Borosilicate Solves Safety Problem In 'Chunnel'
July 1995

The new tunnel between England and France is in fact three tubes - one taking trains from the Dover area to Calais, one taking trains from the Pas de Calais to Dover, and one in the middle to supply services to the other two. All three are 50km (32 miles) long, engineering triumphs of today which may or may not become commercial triumphs tomorrow. Connected to the main tunnels by communication corridors, the central tunnel carries the power supply, safety equipment and maintenance gear. Automatically-guided vehicles provide inspection and safety services.

The obvious need for bright and totally reliable illumination presented technical problems for which no real precedent existed, these tunnels being the longest and most complex ever constructed. The light fittings in the service tunnel had to withstand shock waves of vibration from the trains as well as very high temperatures and corrosive humidity. The safety standards were probably the highest ever specified for lighting.

Originally the plan was to install polycarbonate lamps as were used during the chunnel construction work. This, however, was banned by the international safety committee. In fact, a ban was placed on all use of plastic in the service tunnel. This produced not only a technical crisis but one which might jeopardize the time plan of the whole project.

Comatelec, the French company responsible for light fittings, turned to Schott-Rohrglass of Bayreuth, Germany for tubular casing material which could tolerate enormous heat while providing optimum light spread and minimal light loss. Schott quickly developed a profile glass, subsequently named "Conturax" to met the Comatelec specification.

The new glass tubes have a profile specially designed to strew or spread out light for better illumination. Boric oxide in the glass formulation has three major roles: to improve chemical resistance against the exhausts from the service trains; to provide heat resistance; and to improve the strength of the glass, enabling it to withstand shockwaves from the trains. The glass of the lamps contains 13 percent boric oxide.

This profile glass was cut lengthwise and then further processed. In the new lights, the original square cross-section cover was reshaped and replaced by a semi-cylindrical cover. Also a semi-cylinder module with grooves was cut to measure to reduce the glass's reflectivity, thereby improving light distribution.

By scaling the grooves in the best possible way, the new glass cover has the same optical light refraction and photometric properties as the original polycarbonate lights. But the glass covers needed a firmer and more stable support than the ones made of polycarbonate and therefore were held together with a pressed aluminum base of semi-hexagonal cross section.

These are reckoned to be the safest light fittings known to modern technology. They are stationed at eight and one half meter intervals on the French side of the tunnel and nine meter intervals on the English side.