Important disclaimer:

This section does not infringe any subject matter covered by the Official Secrets Act, nor does it indulge in politics or wild conspiracy theories.


General Qs & As

What are the special telephones we see in the bunker museums now open to the public?

At Anstruther (Scotland's Secret Bunker) there is a "carrier control point" (CCP) on display. This is a grey box containing two red telephone handsets, one black telephone handset and various other gubbins. There's a tiny image of one of these used as an icon at . The carrier control points represented the interface between the national attack warning network (WB1800, alias Handel) and the local warning networks (WB1400). Some 500 of these consoles were installed—two each at 250 (typically) district police stations (one in the peacetime control room, one in a protected basement). The idea is that if an air attack had been detected by intelligence satellite, Fylingdales radar etc., then one of the national warning points (e.g. RAF High Wycombe, the IUKADGE sites, Fylingdales, UKWMO at Oxford, Preston, possibly Lawford Heath, possibly BBC Evesham) would have used the Handel network to alert the police operators at the CCPs. The CCP operator would have heard a squeaking noise and a red light would have flashed next to the two red handsets. (For redundancy, the two handsets were fed by two different circuits known as the X and Y paths.) On picking up the red handset(s) he would have heard the national warning officer say the phrase "national attack warning red". On hearing this, the CCP operator would then sound the "attack warning red" locally. This took the form of (a) a speech broadcast to the speech "carrier warning receivers" located at key points in the locality, and (b) activation of powered air-raid sirens by means of a key on the CCP console. There is a complete working WB1400 equipment chain at Hack Green ( again part of the training kit that was donated very kindly by BT to the museum. We have fired it up at RSG meetings. Maybe we should do this again next year. As yet we have only connected it to a wimpy little fire-alarm type siren. We haven't yet summoned up the nerve/stupidity to connect it to one of the big kick-arse 10kW three-phase sirens. (140dBA is a bit OTT even for us.) [ (Richard Lamont)]


What do the expressions 'Backbone' and 'Ace High' mean?

See these links for Backbone and Ace High.


What is known about the so-called 'Citadel' exchanges in London?

Not a great deal! Probably the first to be built, and the only one to be described in print in any great detail, was the citadel exchange at Faraday building. The POEE Journal's Victory issue (January 1946) states:

It was decided ... at the end of 1940 to proceed with the construction, as quickly as possible, of a “fortress” type of structure on the vacant site adjoining Faraday Building, North Block, which would be proof against direct hits by medium size bombs, and in which the services could be maintained during the attacks.

The “Citadel”, as this structure subsequently came to be known, was an exceptionally heavily reinforced concrete building, without windows, and having walls between 6 ft. and 3 ft. thick and a roof 7 ft. 6 in. in thickness. As the Citadel was intended to function as a separate entity, in the literal sense of a fortress, such comprehensive facilities as dormitories, artesian well, fuel storage tanks, ventilating, air conditioning and gas filtration plants were provided, so that the services could operate without being impaired by the conditions prevailing outside. The continuity of power supply was also ensured by the provision of diesel engine-driven alternators.

The construction of the Citadel was commenced in May, 1941, and completed in the exceptionally short time of seven months for such an elaborate structure. The installation of the considerable amount of equipment and the completion of the line plant arrangements were accomplished just as expeditiously in nine months, and the exchange was finally brought into service in November, 1942.

Incidentally the Citadel was closed in the late 1980s.


Were there others?

Yes. The government constructed a number of heavily protected exchanges in London during the Cold War period of the 1950s and 1960s. FEDeral (old dialing code FED or 333 from the public network, later reached by a standard number on 222 exchange), serving key central government functions, was, as described above, originally a WWII exchange which (take your pick) had its own citadel just south of Horseguards Avenue or was located in the Whitehall tunnel system. One report states that the citadel was absorbed into the present MoD Main Building when it was built, also that the exchange known as Federal moved from there to the Marsham Street site (Rotundas) at some stage [this last item subject to confirmation].

