Ask anyone from England and they will tell you that Australia has the most poisonous and most annoying animals and insects in the world.

Take out all of our poisonous spiders and snakes and harmless but irritating flies and mosquitoes and you are still left with an array of insects and animals that can cause damage to a fibre optic network, namely rodents and termites.

It is because of these two pests in particular that cable being used in underground systems needs special design considerations.

Here we outline design features of a cable structure to assist with long term durability.

Termites have historically been a major problem for installed cables and various methods have been developed to protect cables from termite attack. The most common method now employed is a nylon sheath, or “hard Jacket”. There are different theories as to why Nylon protects cable against termite attack. A commonly held theory are that the nylon is basically hard enough to stop the termites, another is that the nylon is very smooth and does not allow the termites pinchers to get a purchase on the nylon. A further belief. that is an extension of the “smooth surface” theory, is that any scratches on the sheath may then allow the termites an opportunity to bite into the nylon, and that a sacrificial jacket should be added to the cable to protect the nylon from scratches. An opposing theory suggests that the sacrificial jacket can actually give the termite something to push against as it attacks the nylon and as such is counter-productive.

From a historical viewpoint, the main termite barrier that we deploy in Australia for underground Optic Fibre Cables is a simple nylon jacket. A sacrificial jacket, (usually Polyethylene), is sometimes added, but would account for less than 1% of cables installed.

All of the major Telcos deploy cable using a simple nylon jacket, in both Duct and Direct Buried cables, and feedback over the past 10 to 15 years indicates that this simple form of protection has, and continues to be, a very effective barrier to termite attack. The fact that 99% of cable deployed is without a sacrificial jacket seems to suggest that it is not needed. This is purely general market and field feedback at this stage and not backed by any scientific research. This tends to suggest that the addition of a sacrificial jacket may be an unnecessary additional cost.

A Nylon sheath will provide a good level of resistance to white ant/termite attack but will provide little or no resistance to rodent attack. Any cable that needs to be rodent proof will need to be physically armoured with a material that can stop rats and other rodents from penetrating it. The two options found to be most effective under local conditions are a Glass Reinforced Plastic rod (GRP) armour or metallic Corrugated Steel Tape (CST) armour. The first option has the advantage of being non-metallic, so if an all dielectric cable is needed it is the best option. This design of cable is armoured by non-metallic GRP flat rods. It offers very good crush resistance and is rodent proof.   

Figure 1 has been taken from an independent report commissioned to test the performance of the GRP design. The cable at the top of the photo shows the GRP design. The Rats can penetrate through the outer sheath but cannot penetrate through the GRP armour. This means the inner core of the cable is fully protected from an attack by rats. The bottom cable in the photo is a standard loose tube cable and was totally destroyed by rats within 7 days. In fact, some standard test cables were destroyed within 24 hours. It also highlights the fact that a nylon sheath offers absolutely no protection from rodent attack.

An area of concern with some overseas designs is in regards to OH&S. As above, the Australian designed cables use GRP rods to fully armour the cable and offer the robust protection needed to block the rats. Some overseas designs use a tape that incorporates slivers of glass in the tape. The theory is the rats eat into the tape, experience pain as the glass cuts their mouth, and do not proceed to eat it. Working with this tape in the field when terminating the cable may raise OH&S issues. Testing of this cable under Australian conditions has shown that this method of armour is totally ineffective as shown in Figure 2.
 
The final piece to the puzzle is the use of additives (bittering agents) to deter termites eating the outer sheath of the cable. In initial studies some of these have been found to be very effective but have not become widely used as the Nylon sheath with GRP armour has been providing adequate protection.

Always look for cables manufactured for Australian conditions. The easiest way to determine this is to buy product manufactured in Australia.

Craig Buckingham RCDD has been involved with the ICT industry since beginning his career as Telecommunications Mechanic in the British Army 15 years ago. He has worked on all facets of the cabling industry including network configuration, cabling installation, design and consultation. Craig has been with AFC for 4 months now as their Technical Development Manager delivering technical Learning Sessions to Consultants and End Users alike.

Visit www.afcgroup.com.au.