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Laying submarine cables

Damaged submarine cable samples. Artefacts at Porthcurno Telegraph Museum

Underwater cable-laying techniques : it's dangerous down there

Laying a submarine cable is a remarkably complex, hazardous and expensive business. Routes need to be surveyed, technology developed, the cable needs to be laid without being lost, broken or damaged. The seabed is as hilly, rocky and varied as any terrain on land - so undersea cable expeditions have always started with surveys to find relatively flat and unbroken routes.

The next problem is keeping the cable safe from accidental damage. The hazards down there include sharks, earthquakes and volcanic activity.

Fishermen pose a far worse hazard than fish - deep-sea trawls routinely snag and break cables.

These days, new cables tend to be buried using robot submarine ploughs that crawl along the seabed.

Telephone cable sign

Telephone cable beacon (1970s) : warning hidden danger

There's always a danger that submarine cables, out of sight under murky water, could be dragged up by a sailor's anchor or fisherman's net. After ensuring all maps showed where submarine cables lay, this huge 10 foot diamond-shaped sign was about all The Post Office could do to try to steer boats away from causing potential damage.

The sign would be bolted on to a post on the banks of a river or on the coast in the hope that unaware sailors would realise that a submarine cable was laid underwater nearby and that it wasn't a good place to drop anchor or throw out a fishing net.

No doubt the signs have helped avoid some problems, but no solution is foolproof and cables are still regularly dragged up from river and ocean beds today. This monster never managed to see active duty and was placed in the BT heritage collection straight from the storeroom!

Deep Sea Cable Repairs' - PO publicity poster, 1939

Underwater cable-laying equipment : gently does it

Specialised cable ships have tanks below deck to store the cables, 'cable engines' that allow the heavy cable to be paid out at a defined rate and sheaves - grooved wheels - to guide it over the bow or stern.

Cable ships also have systems that hold the ends of cable lengths together to allow them to be attached ('spliced') to each other.

From the 1950s the need to incorporate repeaters (tubular housings containing amplifiers) into telephone cables created a whole new set of handling problems. For repeaters that were too bulky to go through sheaves, The Post Office developed the 'five sheave gear' system that held the ends together under tension with a bypass cable while the repeater was inserted.

Later, an even more ingenious Post Office system used rubber tyre wheels to grip the cable - springing apart to allow the repeater to go through.

Bouy marking undersea telegraph cable

Underwater cable-laying stories (1955) : tales from the deep ...

All cable laying expeditions tend to be difficult - as the story of just one will show.

This was the laying of the first transatlantic telephone cable, TAT-1, in 1955-6.

The Post Office cable ship HMTS Monarch (IV) was selected to do the complicated lay, which involved a westbound cable running from Oban to Rockall, across the North Atlantic to Newfoundland and then on via a land and shallow sea route to Nova Scotia, from where a microwave radio relay would carry the TAT-1 links into the US domestic system. A separate cable on the same route would carry traffic back the other way.

Monarch had to make seven separate 'load and lay' voyages, from the spring of 1955. She had to battle fog, storms, icebergs, lost cable buoys, faulty repeaters and field ice before the final splice was made on the evening of August 14, 1956 - 90 years after the first transatlantic telegraph cable was laid.

Submarine Cable presentation set ; Aden-Natal. Connected Earth artefact, now at the Museum of London.

Cable presentation set - Aden to Natal (c1879) : a cable for all occasions

This demonstration case shows the variety of cables that were used to create the underwater cable link from the Middle East to South Africa, as part of the independent telecommunication links being established across the British Empire.

By this date, laying cable was a carefully planned operation. The different widths of cable were specially chosen for the sort of terrain on which the cable would rest. It was thicker and stronger for rocky areas and near to the shore landings, but thinner for deep water sections, where the danger from human damage was small. A chart was plotted before the cable manufacture began, which would dictate the route and composition of the cable.

Porthcurno, Cornwall

Porthcurno, Cornwall (1870) : wired to the world

Porthcurno was the main gateway for telegraph cables linking Britain to the rest of the world.

The first choice had actually been Falmouth but there were serious concerns that ships coming into port might drag their anchors across the cable. Porthcurno was made as a late substitution. It was an ideal spot offering protection with a sandy beach and storm-protecting headland.

The enterprise was headed by John Pender as a part of his dream to create a cable around the world and Porthcurno (telegraphic code name PK) became his centre of operations. The first successful link was made to the Mediteranean and the first transmission was made in 1870. By 1872 Porthcurno had 27 operators and trainees.

In 1915 a training school was established there, which subsequently became Cable & Wireless's training college until 1993. In the late 1930s the government ordered the station to be relocated into tunnels in the cliffs for added security.

The station closed in 1970 and is now the Museum of Submarine Telegraphy.

Engineer - (transatlantic cable installation) : Peter Newman

Peter Newman is a retired ex-Post Office and BT engineer who worked for the Post Office from 1946 until 1986, mainly on the long distance area working in Faraday building, London and then latterly the international network

He recalls working on the first transatlantic cable installation and the wonder of speaking to people in the USA who sounded as if they were just next door.

audio clip


I was doing clerk of works duties on the installation of the equipment for what was going to be the Kingsway repeater station, and eventually became the transmission system that backed up the transatlantic terminal, which moved into the end of that tunnel system in 1956 when the first transatlantic cable opened. The point was that we were sitting talking to people in the States that sounded if they were in the room next door. When you know, prior to that, operators had to monitor all the time to even calculate when the radio systems available to the public was acceptable or not.

Technical Officer - (first transatlantic cable) : Pat Hastings

Pat Hastings joined the GPO in October 1946 where he worked for forty three years, becoming a Technical Officer.

Pat has been involved with many of the modern developments in telecommunications, from transatlantic telephone cables to satellite communications

He talks about the techniques for laying the first cable joining the USA and Britain and particularly the hazard it faced from the two nations' fishing fleets.

audio clip


In September 1956 a network was opened to America using the this newly laid trans-Atlantic cable. Now the unique thing about this particular cable system was that it had only thirty six circuits on it and it was a two cable system, one cable for each direction of transmission. The cable itself housed repeaters which were actually installed longitudinally along the length of cable so that all that happened was that the cable got fatter when it had needed a repeater and then back to its normal size. The cable of course was an armoured cable and this was laid on the sea bed across the Atlantic. I'm not sure the distance now but it was probably something like three thousand miles. The great advantage of this particular system in that the cable simply got thinner and fatter to house repeaters was that it could be laid in a continuous run by the cable ships and of course our own Post Office cable ship the Monarch was then heavily involved in laying of these cables. In the early days there were a few problems with cable failures largely because fishing boats, who were doing deep sea trawling, tended to catch hold of this cable which was only laying on the sea bed not in the deep sea portions, but in the little continental shelf areas coming off the main continents, particularly in America where the continental shelf extends I believe a hundred and fifty miles or so, and deep sea trawlers would fish up the cable and obviously get it all tangled up with their boards and what-have-you and officially they were supposed to notify the companies, that's AT&T and the Post Office what they had done, but I'm afraid they weren't always very honest and it was on one or two occasions quite obvious that the axe had come out and chopped the cable in half.

fun and games

Can you beat our games? Explode equipment to see what's inside, hear the changing sounds of telecommunications, see how telecommunications designs have changed over time or send an e-postacard.

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