the 'A' wire. This is all on the line side of the hook switch, so is connected to the line when the telephone is 'on hook'.

With pulse dialling (loop disconnect), the telephone is taken 'off hook', looping the line, and the dialled numbers 0-9 are signalled by the appropriate number of pulses briefly disconnecting the loop - except that 10 pulses are sent for number 0. This causes the audible clicks in the earpiece - in fact, you can count the pulses. But there's an issue - the brief pulses of disconnection of the loop get seen by the bells of other telephones on the line, which can cause those bells to tinkle as another telephone dials.

Eliminating the tinkling

The tinkling of other bells is irritating, and so the wiring was modified to eliminate it. In the past, bell tinkle was taken by engineers to indicate that the subscriber had installed their own extension wiring - which used to be prohibited, the telephone wiring in the premises was all the property and responsibility of BT. It was only with the introduction of the NTE5 master socket in 1986 that it became permitted to modify your own home telephone wiring.

The modification to the wiring was to locate the bell capacitor in the master socket, and use an additional wire out to all other sockets to drive the bells of all telephones on the line. The bell capacitor in the master socket was connected at one end to the 'B' wire, and at the other end to the new 'bell wire'. Individual telephones no longer used a bell capacitor, and connected their local bell from the bell wire to the 'A' wire. By itself this doesn't eliminate the tinkling, until it is arranged that when a telephone goes 'off hook' it also makes a connection between the bell wire and the 'A' wire. This effectively short-circuits the bells of all telephones on the line, and prevents any bell from tinkling during pulse dialling.

Effects of eliminating the tinkling

Telephones designed for the UK will use 3 wires - 'A', 'B', and 'bell'. Their bells will be connected between the bell wire and the 'A' wire. If used in an installation without a bell capacitor or without a bell wire, such telephones won't ring their bells. A good example is a plug-in electronic ringer I have - the BT phone plug has only 2 pins - for the bell wire and the 'A' wire.

The US telephone system is different, and each telephone provides its own bell capacitor - but the US doesn't use pulse dialling any more. Telephones designed for that market will ring without a bell capacitor or without a bell wire.

The bell wire complicates DSL filtering. The best solution is a filtered master socket (e.g. NTE5c with VDSL SSFP). If using microfilters on each telephone, some microfilters include a bell capacitor, but some others don't - I've not seen any which properly support the bell wire.

BT plug/socket numbering

This is a complete list of pin usage. Pin 1 is closest to the latch.

Function	Pin number	Notes
1
2	'B' wire	Used to be guaranteed more negative voltage than 'A' wire
3	Bell wire	To distribute ringing current, and eliminate tinkling
4	Local earth	Needed for 'C' wire signalling, which is obsolete
5	'A' wire
6	'C' wire	'C' wire signalling is obsolete