According to one Admiralty file at the PRO, Trafalgar and Victoria (naval, not public) exchanges were housed in or under the Admiralty Citadel. One of these, probably Trafalgar, was previously located during World War II beneath Trafalgar Square, with exits to street level opposite Canada House and in Charing Cross Road near the Tate Gallery; there are photographs of the manual switchroom in the Imperial War Museum. Another report states that the exchange equipment was in tube-type tunnels, close to Trafalgar Square, running off the main Whitehall tunnel system and that photographs exist in the BT Archives. The two reports are not mutually exclusive.

In this way this exchange (Trafalgar) was connected to the deep level tunnels, as were telephone facilities located underground where the Queen Elizabeth Conference Centre now stands in Victoria Street (presumably the original Victoria admiralty exchange). Another deep-level exchange during WW2 was that of the US forces in the Goodge Street tunnel complex.


When did the Post Office first construct deep-level cable tunnels? And where does Federal fit in?

According to the IPOEEJ (January 1946):

This tunnel system... constructed for the Service Departments [i.e. the Whitehall tunnel, generally known as Q-Whitehall] is a comprehensive scheme of deep-level protection in the vulnerable central area for equipment and cables, carrying vital defence communications from the buildings of the Service Departments to other parts of the country. Associated with these specially constructed tunnels are the public and P.O. tube railways.

The ultimate scheme represents the accretion of five principal component schemes, which were proceeded with at various times during the war. The initial scheme, commenced in December, 1939, was a tunnel 12 ft. in diameter and at a depth of about 100 ft., which, intended at the time solely for cable protection, is connected by short lateral tunnels of 5 ft. diameter to the Service Departments and Federal exchange. The latter is a protected exchange in sub-ground accommodation and was provided at the outbreak of the war to give an uninterrupted service for the principal officers in Government Departments.

Access to the main tunnel for Post Office personnel is provided by an automatic lift and emergency staircase in a shaft at an exchange [in fact WHItehall exchange], which is connected to the main tunnel by an 8 ft. diameter lateral tunnel. The cables from the buildings of the Service Departments, after being taken through 12-in, steel bore tubes connected to the smaller lateral tunnels, are terminated on the M.D.F. in the main tunnel.

It was obvious that the main tunnel would afford absolute security for telephone and telegraph equipment, the first installation of which was accordingly proceeded with and completed in the summer of 1941 to meet the increasing requirements for defence communications. This equipment, which has been added to from time to time, and now provides for about 4,000 working circuits, includes among the many constituent items, 71 18-channel V.F. systems, 26 carrier systems, 13 coaxial cable terminals and 864 audio amplifiers.

During 1941-42 major extensions of the tunnel, which more than doubled its length, were carried out, affording underground access between various Service Departments and accommodating a teleprinter switching centre.

In all, a total of 1 mile 740 yards of tunnel has been constructed under the various schemes associated with the tunnel system and six shafts with passenger lifts provided. The tunnel system is connected via the tube railways to the Citadel building.

These specially constructed tunnels and the public and P.O. tube railways have been extensively used to give deep-level protection to cables carrying vital communications. A total of 72 miles of cable has been laid in P.O. tunnels, 116 miles in public tube railways and 20 miles in the P.O. railway.

It is notable that the Q-Whitehall tunnels, following some years of abandonment, are currently being refurbished. From Daily Telegraph, 30th April 2000:

LABYRINTH of tunnels beneath the streets of Whitehall and Westminster is being turned into high-security offices for the Government.

But although several MPs have asked for full details of the work, the Government has refused to supply them on security grounds. Builders working on the offices have had to sign the Official Secrets Act. A former minister said: "We have all heard about the tunnels. I was once told that John Major was taken into one of them when the IRA attacked Downing Street with mortars, but I don't know if that is true or not."

The multi-million pound renovation will transform old disused rooms whose existence is well known in Whitehall, although exact details about them are shrouded in secrecy, even from civil servants and ministers. One tunnel is said to run from the Cabinet Office basement, which connects to 10 Downing Street, to the Ministry of Defence. A renovation of the Cabinet Office basement to create overspill offices to cope with Tony Blair's expansion of No 10 is known to be under way.

