Use of Lead in Fiber Optic Cable Sheathing and its Alternatives

Lead sheathed fiber optic cables are not common and the end-users for such cables are often limited to petrochemical projects. Petrochemical projects, where the presence of many chemicals, acids, etc can not be avoided is a sensitive area of installation of fiber optic cables. The outer sheath of the fiber optic cable shall be capable to resist the harmful effects of chemicals and acids in the outdoor environment.

Lead is known for its acid resistance. Lead can be extruded and can be made a sheath in fiber optic cables that will give enhanced acid resistance characteristics to the cable. Lead (pronounced as ‘led’) which is abbreviated as Pb in the atomic table of elements, is an excellent moisture barrier also. Lead sheathed fiber optic cables perform well when laid underwater. As the lead protection is metallic protection that is fully covered without the use of overlapping unlike corrugated steel tape or aluminum tape, the moisture resistance performance of lead sheathed cables is good.

A lead sheathed cable will have an inner sheath often done with polyvinyl chloride (PVC) or polyethylene either normal or flame retardant grades. The lead sheathing is done over an inner or intermediate sheathing, called bedding in the extrusion industry. The lead sheath is covered with another layer of polyethylene or PVC.

Lead sheathed fiber optic cable

Lead (Pb) is a heavy metal, but at the same time, it is soft. The weight of the lead sheathed cable is a major disadvantage and the installation becomes more difficult. Most often, lead sheathed cables are armored by using galvanized single steel wires that give required pulling strength. The density of extrusion grade lead used in the sheathing of fiber optic cables is approximately 11.3g/cc. The density depends on the alloy type and combination.

Since the density is high, the lead’s contribution to the weight of a wire armored fiber optic cable is almost more than 50%. For example, if the inner sheath over which the lead has to be extruded is 10mm, and the lead thickness is 1.5mm, the weight of the lead will be approximately 615kg per kilometer of the cable. Fiber optic cables are known for their low weight and reduced diameter. The advantage of low weight and reduced diameter makes fiber optic cable installation easier and faster.

There is a concern about environmental pollution from the lead. Lead materials are tagged as poisonous and dangerous. People who argue in favor of lead may disagree with the above statement. The operators who work in a lead extruder have to take extra precautions to avoid the harmful effects of the lead extrusion. Some cable manufacturers provide milk to the operators to drink. Consumption of milk is said to reduce the poisonous effects of lead.

The lead extrusion process is complicated and needs great care during the process. The pipe extrusion method makes it difficult to maintain the thickness of the lead sheath throughout the length. A thickness of 1.5mm nominal is generally provided. Recently the thickness and concentricity of sheathing can be well controlled with the use of the latest technology in the lead extrusion process

Extrusion grade lead comes in the form of ingots are loaded into the electrically heated melting pots. The heating temperature is around 380 degree Celsius. The melting point of lead is approx. 327 degree C.

Lead sheathed fiber optic cables are expensive, but worth it if the installation conditions demand such a high quality cable. An alternative to lead sheathed cables have been proposed by several cable manufacturers. Some of the lead free cable designs include a combination of special polymers and aluminum tape. Polyamides are known for their excellent hydrocarbon resistance. Aluminum tape is a good moisture barrier.

The advantage of lead free alternative designs are the lower cable weight and reduced diameters, which can be beneficial in the installation. The civil construction cost is the highest contributor to the fiber optic deployment project cost. If heavier and bigger lead sheathed cables need extra civil works that will increase the project cost substantially, better to look for competitive and alternative lead free design options. The job here is to ensure the desired performance parameters for the fiber optic cable.

As we wrote above, an advantage of lead sheathed fiber optic cables is their good water resistance. Lead sheathed cables can be laid in water for many years, probably more than the guaranteed life of optical fibers and the outer sheathing materials used to construct the cable. If that performance comes with an extremely high cost, buyers can look for alternative designs.

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