The different types of BT Telephone Exchange Equipment used over the decades
Manual Telephone Exchanges
Manual telephone exchanges varied in size from a small switchboard operated by a single person, often found in a post office to a large manual telephone exchange operated by dozens of operators, often referred to as the hello girls.
- CB1Central Battery No. 1 switchboard
- CB9Central Battery No. 9 switchboard
- CB10Central Battery No. 10 switchboard
- CB12Central Battery No. 12 switchboard
- CBSmCentral Battery Signalling, Multiple type
- CBS1Central Battery Signalling No. 1, non-multiple, lamp signalling.
- CBS1iCentral Battery Signalling No. 1, non-multiple, indicator signalling.
- CBS2Central Battery Signalling No. 2, non-multiple.
- CBS3Central Battery Signalling No. 3.
- CSCountry Satellite exchange
Strowger Automatic Telephone Exchanges
Strowger technology was original invented by a funeral director in the United States who was concerned that the local manual exchange operators were always put calls through to his competitor (the operator was the wife of the competitor). This technology uses mechanical units to progress the call through the exchange.
- IAXIsland Automatic Exchange
- RAX5Rural Automatic Exchange No 5, similar to a UAX5.
- SAXSmall Automatic Exchange. This type of exchange was used after the UAX design. It basically consisted of standard Strowger racks but height reduced 8' 6" rather than 10' 6". They would typically have 3-digit numbering and consist of uni-selector rack, 50 point linefinder rack, group selector rack, final selector rack, meter rack, etc.
- TXSThese were larger scale exchanges, typically used in towns and cities.
- UAX5These were small scale exchanges used in rural areas for up to 90 subscribers. Similar to a RAX5.
- UAX6The UAX6 was a large version of the UAX5 exchange.
- UAX7The UAX7 was the first large capacity UAX, designed for up to 800 subscriber lines.
- UAX8The UAX8 was introduced around 1937 and provided a capacity from 50 to 200 lines.
- UAX12A small rural exchange which was the first GPO exchange to be standardised.
- UAX13These were originally designed for 200 subscribers, extended to 400. Larger exchanges serving over a 1000 subscribers, typically used in rural locations. It was a replacement for the UAX6.
- UAX5, UAX12 and UAX13 Exchanges (additional information)They had a mixture of equipment on each rack. This made it easy to increase capacity by just adding a rack or two at a time. They were initially hermetically enclosed units in unheated buildings. Later they changed to open units and heated the building.
- UAX14The UAX14 was the largest type of UAX and superseded the UAX7. It had a capacity of 200 - 800 subscriber lines. They a bit of a mixture of standard height reduced Strowger racks and mixed equipment UAX14 specific racks. The A rack has linefinders & final selectors. They were 4 digit numbers with digit absorption of the first digit by the group selector. Number range 2xxx & 3xxx.
Crossbar Telephone Exchanges
The next type of exchange was the Crossbar exchange, this was the first common control type of exchange. These units were commonly installed in large town or city exchanges where additional capacity is required. The floor size footprint is around 80% of that of Strowger. This technology was commonly installed in the 1960s to 1970s. The first Crossbar public exchange opened on 23 November 1964 in the village of Broughton, just north of Preston, replacing a CBS2 manual exchange.
- TXK 1This type of Crossbar exchange was typically installed in Non-Director areas.
- TXK 3This type of Crossbar exchange was typically installed in Director areas and other large capacity areas, including London, Birmingham, Manchester, Liverpool, Glasgow, Edinburgh, Newcastle and Belfast.
Electronic Telephone Exchanges
During the 1960s a number of rural exchanges were reaching capacity and a solution was required that was not so mechanical. There was a new electronic component called a reed relay and the TXE exchange was designed around that technology.
