The basic mechanical failure mechanism for optical fibres is the slow to rapid growth of any glass imperfections in the fibre caused by the fibre being under stress. This ‘fatigue’ phenomenon can be accelerated with the presence of moisture (H20) molecules at the glass surface of the fibre. Craig Buckingham and Richard Kaehne from AFC Group explain.

The greater the concentration of water molecules (OH – ions) at the glass surface, and the greater the stress applied to the glass, and subsequently, the more rapidly the surface imperfections will grow. This accelerated fatigue in the presence of OH- ions is similar to “stress corrosion.” The speed of imperfection or “crack growth” in optical fibres is very dependent on the size of the flaw in the fibre.

To insure that no flaws greater than a predetermined size are present in finished fibre, fibre manufacturers subject their fibres to a brief elongation or stress, a process called proof testing.

All manufacturers of fibre optic cables intended for use outdoors must address the issue of protecting the fibre’s glass surface from the presence of moisture. This is because the 250 μm primary fibre coating provides only a 62.5 μm-thick layer of UV-cured acrylate material as basic protection over the fibre’s glass surface. This UV-cured acrylate material is not chosen by the fibre manufacturers for its optimal resistance to water or its minimal porosity. It is in fact chosen primarily because of its fast processing speed, since a primary cost driver for fibre manufacturers is the draw speed, which is steadily increasing. The very thin UV-cured acrylate layer is porous to water molecules and will permit concentration of OH- ions at the fibre surface, if the fibre is immersed in water.

All plastic materials are porous to varying degrees. The general category of thermoplastic materials commonly used in cable constructions will to some extent absorb water; however, thermoplastic materials certainly do not act as a complete water block. Only materials like metals or glass can provide a true “hermetic” seal. Plastic materials are generally characterized with parameters such as water absorption and absorption of other common solvents such as oils, gasoline, kerosene, etc. This being the case, water molecules cannot be eliminated from the glass surface of any fibres incorporated in a cable having plastic jackets. The issue is to minimize the concentration of water molecules at the glass surface so that stress crack growth effects are minimised.

There are two different design approaches to water and moisture protection in fibre optic cables. The loosetube gel-filled cables must prevent water from reaching the 250 μm coated fibres.
The approach is to “waterproof” the cable by “filling” the empty spaces in the cable with gel, theoretically preventing water from reaching the 250 μm coated fibres. To insure that this is accomplished, the “filled” cables are generally subjected to a hosing test to show that water will not flow through a short section (one meter) of cables. The fact that gels can move, flow, and settle, leaves an uncertainty of the filled level of any particular point of a loose-tube gel-filled cable. This uncertainty of the filling is highlighted by the routine practice of water-blocking the loose-tube gel-filled cables at the entrance to splice housings to keep water from migrating from the cable into the splice housing.

The tight-buffered, tightbound indoor/outdoor cables utilise an entirely different design approach to deal with the moisture issue. Rather than attempting to be “waterproof,” they are designed to be water tolerant.

Recognising the porosity of plastic materials and the inherent problems of waterproofing a cable, the moisture protection is concentrated at the fibre surface where it is most needed.

Correctly designed harsh environment tight-buffer systems consist of extremely low moisture absorption coefficient materials at the fibre coating. This provides a buffer system thickness of 387 μm over the glass which is more than six times as thick as the 62.5 μm coating found in the loose-tube cables.

Buffer materials are low-porosity plastics with excellent moisture resistance. This construction very effectively minimizes the water molecule and OH- ion concentration level at the glass surface and virtually eliminates the stress corrosion phenomenon. The tight-buffered design also has the great advantage of being a solid, non-flowing, non-moving structure. The same level of protection remains in place all along the fibre, regardless of installation conditions, environment, or time.

The balance of the tight-buffered, tightbound cable designs is such that it minimizes the open spaces available in the cable structure in which water can reside. Even if an outer cable jacket is cut, or water otherwise enters the cable structure, only a very small percentage of the cross-sectional area is open to water.

In Australia, we are guided by AS/ACIF S008 2010 which refers to “Water Penetration specified in Clause 25, Method –F5B of IEC 60794-1-2 [28]. Of particular interest is the following:

1. Water penetration refers to the effectiveness of a cable in restricting the longitudinal movement of water or moisture along the core. This requirement is primarily intended to localise any water penetration to minimise the adverse effect on cable performance and to prevent water or moisture leaking into joints and terminations that may cause corrosion problems.

2. Additionally, cable installed underground should have a high density compound sheath material (such as polyethylene) that provides an adequate barrier to moisture entry to the cable core. The addition of a lapped metal tape (‘moisture barrier’) and/or grease or gel within the core (‘filled’ or ‘flooded’ cable) provides even higher protection against moisture entry.

The above considerations are very important and should always be considered. The most critical consideration is that the cable used is fit for purpose. Always refer to the manufacturers Specification Sheet and follow their installation instructions.