Work is also in progress on a mile-long tunnel under Whitehall, which is about 40 feet underground. It is expected to take nine months to complete, according to an official close to the project. New tiling and fire doors are being installed along the walls of the tunnel, which is 7ft 6in wide and snakes below the heart of London. Small offices open on to the tunnel every 200 yards in which workmen are installing computers and video surveillance cameras. Workmen enter the tunnel through a British Telecom telephone exchange in Whitehall and climb down three flights of stairs. Maps and plans cannot be removed from the site and each worker must hand in their copy of the plans at the end of each day.

Staff have been warned that if they use the designated fire exits without a genuine emergency they could be sacked. After the Whitehall tunnel is completed it is hoped that it will be connected to others, including a warren of passages built under Parliament in the 17th century.

The Whitehall tunnel system was constructed by William Halcrow, a civil engineer, in the Thirties. He was also responsible for boring the Victoria and Jubilee tube tunnels. His Post Office tunnel under Broad Sanctuary and Great Smith Street was designed to carry cables between government departments and built to be strong enough to withstand the pounding of air raids.

By the Fifties, military specialists agreed that the tunnels were no longer an adequate defence system because of the threat of nuclear weapons and they were largely abandoned. Many of the underground passages in central London connect with another Post Office tunnel built in 1954 which runs as far as Paddington to the west and Moorgate in the east.


What other measures were taken to protect line plant starting in WW2?

 The IPOEEJ article already quoted states:

Although some measure of security was achieved by the provision of alternative routing, ‘the risk of extensive damage to the heavy concentration of trunk and junction cables in the central area was so serious. as to make it imperative to adopt an exceptional and notable safeguard by diverting a number of the cables to the public and Post Office tube railways, and a deep tunnel which was specially constructed during the war to accommodate Post Office plant.

The construction of this tunnel, which is 7 ft. in diameter, and at a depth varying between 70 ft. and 100 ft. below the surface proceeded from three working shafts, one of which was retained as the leading-in point for the cables. A lift of sufficient capacity to take full size cable drums was installed in this shaft. Owing to the need for conserving iron during the critical period of the war, the greater part of the tunnel was lined with reinforced concrete segments instead of the customary cast iron. Plant was installed for the ventilating, draining and lighting, of the tunnel. At one point an enlarged offset was constructed to accommodate loading pots. Among the arrangements for cabling was the provision of specially designed roller skates, which were attached to the cables to facilitate their movement to the appropriate section of the tunnel. A total of 60 cables with an aggregate mileage of 62 was installed in the tunnel, in which about 150 cables can ultimately be accommodated.

Protection for defence communications was in general more elaborate and certain in its function than that provided for public communications, owing to the fundamental importance of maintaining the continuity of their service. In London this objective was finally achieved by putting the plant deep underground, or in exceptionally strong reinforced concrete structures having walls several feet In thickness. The most notable and comprehensive scheme of this character was the tunnel system constructed for the Service Departments.

In London the tube railways were used extensively to give deep-level protection for a few miles for the cables radiating from the central equipment. Beyond the emergent points from the, tube railways, however, the cable routes were as vulnerable as any other underground plant at shallow depth, and with the persistent and widely dispersed bombing in 1940-41, the incidence of damage to these routes was sufficiently serious to require special measures to mitigate the effects of the interruptions.

The scheme adopted entailed the linking up, by circumferential cables, of the radial cable routes of the tube railways at a number of selected interception centres located not far from the emergent points. Included in the scheme were many of the surface trunk cables, which were intercepted at exchanges adjoining the main routes. At several places where the circumferential and radial routes intersected, and exchanges were not conveniently situated to intercept them, substantial pill-box structures, in which an interception frame was installed, were constructed. Interruption by bomb damage to any of the radial cable routes could thus be readily restored by suitable re-routing of the circuits over the circumferential cables at the interception centres.

The London scheme, started in the late autumn of 1940 and completed during the following year, involved the laying of 250 miles of loaded cable of various sizes. An interesting feature in the cabling work was the completion of the circumferential cable system across the Thames by using the 12 ft. diameter pilot tunnel at Dartford, which had been constructed before the war in preparation for building the main vehicular tunnel.