- TXE2 Type 1This was the first type of electronic exchange and there was only ever three of these. The first was Ambergate in the Ripley charge group, the second was at Shirebrook in the Mansfield charge group and the third was in the East Anglia area. The Ambergate unit was installed in 1966 and lasted until 1989. The Shirebrook did not last too long. The maximum number of subscribers was 4000. Also, these exchanges could not be enhanced to be part of a linked numbering scheme. These were typically painted battle ship grey.
- TXE2 Type 2The type two TXE2 was the main production version which was produced by the Plessey company. There was a number of enhancements made with these and the newer models were capable of working as part of a linked number scheme. Again, capacity was up to 4000 subscribers. TXE2 exchanges normally required a new building as the rural UAX type of building was too small for the replacement to be installed whilst the Strowger was still in service. These were typically painted light straw cream.
- TXE2 Type 3Next was the type three. The biggest difference was that potentially 7000 subscribers could be connected to a unit. Also, there were minor differences between the Plessey, STC and GEC types. They could all work within a linked numbering scheme.
- TXE4RDThe next type of common control read relay exchange was the TXE4RD. The code RD refers to Rectory Design as the first exchange was at Rectory exchange in Birmingham. These where capable of up to 40,000 subscribers and were used in town and city exchanges. Often these would be installed as replacements to a Strowger exchange in extensions to original Strowger building. Occasionally a new site had to be found for the TXE4. Like the TXE2 exchanges the programming of these exchanges was by threading a thin wire through a series of ferrite cores.
- TXE4AA further enhancement to the TXE4 was the use of shift registers programmed by a keyboard and screen, backed up on a reel of computer tape. There was also a space saving as well with racks having 720 subscribers instead on 480 on the RD version. Each type had a “set” of 5760 subscribers, so the RD needs 12 racks and the A needs 8 racks. • The 4A were often used to extend large rural exchanges to take capacity up to 10,000 where the Strowger was considerably less. In the case of Burton on Trent and Willowcroft in Derby, the TXE4A were built in a new building adjacent to the old exchange. There was a link corridor which carried all the cables to the MDF retained in the old building. When these exchanges were replaced by a digital unit these building could be physically disconnected and sold off.
- TXE4IWhere additional capacity was required, it was possible to extend a TXE4RD with a TXE4A. these hybrids are called TXE4I exchanges. Such examples existed at Corby and Warwick exchanges..
- TXE4 EnhancementTo complete the analogue story, STC designed an enhancement which provided C7 signalling allowing level 0 traffic to be digitally transmitted to the digital core exchange. The enhancement also replaced the conventional metering with a computer-based billing system. From this itemised billing was available for subscribers.
- TXE4 Gate ReplacementOne further enhancement to the RD exchanges was the replacement of the programming gates with a computerised version.
Digital Telephone Exchanges
The next generation of exchanges are the digital exchanges coded as TXD. Occasionally a new building had to be built for the digital exchange as there was no spare capacity in the old building.
- System XBT worked with the Plessey company to design and build a System X exchange. This was BT’s preferred replacement to all the analogue exchanges. Gradual replacement started in the late 1980s but was ramped up with the cable company competition.
- AXE10 (System Y)Plessey were unable to deliver at the speed BT required so BT looked at the Ericsson AXE10 off the shelf solution. Although the software was a little behind that of the System X it had a couple of advantages. It was slightly smaller in size, and the racks where common (i.e., did not need to be specified as part of the build) and they were single sided whereas the System X is dual sided. They were based on a modular system with magazines being installed (with four screws) into the empty shelves. The cables were connected to the front of the magazines (cards). This was unlike System X where the cables were connected to a wire wrapped back plane. AXE10 was often selected where there was little spare space in the exchange building.
- UXD5 (Unit Exchange Digital No. 5)The UXD5 is a small digital switching telephone exchange for rural communities. It is based on the Monarch 120 PABX. It replaced mechanical exchanges such as the UAX12. A UXD5A could serve up to 150 lines, the larger UXD5B version could serve up to 600 lines.