The Emergency Manual Switching System - E.M.S.S.

The Emergency Manual Switching System was designed to provide a skeleton long distance telephone service for Civil Defence, Military and Emergency Services should the normal public switched network be affected by enemy action. The system was closed at the end of the cold war in the early 1990's.

Background to the UK Analogue Telephone Network

Following the Nationalisation of the telephone companies in 1912 the UK telecoms infrastructure was run by the GPO / Post Office Telephones and in 1984 an Act of Parliament changed it into a public limited company British Telecom (BT).

At the start of the cold war, the GPO analogue network was very different from today's digital networks. There was no competition from operators in the market such as Mercury, Sky, TalkTalk or Virgin, as there is today. The GPO was the sole provider of telephone communications for the public and military.

1950's Telephone Exchange

Avoncroft Museum UAX13

Less than a quarter of people had a telephone in their own home. Due to the lack of government investment in the telephone network, there was often a waiting list for a telephone, two residential customers may have had to share a line, with no privacy between them. Unbelievably, there were advertisements to discourage calls at certain times of the day and encourage users to keep the calls to a short duration.

At the start of the cold war, many towns still had a manual system, requiring the user to pick up their phone and ask the switchboard operator for the number they required. In areas with automatic exchanges, local calls (classified as within a radius of 20 miles) could be dialled directly. Automatic long distance dialling known as Subscriber Trunk Dialling (STD) was introduced at Bristol in 1958. Full national rollout took in the region of 20 years. Before STD, automatic exchange customers wanting a call over 20 miles dialled either '0' or '100' for the operator who then connected the call. Even after the introduction of STD it was a number of years before every part of the country could be reached automatically, requiring those calls to be connected by the operator.

Major towns had an analogue automatic telephone exchange known as a Group Switching Centre (GSC). This exchange switched customer dialled calls within the local area. It housed the group routing and charging equipment for STD calls within that same area. Often there would be a co-located Auto-Manual Centre (AMC) housing operator switchboard positions handling long distance calls that couldn't be dialled directly, directory enquiry and 999 calls for the local area.

During the nineteen seventies, home telephones gradually became more common, so by the eighties the majority of homes had one. Analogue mobile telephones the size of a brick only started to appear in the early nineties just as the Cold War was ending, but the high price of handsets and calls restricted their use to mainly business people.

Starting in the mid eighties, the BT network was slowly modernised from an analogue to a digital system, culminating with a totally digital network by 'Phoneday' on April 16th, 1995. The cold war having finished a couple of years earlier.

Predicted Wartime Telephone Network Overload

All telephone networks are very susceptible to overloading caused by a local or national event, this can be anything from bad weather to a radio phone-in. The networks are only designed to cope with average number of calls. With today's modern digital networks, calls can be rerouted to avoid local problems. During the Cold War period the analogue network didn't have this resilience.

It was assumed overloading would occur during the period of build up to war as people tried to call relatives. Damage caused by war action itself would severely disrupt the network. To prevent overloading affecting essential services and the war effort, a system of disconnecting less essential telephones was introduced in the analogue network, which has been carried forward into the digital and mobile networks too. The disconnection facility is known as the Government Telephone Preference Scheme (GTPS).

This may sound harsh but in the sixties and seventies the network often could not cope with normal traffic and engaged tone was very common. In a war situation it was unlikely to cope with the predicted surge of calls, so by invoking the GTPS, the capacity could be reserved for government, military and civil defence lines

At the time of the Electricity Generator workers' industrial dispute in the early 1970's I recall working by battery lights during a power cut to match the correct level of preference category for emergency lines, compared with the list provided by the 'Area War Group' (AWG). However it was never necessary to use the Preference Scheme in Kettering as our emergency generator never failed.

Cold War - Government Telephone Preference Scheme

This scheme works by separating telephone lines into three preference category groups (Pref Cat).