The inclusion of the surface trunk cables in the scheme enabled not only defence circuits to be rerouted, but also important trunk circuits which had been interrupted by bomb damage to the radial routes, a facility which exemplified the duality of purpose of the Post Office telecommunication network in the national prosecution of the war.


What is or was Autovon?

AUTOVON (standing for AUTOmatic VOice Network) was the switched voice communications system of the US military, now replaced by a digital system called DSN. The Autovon network covered all locations where US forces had a presence and there were a number of Autovon exchanges in Britain. This was an extremely high-security system, although through a loophole it was possible for a time to gain unauthorised access to Autovon from the normal public network (if you knew the dialling code!).

Autovon was notable for having four-wire (not two-wire) transmission (although a number of standard two-wire telephones were also connected). Telephones had four extra buttons on the right of the keypad, using the so-called A, B, C, D touch-tone (DTMF) frequencies to allow privileged users to set class of service for each call. These were "ROUTINE", "IMMEDIATE", "FLASH", and "FLASH OVERRIDE" priorities, with ROUTINE being the lowest and FLASH OVERRIDE the highest. To dial higher priority phone calls than routine, special authorisation was needed.

Each military installation had its own prefix for use in the Autovon and not all PABX extension telephones at military locations had the capability to call other military installations via Autovon. However, they could all receive Autovon calls coming from another installation apparently. Dialling codes and numbers were similar to the US civilian system but not the same; the last four digits were, however, always identical.

A detailed description of Autovon will be found in US MILITARY COMMUNICATIONS by Ricci and Schutzer (Computer Science Press, Inc, 1986).


What is EMSS?

EMSS is the emergency manual switching system. part of the telephone system designed to function post-nuclear attack. It consisted of manual phone switchboards in some major exchanges, usually with some minimal protection and accomodation. Once the attack had knocked out most of the network the EMSS could provide a very basic trunk system to connect places not on the emergency communications network (or ECN) eg a regional bunker could talk to a power station, until the exchange's batteries ran out. [Steve Fox]


What was the preference system?

It is well described in Duncan Campbell's book War Plan UK and enabled telephone service to be restricted to high-priority users only in times of civil and national emergency or natural disaster. Today both landline and mobile phone communications users can be given priority switching. See answers 2 and 3 in


Where can I visit former secret locations that are now museums with one-time secret communications equipment on show?

Go to this page.


And where can I learn more about the background to the Cold War?

A list of books, societies and other resources can be found here as well.

Records on the Internet

1. Kingsway telephone exchange (London)



2. Manchester deep level tunnels and Guardian exchange



3. St. Margaret's Bay repeater station, near Dover, Kent




4. Information on the Rotor scheme will be found here:


5. Government Emergency Communications Network for mention of ECN


6. UK Nuclear Attack Warning System provides a definitive description (including HANDEL, the various WB and hilltop radio systems)


7. Autovon for illustrations of an actual Autovon exchange in Britain.

A large number of articles on Autovon can be found using a Search Engine; probably the most useful (and accurate) are:


8. USA cold war communications     (AT&T's microwave and coaxial cable networks)   (Washington DC area's Cold War infrastructure) Cold War Comms, devoted to the C3I networks, systems, and facilities of the Cold War era.  There's a lot of Bell System discussion on the list, since AT&T was so heavily involved in national security communications (AUTOVON, hardened switching centers, etc.).

Websites concerned with a broader selection of Cold War-related technology topics (from a British viewpoint) include: (Research Study Group) (further links on subjects such as Communications, Warning and Intelligence; Emergency Planning and Civil Defence; Museums (such as Bletchley Park); Nuclear Weapons and Weapons Effects) (includes FAQ on subterranean subjects)

Booklists and information on former secret places now open to the public.

The Defence of Britain Project website is at


If you are interested in twentieth century military archaeology, or in related historical or conservation subjects, you may like to join one of the clubs and societies listed below. Please enclose SAE with letters.


