The Channel Tunnel (nicknamed the Chunnel) between England and France is in fact three tubes:
- One takes trains from the Dover area to Calais
- One takes trains from the Pas de Calais to Dover
- One in the middle to supply services to the other two
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. At more than 31 miles (50 km), it is not the longest tunnel in the world, it does have the longest undersea portion (23.5 miles or 37.9 km). Its complexity and capacity make it truly an engineering triumph of the modern age.
Light Fittings: Specialized Requirements for Safe, Reliable Illumination
From the beginning of the tunnel’s construction, the obvious need for bright, reliable illumination presented technical problems for which no real precedent existed. 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 like those used for the construction work. This solution was banned by the international safety committee. In fact, a ban was placed on all use of plastic in the service tunnel, resulting in a technical crisis that might jeopardize the timing of the whole project.
Comatelec, the French company responsible for light fittings, turned to Schott-Rohrglass of Bayreuth, Germany for tubular casing material that could tolerate enormous heat while providing optimum light spread and minimal light loss. Schott quickly developed a profile borosilicate glass, subsequently named Conturax, to meet the Comatelec specification.
Borosilicate Glass Offers a Brilliant Solution
The new borosilicate glass tubes had a profile specially designed to spread out light for better illumination. Boric oxide in the glass formulation has three major roles:
- Improve chemical resistance to exhaust from the service trains
- Provide heat resistance
- Improve the strength of the glass, enabling it to withstand shockwaves from the trains
The glass of the lamps contains 13% boric oxide.
The 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' reflectivity, thereby improving light distribution.
By scaling the grooves in the best possible way, designers enabled the new glass cover to provide 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. The resulting light fixtures are among the safest light fittings known.