The first stage of disconnection prevents Pref Cat III customers from making calls. The second stage of disconnection prevents both Pref Cat II & III lines from making calls. The Pref Cat I lines have no disconnection facility. All lines regardless of their category can receive incoming calls. So even with the maximum level of disconnection enabled, a Pref Cat I line would function normally as they could still make a call to a lower priority line with Pref Cat II or III.

The gallery shows the preference control slide in units at a TXE2 exchange on display at Avoncroft Museum, Bromsgrove. This type of electronic exchange with a reed relay switching matrix was introduced in 1968. The preference keys are mounted on the top shelf of the Calling Number Generator rack, disabling its function in allocating customers to the equipment that receives the number dialled, thereby stopping them making calls.

The red keys have little spring plates to stop them being accidentally operated. The nine keys on the left-hand side are the local control of the preference level. The other unit with a single 'Test' red key is the remote control unit, allowing the preference to be set firstly by calling an ex-directory number to prime the system and then dialling a second ex-directory numbers within three minutes that switch the preference on or off. In all cases, the master keys, top left on the group of nine must be operated to enable both local or remote control.

Within each GPO Telephone area, a small group of employees in the 'Area War Group' (AWG) liaised with government and the military to maintain a list of lines in Pref Cat I & Pref Cat II. When these lists changed, engineers were instructed to take action to modify the customers exchange line in the exchange.

At the time of the Electricity Generator workers' industrial dispute in the early 1970's I recall working by battery lights during a power cut to match the correct level of preference category for emergency lines, compared with the list provided by the AWG. However it was never necessary to use the Preference Scheme in Kettering as our emergency generator never failed.

Telephone Service Restrictions, Post Cold War

Even as network capacity has increased to present day standards, voting for a reality television shows can easily overwhelm it. In the present day digital network an additional software tool known as call gapping is used to reduce the peaks of calls to a specific number(s). This wouldn't work in a war emergency situation where calls are made to friends and family but not to one specific number. The GTPS still exists in the landline network and a similar arrangement, the Mobile Telephone Privileged Access Scheme (MTPAS) is in place on the mobile phone network. They may be used if we suffer a terrorist attack or a natural disaster such as flooding or earthquake jeopardises the telephone network.

Even after the end of the cold war, the GTPS and MTPAS for mobile phones still exist and are likely to continue in one form or another. This should not been seen as anything sinister. In the past it has been necessary to use the GTPS when telecommunication equipment has been accidentally damaged to ensure the emergency services can still function.

The Trunk Subscriber

A special type of customer's line existed well before the Cold War period and continued until the telephone network became digital. These customer lines terminated directly on the GPO public switchboard and were known as 'Trunk Subs' deriving their name from Trunk Exchange and Subscribers (the old name for customers). Typical customers would be the Police, Fire and Ambulance control centres dealing with 999 calls. When someone dialled 999 and asked the GPO operator for the Police, to save the time taken to dial the control room's telephone number, instead the operator would plug into the Trunk Sub and be directly through to the Police control centre.

Trunk Sub lines could also make calls, if they picked up their telephone, or plugged into it on their private switchboard, it called the GPO switchboard and was immediately answered by the operator.

Additionally in the cold war period, a variety of privileged customers were connected as Trunk Subs. Military Establishments such Armed Forces HQ's (AFHQ), U.K.W.M.O. Group controls, Local Authority Emergency Centres (LAEC) and Regional Government Headquarters (RGHQ) were typical users. This gave them priority access to the GPO switchboard operators and were not reliant on the automatic network. There were other uses for Trunk Subs such as special phones at exhibitions to the Queen's Train making an overnight stop in a siding.

These privileged customers lines were given an anonymous Trunk Sub number. This meant that GPO operators never needed to know the identity of the Trunk Sub, as incoming callers would just ask for a Trunk Sub number in a certain town. For example, Mytown Trunk Sub 11 would ask for 'Anytown Trunk Sub 9' The call would pass through the GPO network from 'Mytown' until the 'Anytown' operator connected the call to its Trunk Sub 9. Neither operator needed to know the identity of the caller or called line.

