Declassified Home Office files reveal a network of resilient cable routes the length and breath of the U.K. with not only underground cables but underground amplifiers and terminal equipment.
By the sixties the long distance cable network contained many carrier cables, mostly between the major cities but others routed to avoid the centres of population that might be a target in a nuclear war. When introduced in the nineteen thirties, the carrier system allowed 12 separate circuits to be carried on two pairs of wires, one pair in the 'Go' direction and the other pair in the 'Return' direction. Usually all the 'Go' pairs were in one cable and all the 'Return' in a second parallel cable to avoid interference between the two directions. Further enhancements post WWII allowed two groups of 12 circuits, thereby doubling the capacity to 24 circuits per pair, giving a maximum capacity of 576 circuits.
Cables working at carrier frequencies exhibit greater losses than those working in the audio frequency band. Repeater stations were built at approximately 15 mile intervals along the length of the cable to house the amplifiers needed to overcome these cable losses. Sizable buildings were required to accommodate the bulky thermionic valve amplifiers, one per cable pair, 48 in total. As well as the amplifiers themselves, a large power plant and standby generator was required to supply the considerable amount of energy consumed by the thermionic valve heaters and high voltage anode supply.
These repeater stations were vulnerable to blast damage, so in order to protect the circuits in the core infrastructure of the Emergency Manual Switching System (EMSS), a scheme known as 'All Underground' was developed. In contrast to the normal carrier systems employing thermionic valves, this network used newly developed Transistorised 24-Channel Carrier Equipment in protected basements at terminal stations and transistorised amplifiers at the repeater stations.
I have redrawn the map above, found in Home Office file HO 322/824 in the National Archives at Kew, and adding context by showing the location of EMSS Tier 1 & 2 node sites. Being 'Top Secret', the Central Government War HQ at Corsham was not shown even though it was part of the 'All Underground' network.
Intermediate Repeater Stations
The 'All Underground' network intercepted two pairs of wires in each carrier cable before they entered the repeater station and used transistorised amplifiers fitted inside robust steel cases located in a jointing chamber in the grounds of the repeater station. The two 'Go' amplifiers were housed in one case and the two 'Return' amplifiers in a second case. Another jointing chamber housed two sets of batteries, one feeding each amplifier case.
The remaining 22 pairs (3 to 24) continued to be amplified inside the repeater station with thermionic valve amplifiers. BT Archives contain a photograph of model of the jointing chamber containing the amplifier cases. At the terminal stations, the two pairs of the 'All Underground' network were terminated on self contained transistorised multiplex equipment, housed in the basement. The remaining 22 'normal' circuits continued upstairs to the repeater station in the telephone exchange building.
The two batteries consisted of 8 lead-acid cells each, providing 16 volts to power the amplifiers. These were charged from a 18.1 volt mains power unit (Power Plant 129) located in an underground room in the repeater station. The amplifiers consume 460 mA from each battery. Battery capacity of 141 Ah giving about 300 hours of reserve power.
The jointing chamber containing the two batteries required ventilation to avoid the buildup of hydrogen gas and acid vapour. The ventilation was afforded by two vent pipes fitted with cowls facing in opposite directions. The specification calls for the battery chamber to be sited within 3 feet of a property boundary and the lid raised 1.5 inches above the ground. With the help of Google Street view, these long redundant pipes and manholes still can be seen at some repeater stations.
Digital Upgrade circa 1986
In its basic form, the 'All underground' network provided up to 48 individual circuits between terminal points. These were analogue speech circuits carried over analogue multiplex.
During the eighties, the sixties analogue multiplex equipment was upgraded to digital multiplex. Now each pair of go and return wires could carry 120 speech channels instead of the previous 24 on the analogue multiplex. The part of the network descending south from Worcester was made digital by 1-September-1986. The Peterborough section northward to York, target completion date was 17-October-1986.
The photograph gallery below shows the digital multiplex equipment housed in a copper clad room in the basement adjoining the EMSS switchboard. The cable interface equipment is known as 'Equipment Digital Line 8A' (EDL8A) each providing four 2-Megabit ports which are connected to standard digital multiplex equipment in the adjoining racks. Each multiplex breaks down one 2-Megabit digital signal into 30 analogue speech channels.
Similar EDL8A racks can be seen in the Central Government War HQ at Corsham. This secret location was not marked on the map at the top of this page however its identifying Q-Code of QQCF can be seen at Worcester and the photographs of the Corsham bunker's racks.
The digital transmission network required amplification at shorter distances. Instead of using batteries in manholes, the amplifiers were powered from terminal stations by feeding the power down the same wires that carry the digital signal. The terminal stations operated at 50 volts, making use of the 50 volt reserve batteries for the EMSS and the standby generators at those locations.
Signal regeneration was required at approximately 3 Km intervals, the regenerators being located in two robust cases, one for each direction, and a third holding maintenance spares. The three robust cases (Case, Repeater Equipment No.1A) sited in a convenient manhole as in normal practice.
Closure of All Underground
Cabinet Papers from 10th June 1993, held in the National Archives at Kew, give a closure date. This decision was prompted by the Government Review of Emergency Communications (GREC) report. The report was further scrutinised by the Post Review Working Party (PRWP).
CAB 134 / 5766 extract . . .  Two separate, BT provided, EMSS transmission systems, the 'Backbone Radio' (BBR) and 'All Underground' (AUG) schemes, had also been reviewed by GREC. In accordance with its recommendations . . . . the obsolescent cable-based AUG was closed in January 1992. The GREC recommendation that the AUG pathways could, if refurbished with optical fibre, provide resilience to EMP for other emergency communications networks including the enhanced ECN, had been rejected by the PRWP as too costly to implement.