GPO / BT Defence Network

G.P.O. / BT Structure

G.P.O. Telephones, up to and for a number of years beyond privatisation, was organised with Telephone Headquarters in London setting the company policy and budgets. The country was further divided into Regions who were responsible for their part of the budget and exchange and lineplant planning. Each Region was further subdivided into Areas. Each of these areas had a 'Telephone Manager' to who the various departments were accountable. The majority of these departments were located in a 'Telephone Managers Office' a large building or collection of buildings housing hundreds of staff. The map below shows the Area boundaries and the location of both Area and Regional Headquarters.

GPO Regions and Areas — Telephone Areas within Regions

The Post Office War Organisation (POWO) was organised to reflect these Regional and Area boundaries. A national emergency centre was located in Oswestry coordinating the response.

Post Office telephones was privatised in August 1984 and the POWO became the British Telecom War Organisation (BTWO) In the years after privatisation, during the 1980s there were frequent structural changes and reduction in staffing numbers as digital modernisation took place. Regions were abolished and areas amalgamated to create Districts. Later those combined to form larger districts almost the same size as Regions had been, but without the bloat of a regional organisation between them and headquarters. Fearing the Government would split the company into smaller units, some departments moved into a 'National Networks' organisation (NN) and the remainder in 'Local Communications' sector (LCS).

The communications network to support the POWO was known as the Post Office Defence Network (PODN) becoming the British Telecom Defence Network (BTDN) after privatisation.

The BTDN at the time of its closure in the nineties contained a mishmash of various circuit markings on switchboards reflecting the original PODN and the various company reorganisations during its life as BTDN. This makes identifying the underlying structure difficult for the researcher. The BTDN also contained a telegraph network enabling hard copy messages to be sent to BTWO locations all over the country.

The Defence Network Structure

The Post Office Defence Network (PODN) renamed the BT Defence Network (BTDN) used manual switchboards to link important operational buildings, such as larger telephone exchanges, repeater stations and microwave radio stations with coordination points in the wartime management structure of Area War Groups (AWG) and Regional War Groups (RWG). When post-privatisation reorganisations removed the regional tier of BT's peacetime management structure, the RWG were replaced with a number of control centres around the country that reflected the Home Office Regions.

Wartime dispersed civilian government was planned around the Home Office regions, each region having two Regional Government Headquarters (RGHQ). The regions are numbered 1 to 10 in England and Wales, in Scotland its not the Home Office but the Scottish Home and Health ministry responsibility, this is region 11. Military Home Defence supporting the RGHQs was also aligned to the same region boundaries, employing its own regional VHF radio communications scheme code named MOULD, which worked independently of the landline network.

At the majority of sites a suite of switchboards would serve both the Emergency Manual Switching System EMSS and the bottom tier of the BTDN. In other towns the BTDN switchboard was completely separate either located in nearby rooms or in completely different buildings, such as Chester and Worcester.

The BTDN part of the combined switchboard generally has very few circuits. One circuit to its own Area War Group (AWG) and maybe other nearby AWG too. Circuits marked 'XX SEC' where the 'XX' is an exchange traffic division code for the location of the Subordinate Engineering Control (SEC). The SEC was a localised engineering control for maintaining the Public Switched Telephone Network (PSTN) plus any government facilities. Beneath each SEC was a group of MEPs or Minor Engineering Points, where small teams of engineers would be stationed to carry out any actual maintenance work.

At a combined BTDN and EMSS, the BTDN circuits are not used for connecting customers, but the EMSS circuits may be used to connect BTDN services through to departments in other parts of the country.

switchboard jacks — Simplified BTDN Network

The Cabinet Office file refers to the BT Wartime Network Management Centre at Oswestry, this is co-located with the Network Operations Centre as it was known then. Located at Park Hall, Whittington, SY11 4TB Lat / Lon 52.871399, -3.030835

Regional Support Group

The BT Support Group No.8 serving the whole of Wales was located in a former AAOR bunker at Coryton, near Cardiff. As stated earlier each Home Defence Region would have its own BT Support Group (BTSG). It has two small switchboards serving the bunker. Similar switchboards were used at Area War Groups and some EMSS too, although most EMSS were upgraded in the eighties to the Sleeve Control types used in the public network.