Emergency Manual Switching System (EMSS) Overview

The Emergency Manual Switching System (EMSS) consisted of a network of 300 manually operated switchboards situated in the basement protected areas of Auto Manual Centres. They were sited here as they needed to be operated by trained operating staff who would normally staff the public switchboard.

In the Post-Strike phase the EMSS would operate from the switchboard in the basement of the exchange building, providing telephone service to essential users during the recovery phase or it may be used during the transition to war, if normal telephone service were suspended or restricted by the GTPS covered in the topic above. There was an option to handle the EMSS calls on the 'Normal' peacetime switchboard. The number of protected switchboard positions provided depended on the estimated privileged traffic through that exchange.

Many essential users had EMSS Trunk Subs, these include all UKWMO Group Headquarters, Regional Government Headquarters, Council emergency centres (Main and Standby). The regional Armed Forces Headquarters (AFHQ) were connected to two different EMSS locations.

EMSS Network Structure

EMSS Structure

EMSS Network

I am most grateful to Dave McKay G1JWG who forwarded an Annex to a National Archives File HO322/824 entitled Machinery of Government in War Communications Working Party, which he recognised was a sketch of the whole EMSS Network, saying that it appeared to be orphan unrelated to the rest of the file. This chance discovery has enabled me to create the network diagrams shown here.

The EMSS network is built in three levels, the top tier comprises of six Emergency Through Switching Centres (ETSC) all fully interconnected. These handle long distance calls passed up from the tier below.

The second tier has thirty five Emergency Zone Centres (EZC) Each is connected to two ETSC switchboards and has sideways routes to one or more nearby EZC.

The third tier of three hundred Trunk Group Centres (TGC) would appear to be connected to two switchboards in the higher tiers. More information about the third tier of the network would be most helpful, but from another PRO file HO322/1220 it is possible to identify one hundred of those three hundred nodes in the network, as shown by Network Gallery Image 6.

Due to the lack of data we are unable to show the other routes from the twelve tier 3 TCG's connected to Worcester (Network Gallery Image 5) that must have existed, these being sideways routes to other TCG's and upward to Tier 2 EZC nodes. It is likely that most tier 3 TGC would have connected to two tier 2 EZC zone switchboards, as otherwise the six tier 1 ETSC would each have needed around 50 tier 3 TGC's connected to them.

Trunk Subscribers ( Tk Sub ), the end users of the EMSS network, are connected to their nearest EMSS no matter which tier it is on. It may be that certain important establishments are connected to tier one or two switchboard sites. Again specific information is lacking, but in the case of Drakelow RGHQ 92, it had a Worcester TkSub, that bypassed its nearest EMSS, a TGC at Kidderminster.

Worcester is the only ETSC for which we have circuit quantities at closure of the network. To other ETSC switchboards: Lancaster 6, Perth 6, Peterborough 12 and Reading 12. To the EZC layer below: Bath 5, Bournemouth [was Salisbury] 8, Burton-on-Trent 7, Chester 7, Colwyn Bay 3, Exeter 5, Gloucester 8, Leamington Spa 6, Merthyr Tydfil 4, Portadown 3, Shrewsbury 5, Taunton 5 and Truro 3.

There is evidence of routes being ceased before closure, but there is no sign of Carmarthen shown on the map as a Tier 2 node. Perhaps like Salisbury which was replaced by Bournemouth, Carmarthen may have been replaced by another switching centre or amalgamated into Merthyr Tydfil.

Calls could be connected across the whole country by only using the EMSS network. But in order to connect calls to people and organisations not connected directly to EMSS, circuits were also provided into the normal public network. At Worcester a group of 10 outgoing lines to gave access local area customers and a further 10 outgoing lines for National Number Dialling. A small quantity of Operator Assistance 100, all Priority Answer and all Emergency 999 circuits could be answered in the basement if it was necessary to stop using the normal operator switchboard on the top floor of the building.

