Build your outdoor fibre optic cables with the best suited jacket
Outdoor fibre optic cables come in a variety of constructions or finishes. Each have their own particular characteristics and applications. The difference between these finishes is sometimes difficult to understand. In this article we attempt to throw some light on the differences and what advantages each one offers.
For many years the standard finish for a telecommunications cable was a single polyethylene jacket, also referred to as PE or polythene. This material replaced lead as the material of choice for outdoor copper cables in the 70’s. As the telecommunications network in Australia expanded, it became evident that termites were a problem. They could easily chew their way through PE and make nests inside the cable. The interstices between the conductors provided a ready made home for them. Extensive testing by Telecom Australia identified that a smooth and glossy coating of Nylon 12 provided a suitable long term barrier to termite attack. The hardness of the material in combination with its glossy finish prevented a termites mandibles from achieving a grip on the material. This helped in stopping them from chewing the cable. PE or polythene, therefore became the standard finish for fibre optic cables in Australia. More than 70% of fibre optic cable for outdoor applications in Australia is thereby PE/Nylon jacketed cable.
What is a Sac Sheath?
When installing fibre optic cable, it is often hauled or pulled through conduits or channels. This can result in scuffing or scratching of the outer Nylon jacket which removes the shine from the Nylon. If the scuffing or scratching is deep, termites can now achieve a hold on the jacket. This opens up the cable to termite attack. To combat this situation, an application of a PE layer over the nylon, protects the cable from damage during handling and installation. This is referred to as a Sacrificial or Sac sheath. The normal designation is PE/Nylon/PE or Sac sheath.
Corrugated Steel Tape Armour jacket
In addition to a termite attack, outdoor fibre optic cables can also suffer damage from rodents and other vermin. The nylon layer in a standard PE/Nylon cable is considered quite tough. To sustain an attack from rodents however, it is nowhere near robust enough. Their teeth are sharper and their action is more of a gnawing action as opposed to a simple bite or chew. As a result, the cable will require a supplementary barrier. The best and simplest barrier is a longitudinal steel tape. It is often corrugated to improve its flexibility. CSTA (Corrugated Steel Tape Armour) is this type of barrier, and it is usually applied under a PE layer.
Low cost CSTA construction
The CSTA/PE/Nylon finish has the nylon on the outside and is the cheapest to construct. When the cable is attacked by a rodent, it will eat through the nylon and PE before hitting the CSTA. CSTA can stop it going any further. As the rodent gives up and leaves, the cable is now susceptible to termite attack because the Nylon barrier has been broken . Not only does this result in rodent damage to the underlying PE, there is now also a negligible protection for the fibre optic cores. Despite this, it is a common and low cost construction of outdoor fibre optic cables.
More secure CSTA construction
A better approach is to put the termite barrier under the rodent protection which leads to the most secure approach of a PE/Nylon/CSTA/PE cable. Costing more than it’s CSTA/PE/Nylon cousin, it has the advantage that any rodent attack does not degrade it’s termite resistance or cable robustness. PE/Nylon/CSTA/PE designs are the recommended option for long term protection from rodents and termites.
Non-metallic rodent barrier jackets
In some instances, it is not possible to use a metallic barrier for rodent protection. This is often the case when there are power cables in the near vicinity or where lightning strikes are a regular occurrence. For these types of settings, a non-metallic rodent barrier is required. There are two options available, both of which use glass fibres as a rodent repellent barrier.
1- Glass Yarn
The first option is to use a flexible yarn made of, or incorporating, glass slivers in its construction. When chewed the glass yarns repel the rodent resulting in protection for the cable. It is common to put the glass yarns under a PE jacket, and again you have the option of putting a nylon barrier either under the yarns or over the PE barrier. So you can have a finish that is PE/Glass yarn/PE/Nylon or PE/Nylon/Glass Yarn/PE. The later option, similarly to PE/Nylon/CSTA/PE provides some protection to the Nylon from rodent attack.
2- Glass reinforced plastic/Glass composite ovals
A second alternative for a non-metallic rodent resistant cable is to use glass reinforced plastic (GRP) rods. These are usually round in cross section, but can also be oval in which case they are called glass composite ovals (GCO). As well as reinforced with glass, they are also very tough (akin to fibre glass) and offer additional mechanical protection to the cable. The location of the GRP is optional so you can have PE/GRP/PE/Nylon which can suffer from a rodent attack that will render the nylon barrier ineffective, or a PE/Nylon/GRP/PE version instead, where the rodent protection is over the nylon.
Both of the glass yarn and GRP/GCO forms of protection are non-metallic rodent resistant designs. They provide protection from rodent attack, but do suffer damage during the attack. Both can be breached if the cable is experiencing consistent rodent attacks at the same location. In the case of infrequent rodent attacks, they can last about as long as a CSTA design. Where the rodent population is high or the cable suffers persistent attacks over a longer time frame, they are susceptible to eventual failure. In such an instance, a metallic CSTA type offering is of preference.
Low Smoke Zero Halogen jacket
For tunnel work or restricted access sites, the propagation of fire is a concern. In such cases a fire retardant cable construction can be deemed necessary. Since PE is part of the same family as paraffin oil, it is a flammable material and not suitable for such applications. A low smoke zero halogen material (LSZH) substitution is necessary. All the constructions listed above can therefore be reproduced whereby the PE is replaced with a LSZH material.
In the end, it is up to the installer and the customer to determine which finish is best suited to their outdoor fibre optic cable project. From the standard PE/Nylon finish to Sac sheath, CSTA, non-metallic armoured and LSZH cables, there is an option for every application. Where the installer or customer cannot decide or are unsure, the cable supplier should be able to provide guidance and recommendations to meet their end goal.
See our range.
Madison offers a range of finishes as well as custom designs for specific applications. Contact your local sales representative for further information.