The switchboard is the common type of office / factory switchboard. Known within GPO/BT as a 'Switchboard N1070' this was modified to diagram number SA2054 for use in small EMSS exchanges and the BTDN. This was superseded by 'Switchboard AT3796' almost identical in appearance to the untrained eye.

dolls eye switchboard jacks — Dolls Eye Switchboard

This type of switchboard although designed in 1935 was still being installed in customers premises in the early nineteen seventies, it has a capacity for 10 exchange lines and 50 extensions. The exchange lines have drop down flaps to indicate an incoming call. The extensions have eyeball indicators that roll downwards, giving the switchboard it's familiar name of 'Dolls Eye Switchboard'. At Coryton the top row of extensions (41-50 & 91-100) are used for inter-bunker circuits.

Switchboard No.1
Jack Number	Designation	Notes
41	CF AWG	Cardiff AWG
42	CS AWG	Chester AWG
43	SY AWG	Shrewsbury AWG
44	SS AWG	Swansea AWG
45
46	CF SAWG	Cardiff Secondary AWG
47	MW HQ	-
48	BTSG10	North West SG
49	BTSG7	West Country SG
50	BTSG10	North West SG
31-40	Extensions 31-40	Phones on desks in this bunker
21-30	Extensions 21-30
11-20	Extensions 11-20
1-10	Extensions 1-10
Exch 1	Cardiff 391336	PSTN PBX Line 1
Exch 2
Exch 3
Exch 4	Machen 440929 /1	PSTN PBX Line 1
Exch 5
Exch 6	Hengoed 816404	PSTN Line
Exch 7	Hengoed 816446	PSTN Line
Exch 8
Exch 9	MT TkSub 1	Merthyr EMSS
Exch 10	CF TkSub 1	Cardiff EMSS

Switchboard No.2
Jack Number	Designation	Notes
91	CF AWG	Cardiff AWG
92	CS AWG	Chester AWG
93	SY AWG	Shrewsbury AWG
94	SS AWG	Swansea AWG
95
96	CS SAWG	Chester Secondary AWG
97	RW HQ	-
98	BTSG10	North West SG
99	BTSG9	Midlands SG
100	ZHQ 82	RGHQ Brackla
81-90	Extensions 81-90	Phones on desks in this bunker
71-80	Extensions 71-80
61-70	Extensions 61-70
51-60	Extensions 51-60
Exch 1	Cardiff 391337	PSTN PBX Line 2
Exch 2
Exch 3
Exch 4	Machen 440929 /2	PSTN PBX Line 2
Exch 5
Exch 6	Hengoed 816434	PSTN Line
Exch 7
Exch 8
Exch 9	MT TkSub 2	Merthyr EMSS
Exch 10	NPT TkSub 2	Newport EMSS

The BTSG8 has seven exchange lines from three different exchanges to provide resilience. Each of these exchanges is parented on a different main switching unit used to make and receive Subscriber Trunk Dialling (STD) calls. Additionally BTSG8 has four lines to three different EMSS to enable it to make and receive calls on that network.

There are two private circuits to each of the four Area War Groups within Wales at Cardiff, Chester, Shrewsbury and Swansea plus a single circuit to the secondary AWG serving the Cardiff and Chester telephone areas. The AWG are the next tier down, under the regional support group.

The three adjacent BTSG are also connected to Coryton by a private circuit. BTSG7 serves the region comprising of seven Telephone Manager's Areas, Bournemouth, Bristol, Exeter, Gloucester, Plymouth, Southampton and Taunton. BTSG9 serves the Midlands region of Birmingham, Coventry, Stoke-on-Trent and West Midlands. BTSG10 covers the Telephone Areas of Blackburn, Lancaster, Liverpool, Manchester and Preston.