A quick look in the Trunk Sub records for Worcester showed the Blue light services, Police Tk Sub 1, Fire Tk Sub 8 and Ambulance Tk Sub 5, additionally for local government Tk Sub 20; Worcester County Hall (Local Authority Emergency Centre) and Tk Sub 32 ; the Regional Government HQ RGHQ 9.2 at Drakelow Tunnels, Kinver and some others to the County Showground at Malvern, presumably for the special telphone caravans the GPO would send to such events.

EMSS Switchboards in Protected Accomodation

In the Worcester switchroom which features in the gallery there are 12 switchboard positions. All the incoming and outgoing lines are repeated along the length of the switchboard suite so that all are within easy reach of all the operators. There are two and a half faces per switchboard. The lower part of the face, the answering section contains the incoming circuits, these repeat themselves every six faces, A0 - A5. The outgoing line in the upper section of the face repeat every four faces.

Each operator position has fourteen pairs of plug ended cords, as seen in gallery image 3, limiting the number of simultaneous calls to that number. Unlike their public switchboard counterparts, the cord pairs do not have a time clock associated with them as calls are not charged. The operator answers an incoming call with answering plug, which is in the row nearest to the switchboard face. The calling plug of the pair (row nearest to the operator) is placed into the appropriate outgoing line in the top section of the switchboard face.

Within the switchroom is a test jack frame. Its purpose here is to switch the circuits from the normal peacetime switchboard to the apparatus room within the protected accommodation. Operating staff can switch from the normal switchboard to the emergency switchboard by moving the red plugs up from the 'S' row where they are currently situated to the 'N' row above. The third 'M' row is for monitoring and test purposes.

The gallery shows a number of images of the apparatus room. The relaysets convert the signalling conditions sent down the external lines to those needed by the switchboard. Due to the importance of the Worcester EMSS as just one of six main centres, it has its own set of batteries and standby generator independent of the exchange's own batteries and generators.

UKWMO Group Controls on EMSS

Now declassified documents in the Public Records Office (PRO), reveal that all ROC Group and Sector Controls in the UKWMO were connected as Trunk Subscribers into the EMSS Network in 1972.

Due to the "Confidential" security classification of the EMSS, little was known about the system by its potential users, the PRO file HO332/1220 shows this was of concern to the Civil Defence College at Easingwold as it inhibited the subject being included in their training courses. In February 1978 Mr Cumings, Deputy Director of the UKWMO wrote to Sectors. From time to time ignorance is expressed regarding the working of the Post Office Emergency Manual Switching System . . .

It is essential that EMSS users know the relevant Trunk Sub numbers of those they wish to connect to. As stated in the Trunk Subscriber topic above, the GPO operator did not know the identity of the called line, the caller just quotes an exchange and Tk Sub number, it wasn't possible to ask for example "Bedford UKWMO Group". With this in mind, the UKWMO circulated a list of the ROC Group and Sector controls along with their Trunk Sub number, this is reproduced opposite.

Central Government War Headquarters at Corsham


EMSS Network to Corsham

De-classification of documents relating to the Central Government War Headquarters (CGWHQ) at Corsham, reveal that in 1968 it had a total of 37 lines to EMSS switchboards around the country. The map shows the nine E.M.S.S. locations connected to CGWHQ, four are Tier 1 ETSC centres and five are Tier 2 EZC.

EMSS        Lines  |    EMSS        Lines
Reading     10     |    Carmarthen  2
Worcester   10     |    Exeter      2
Bath         3     |    Lancaster   2
Salisbury    3     |    Perth       2
Taunton      3     |

The CGWHQ at Corsham was a very well kept secret despite it having a large switchboard connected to the nine EMSS sites listed above. I had expected it would simply have a Trunk Sub number at each of the interconnecting EMSS's. That could be true, but at Worcester, there didn't appear to be a group of 10 Trunk Subs for those corresponding circuits. However on the face of the switchboard there are a series of 10 outgoing jacks marked as 'PW1151--' and each jack has an individual number. These numbers correspond with the 10 incoming jacks and calling lamps, marked PW115159 to PW115168. See Worcester's Jack field Maybe these were the Corsham circuits?