I am asked all of the time how long fiber lasts. People want to know if they make the big investment in a fiber network whether they will get their money’s worth. And there is at least some reasons for the people to be confused. For example, I’ve seen opponents of municipal fiber networks say publicly that a new fiber network won’t last twenty years.

So how long does a fiber network last? The answer is – it depends mostly on how it is installed. While fiber is tough, it can be stressed during construction, which can significantly shorten its life. The number one cause of fiber damage during construction is damage caused when pulling fiber through ducts. There is almost no damage caused by either blowing or pushing fiber, making those the safest installation techniques. But it’s possible to overstress fiber when pulling, which will eventually result in it developing opacity. The opacity in fiber grows over time as very tiny cracks and stress points in the fiber grow larger and start deflecting light..

The second most common flaw in installed fiber is at splice points. Over time, as fiber expands and contracts from temperature changes, the splice points can shift tiny amounts and degrade the connection. Luckily, most damage from shifting splices can be fixed by re-splicing the fibers as they go bad over the years.

This does, though, speak to the issue of water damage to fiber. Water in itself doesn’t harm fiber, but if water gets into the sheath and freezes and melts over time it can either break splices or it can cause tiny flaws in the walls of the fiber to grow into bigger flaws. Over time, with enough little cracks and flaws, a given fiber can become unusable. So, just like with most other kinds of buried wires, care must be taken to keep water out of fiber lines in areas that will experience freezes.

But changing temperatures alone can do the same damage over time, so fiber that experiences wild temperature swings is not going to last as long as fiber that is protected from temperature extremes. Of course, burying fiber deep enough is possibly the best way to insure steady temperatures over time.

Fiber has gotten better over the years as manufacturers have improved manufacturing techniques. Today’s fibers are nearly perfect out of the manufacturing process and ought to last longer than fibers made thirty or forty years ago. The manufacturers have adopted techniques such as pre-stressing fiber during the manufacturing process (pulling it slightly) which pulls out any tiny flaws to keep them from getting bigger.

But none of what I said answers the question asked – how long will fiber last? Material scientists have been studying fiber since the 1980s and they have built models to predict how long fiber will last if properly installed. They look at all of the factors that can cause failure – how it was made, the presence of tiny flaws, factors that can cause cloudiness, the protection provided by the sheath, etc.

And what they found is very reassuring. Studies have shown that properly installed fiber will only have a chance of failure at a rate of 1 in 100,000 per kilometer per year between years 20 and 40 after installation. Statistics are funny things and that kind of rate is not easy to apply for a layman, but it ought to be obvious that this means very few failures for a normal fiber installation during that time frame. This means that fiber ought to easily last forty years and far beyond. Nobody will yet say how much further beyond, but I talked once to a few engineers from Corning and they told me that as long as it’s treated well that their best guess is at least 75 years. We’ll have to wait around to see if that is true.

The same scientists have studied real life applications of fiber and have calculated that the chances of buried fiber being cut is 1 in 1,000 per kilometer per year. This means it is 100 times more likely for a fiber to be cut than to have it fail from inherent flaws. Again, statistics like this aren’t straight-line ratios, but it you operate a 500-mile fiber network, this tells you that you can expect a fiber cut every year or so. And of course, some networks do worse than that. Outages from fiber cuts and the consequent weaknesses created by the repair splices are a far larger threat to your fiber network than any degradation of the fiber. So bury it deep!