War Emergency Procedure

Alex, a non-BT person discovered a wall chart in what he thought was a secret bunker at Abergavenny telephone exchange, this is reproduced below. Abergavenny along with all large exchanges built post-WWII have protected accommodation in their basement which might seem like a bunker to the uninitiated.

Abergavenny is thought to have once been the wartime command centre for the telephone region. The chart refers to BT Instruction numbers, but those are not to hand. Unfortunately some actions make little sense without seeing the details they refer too in the instruction.

Instruction Number	Action	Notes
EM 001	Review plans
EM 111-2	Designate staff
	Review transport
	Review essential records
	Review access arrangements
	Finalise Instructions
EM 112-1	Alert advance staff
	Prepare passes
EM 113-1	Alert EM Staff for ZHQ	10
	Confirm transport
	Despatch advanced staff
	Alert common services
	Alert EMSS Staff	4
	BTDN Ready	1
	Provide funds
EM 114-2	Despatch common services
	Erect F.O.P.
	Engineering Stores (BTDN)	1
	Alert Main Staff
	Despatch EM Staff to ZHQ	10
	Provide transport
	Man BTDN	1
	Advise BTHQ	2
	Despatch main staff and records
	Despatch ZHQ Staff	10
	Advise BTHQ	2
EM 116	Issue instructions
EM 131	Review non-essential work
	Review essential supplies
	Review dispersal
	Review leave
EM 134	Cease training
	Cease planning
	Confine maintenance
	Disperse supplies
	Top-up fuel
	Cancel leave
	Advise BTHQ	2

Instruction Number	Action	Notes
EM 135-2	Requisition
EM 27	Provide Comms
EM 803	Prepare staffing schedule
	Alert staff
EM 805	Switch ECs	9
	Provide Staff
	Review access arrangements
	Finalise instructions
EM 811-2	Review KPs and Protect	6
EM 812-1	Complete protection
EM 831-1	Review RADIAC	8
	Designate BTLOs
EM 832-1	Alert BTLOs	3
	Collect RADIAC	8
EM 835-1	Despatch BTLOs	3
	Distribute RADIAC	8
	Advise BTHQ	2
EM 852-2	Control News Releases
EM 862	Switch BBC ECs	9
EM 871	Review plans
EM 874-1	Erect F.O.P.	5
	Make EMSS Ready	4
	Confirm staff alerted
	Introduce PREF CAT II.	7
	Prepare for PREF CAT I.
	Advise BTHQ
EM 874-2	Prepare LR
EM 875	Activate EMSS	4
	Introduce PREF CAT I.	7
	Advise BTHQ	2
EM 876	Activate LR
	Advise BTHQ	2

Author's notes

These do not form part of the wall chart, but added to explain some details. The use of BT implies its predecessor the Post Office / G.P.O. too.

BTDN BT Defence Network, telecoms to support the work of the BT War Organisation in the event of the normal public network failing.
BTHQ British Telecom war headquarters.
BTLO BT Liaison Officer. An engineer embedded within a UKWMO, regional government or council bunker to maintain its communications systems.
EMSS Emergency Manual Switching System, a skeleton manually operated telephone system supporting the military and civil defence organisations. This is described in it own chapter found in the Chapter Index on the page menu bar.
F.O.P. Fall Out Protection, special protection often in the form of dry interlocking construction blocks used to create a wall, to shield staff from the radiation given off by nuclear fallout.
KP Key Point, important parts of the telecoms network requiring protection from sabotage during the period of tension. Protection provided by the police and military.
PREF CAT an abbreviation commonly used for Preference Category. The three categories are explained in the Area War Group topic below.
RADIAC Devices for measuring the radiation levels after a nuclear attack to see where was safe to go when clearing faults or attending work.
Switch EC, Emergency Circuits (EC) are normal lines between exchanges during peacetime carrying our calls around the country. At times of war and exercises these lines are switched to create private circuits for special services. Prior to the nineteen eighties communications modernisation, the UKWMO relied heavily on ECs. The BBC ECs, would carry special War Time Broadcasting Service programs to their transmitters all over the country from a central underground studio at Wood Norton, Worcs.
ZHQ Zone Headquarters, formerly known as RSoG (Regional Seat of Government) or RSG and RGHQ (Regional Government HQ) At times of war, GPO/BT Engineering Maintenance (EM) staff would be stationed within the ZHQ buildings to maintain and clear faults on telecoms equipment.