Airfield Research Group

Research into airfields (military and civil) and associated features (munitions dumps, units, memorials etc).

Contact: 33a Earls Street, Thetford, Norfolk, IP24 2AB.


Bawdsey Radar Museum

Contact: Jim Reynolds c/o Bawdsey Estate Office Bawdsey, IP12 3AZ. 020-8948 3436


British Association for Local History

Shopwyke Hall, Chichester, PO20 6BQ. Tel: 01243-787639.


Brixham Battery Gardens

Restoration and public display of Brixham Gardens Battery

Contact: Robbie Robinson 2 Nelson Road, Brixham, TQ5 8BH.


Fort Amhurst and Lines Trust

Restoration of the fort and associated lines.

Contact: Fort Amhurst, Dock Road, Chatham, Kent ME4 4SG.


Fort Perch Rock Society

Established for the preservation of the fort.

Contact: Fort Perch Rock,  Marine Promenade, New Brighton, Wirral, Merseyside,  CH45 2JV.
 Tel: 0151 630 2707


Fortress Study Group

Active in research and recording of artillery fortifications worldwide of all periods.

Contact: Denis Quarmby (Secretary FSG), Blackwater Forge House, Blackwater, Newport, Isle of Wight PO30 3BJ.


Grey Point Fort Committee

Restoration, preservation and public display of Grey Point Fort, Belfast Lough.

Contact: Grey Point Fort, Helen's Bay, Co Down, BT19 1LE.


Historical Radar Archive

Study of UK radar network

Contact: Ian Brown, 3 Kingsmuir Crescent, Peebles, Scotland EH45 9AB.


Kent Defence Research Group

Formed to promote interest and study of Kents defences, but with wider membership and spread of interests.

Contact: David Burridge (Group Secretary), 59 Markland Road, Dover, Kent CT17 9LY.


The London at War Study Group

Contact: Sid Holyland 30 Claremont Close, Claremont Square, London N1 9LU.


Mercia Military Society

Military History discussion group.

Contact: Paddy Griffith, 24 Callendar Close, St Nicholas Park, Nuneaton CV11 6LU.


Mid-Wales Military Society

Contact: DE Davies, 10 Colwyn Terrace, Hundred House, Llandindrod Wells, Powys LD1 5RY.


Palmerston Forts Society

Study and recording of Victorian fortifications.

Contact: D Moore, 17 Northcroft Road, Gosport, Hants PO12 3DR.


The Pillbox Study Group (

Study of pillboxes, local defence sites and voluntary civilian aspects of war (ROC, Home Guard etc)

Contact: John Hellis, 3 Chelwood Drive, Sherford Park, Taunton, Somerset TA1 4JA.


Ridgeway Military and Aviation Research Group

Researches army and airforce use of the Ridgeway area; restoration, vehicles, re-enactment

Contact: Collins Farm, Abingdon, OX13 5NX.


Scottish Military Collectors Society

Study of Scottish military history

Contact: 4 Hillside Cottages, Glenboig, Lanarkshire ML5 2QY.


Society for Army Historical Research

Study of the British Army

Contact: Dr Peter Boyden, National Army Museum, Royal Hospital Road, London SW3 4HT.


The 1940 Association

Study of British history 1939-45

Secretary: Michael Conway, 43 The Drive, Ilford, Essex.


UK Fortifications Club

Research and recording of UK military sites of all periods.

Contact: Peter Cobb, 4 Mablethorpe Road, Portsmouth PO6 3LJ.


Victorian Military Society

International scope. Interests cover period 1837-1915.

Contact: 62 The Links, St Leonards-on-Sea, East Sussex TN38 0UW.


The Western Front Association

Encourages study of the period 1914-18 in order to perpetuate the memory of victims of the Great War.

Contact: Mr David Harrison, 16 Conway Rd, West Wimbledon, London SW20 8PA.


World War II Railway Studies Group

Concerned with all aspects of military railways and logistics in the UK.

Contact: Greg Martin, The Secretary, 17 Balmoral Crescent, West Molesey, Surrey, KT8 1QA.

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