Area War Group

Reflecting the GPO's Regional and Area structure when the war organisation and its communications network were established in the nineteen sixties, each telephone area had an Area War Group (AWG). This was responsible for co-ordination within its area and liaison with Emergency Service customers. One role was to draw up a list of lines with enhanced telephone service preference, known as 'Preference Category I' and 'Preference Category II' and supply this to the Engineering staff to ensure customers lines terminated on the appropriate type of exchange equipment.

GPO/BT Telephone lines have three categories of preference. Ordinary domestic and most business customers are Category III. Important (to the functioning of the state) business lines and Public Telephone Boxes are Category II. Important customers such as Police, Fire and Ambulance stations, Hospitals, Civil Defence and Military have a Category I status. Introducing category two, prevents category three customers from making calls. Introducing category one, additionally prevents category two from originating calls. All three categories can receive calls.

Communications

The switchboard at a stand-alone AWG is the common type of office / factory switchboard modified for its role in the BTDN.

Stand alone switchboard — AWG Telephone Switchboard and Interface Circuits

Typically the AWG switchboard would have a small group of ex-directory telephone lines, lines to adjacent Telephone Area's AWG, A Trunk Subscriber (TkSub) to the EMSS and Private Wires (PW) or Emergency Circuits (EC) to its Regional BT Support Group (BTSG). An emergency circuit is a private circuit that is switched through in an emergency or war situation, but in peacetime carries normal calls from the general public between exchanges whereas a PW is permanently connected for exclusive private use.

As well as telephonic communications, the AWG had access to the public TELEX system and a private BTDN Telegraph network for exchanging typed messages. In this photograph the left hand teleprinter is on the TELEX network and the other on the BTDN system.

The two devices here are the British Telecom Cheetah, known as a 'Teleprinter 87C' and probably having 32 kilobytes of memory for composing messages. Introduced in the early eighties, the Cheetah's life was short lived due to the influx of personal computers and email. The BT public TELEX Network was finally closed in March 2008.

Leicester Area War Group switchboards

The Leicester AWG switchboards have found their way to the Secret Bunker Museum in Anstruther, Fife and form part of the fake 'Main Switchboard'. Not all the circuit designations are visible but give an insight to the typical connections at an AWG

Switchboard No.1
Jack Number	Designation	Notes
41-45	-	Unused Jacks
46	STS	Senior Telecomms Superintendant
47	TS	Telecomms Superintendant
48	TS	Telecomms Superintendant
49	Comm Control
50	unreadable
31-35	-	Unused Jacks
36	Stores
37	Customer Inst'n	Installation
38	PE	Peterbough
39	External Mtce	External Maintenance
40	unreadable
21-25	-	Unused Jacks
26	GM	General Manager
27	DGM	Deputy General Manager
28	Radiac Officer
29	ASR	Assistant (switchboard) Supervisor
30	LE RSC	Leicester Repair Service Control
11-15	-	Unused Jacks
16	blank	no longer used
17	BT SEC	Burton-on-Trent SEC
18	BT EMSS	Burton-on-Trent EMSS
19	KZ SEC	Kettering SEC
20	KZ EMSS	Kettering EMSS
1-5	-	Unused Jacks
6	Copt Oak RS	Microwave Radio Station
7	SG	Support Group
8	SG	Support Group
9	BM AWG	Birmingham AWG
10	DWHQ EM	District War HQ (East Mids)
Exch 1	Leicester 50425	PSTN PBX Line 1
Exch 2	Leicester 50427	PSTN PBX Line 3
Exch 3	Leicester 50970	PSTN Line
Exch 4	GMO 4898	Extension 4898 (see note)
Exch 5		Unused exchange lines
Exch 6
Exch 7
Exch 8
Exch 9	TKND1	Outgoing only line
Exch 10	TKND2	Outgoing only line

Switchboard No.2
Jack Number	Designation	Notes
141	Comm Control
142	Comm Control
143	Rest'rant	Restaurant
144	Male Dorm	Dormitory
145	Female Dorm	Dormitory
146	Records Room
147-150	-	Unused Jacks
131	Internal Mtce
132	Internal Mtce
133	PE	Peterborough
134	External Mtce
135	blank	no longer used
136	External Mtce
137-140	-	Unused Jacks
121	blank	no longer used
122	unreadable
123	T & G	Trunking and Grading
124	Internal Planning
125	Transport
126-130	-	Unused Jacks
111	LO SEC	Loughborough SEC
112	unreadable
113	blank
114	blank
115	MY Auto	Melton Mowbray ATE
116-120	-	Unused Jacks
101	AWG NC	Norwich AWG
102	unreadable
103	blank
104	AWG SK	Stoke-on-Trent AWG
105	OK RS	Oakham Microwave Radio Station
106-110	-	Unused Jacks
Exch 1	TKND3	Outgoing only line
Exch 2	GMO 4899	Extension 4898 (see note)
Exch 3		Unused
Exch 4	Leicester 50426	PSTN PBX Line 2
Exch 5	Leicester 50428	PSTN PBX Line 4
Exch 6	Leicester 50729	PSTN Line
Exch 7		Unused exchange lines
Exch 8
Exch 9
Exch 10

The Leicester AWG has six Public Switched Telephone Network (PSTN) lines, a group of four form a Private Branch Exchange (PBX) group with an ex-directory number of Leicester 50425. Two other single PSTN lines and three outgoing only lines TKND 1 to 3, which would allow national calls to be made bypassing the STD system. There are two lines to the General Manager's Office PABX, GMO 4898 & 4899. These allow contact to and from office based staff in the GMO building, and from outside by prefixing the extension number with Leicester 53. (e.g. 534567 was the external number for extension 4567)

Two microwave radio stations, Copt Oak and Oakham have direct connections to the AWG, a similar arrangement is seen on the Oxford AWG with lines to Stokenchurch RS and Sparsholt Firs RS, both being part of the 'Backbone' radio system (BBS). Copt Oak is the only Leicester one on the BBS.

There are lines to adjacent AWG, Birmingham, Norwich and Stoke can be seen but others may be amongst the unreadable circuits. Its likely to have included Coventry, Nottingham and Peterborough AWG. There are two circuits marked Peterborough (PE) with no suggestion to what they might be, AWG or EMSS.

Note The old title of Telephone Manager was later replaced by the term General Manager, the Telephone Manager's Office became the 'General Manager's Office (GMO). During the Post Office era, Engineers vans had the words Post Office Telephone Manager (Town Name) on both the driver's and passenger door. Some customers actually thought the engineer was the 'Telephone Manager' thereby promoting them by five tiers of management.

Staffing in an Emergency or War

During a civil emergency or war it would be necessary for staff to be based at key points within the telephone network infrastructure. These may be important exchanges, repeater stations, radio stations and EMSS / BTDN locations. The welfare of the staff must be considered by providing meals, sleeping, washing and toilet facilities.

Ration boxes for the switchboard operators and engineering staff were stored on site even during peacetime. The examples below can be dated by the British Telecom dotted 'T' logo to between 1981-1991 when it was replaced by the piper symbol. By then the cold war had ended and EMSS wound down, its unlikely there were any rations carrying the piper logo.

Emergency locations were often situated in below ground sections of a building where possible. Many key locations were totally above ground such as repeater or radio stations which offered little or no protection from the radiation given off by nuclear fall-out. Even basement locations may not give sufficient protection.

In order to create radiation protection for staff at these locations, the Ministry of Public Building and Works designed interlocking high density concrete blocks, that could be assembled by untrained staff without the need for mortar. Walls or enclosed spaces could be created or window openings blocked. The density of the nine inches of concrete gave the required level of protection.

Plans were drawn up showing how the blocks were to be assembled, floors marked with paint and the required number of blocks stored on site so they could be moved into place should the need arise.

The blocks are very cleverly designed to interlock both horizontally and vertically, for added stability the E-Type block can be rotated vertically and interlocked. The nose of the chevron is flattened to allow the blocks to better mesh together. The majority of the wall is constructed with 'E' type blocks, but other designs the K-Type & L-Type create corners. The A-Type design has a flat bottom to spread the load when laid directly on a floor. If a protective 'roof' is required, blocks with flat tops allow planks to be laid across two parallel walls to support concrete slabs.

Block E Design — The Shape of the E-Type Fallout Block

The photograph below shows a safe area constructed within an important microwave radio station on the Backbone (BBR) network. The structure is 2.2 metres (7ft 4in) tall. As radiation travels in a straight line from radioactive fallout on the ground outside this is sufficient to protect an occupant while standing inside the refuge. It should be stressed the block wall offers no protection against bomb blast.

Closure of the BT War Organisation (BTWO)

Extracts from a Home Office report dated 25th May 1993 to be considered at a Cabinet Office meeting on 10th June 1993. Revealed within the contents of declassified Cabinet Office file CAB134-5766, held in The National Archives at Kew. Paragraph 14 briefly describes the department's purpose.

14. The BTWO which would have been dispersed to protected sites, was intended to facilitate BT's restoration of damaged communications in time of war. It was interlinked by their bespoke communications system, the BT Defence Network (BTDN). The GREC recommended that the BTWO be maintained, and its size and arrangements reviewed. A review was initiated by the PRWP in 1989, but suspended pending the outcome of BT's Project Sovereign re-organisation. A joint BT/Home Office working group was then set up by the PRWP in early 1991 to carry forward the review.

15. Once again, the changed political climate and diminished threat of nuclear war were major factors taken into account by this review. Other significant considerations were: the enhancement of the ECN and closure of the EMSS, BBR and AUG; the updated requirements of other Departments; the absence of comparable facilities for other essential services; and the high cost of modernising the BTDN. In July 1992 the PRWP accepted the Home Office's conclusion that the BTWO should be considerably reduced in size to a central core, but that BT's designated wartime Network Management Centre at Oswestry should be retained. The closure process is continuing, and the BTDN was shut on 28 February 1993. The Home Office will pay for the cost of BT's connection to the ECN.

Abbreviations Used

ECN Emergency Communications Network, a government voice network, during the Cold War period linking Regional Government HQ, Local Authority Emergency Centres (LAEC) and UKWMO Groups and Sectors. Post Cold War, after the closure of the RGHQ and UKWMO sections, an enhanced ECN was extended to link together the LAEC with many other user groups. The ECN is described in more detail on its own page, linked from the bottom of this page or found in the A-Z Index in the top of page menu bar.

GREC Government Review of Emergency Communications.

PRWP Post Review Working Party - implementing the GREC Review.

The next three abbreviations are systems operated by BT, each has a subject page of their own on this website.

EMSS The Emergency Manual Switching System
AUG The All Underground Network
BBR Backbone Radio part of the GPO/BT Long Haul Microwave Network

The three pages are linked from the bottom of this page or the topic may be found using the top of page index bar.

Project Sovereign A major structural reorganisation of BT which included loss of staff through voluntary redundancy.

G.P.O. / BT War Organisation