For repairs I’m guessing there will be no or very limited splicing you can have connectors at each pole and simply run a new fiber if you have a fiber break mid line the main reason for splicing fibers today in below ground installations is that pulling them out requires you to dig up a whole bunch of dirt if it’s just plug and let the robot rewrap the fiber along the power line I think it’s cheaper to do just that.
On the illustration the winding is very sparse, it definitely does not look like running into minimal bend radius problem or adding much length.
If the winding is sparse, let’s say 1 turn every 1m on a 38mm diameter cable, then the amount of length added will be ~1% of the total linear length (neglecting the cable sag). So the amount of added length due to winding it will negligible.
At 1000 bends per km, if we assume loss is 0.001dB per bend, then we’d add 1dB per km.
http://www.prioritywire.com/brochures/Medium%20Voltage%20Pow...
Thanks for sharing it. It gave me a very interesting perspective on the problem.
Wondering if there's any advancement in fiber production that reduces these problems (cracks, etc), or reduces dB of loss over distance.
OP is concerned about extra cable length that is needed with this approach (completely unfounded), drops a few attentuation loss over distance calculations, but really, the practical engineer says: "we'll have only as many winds of fiber per meter as is neccessary"
I did a quick calculation with a simple assumption and admitted that my initial estimation of added length was incorrect and it’s not an issue.
The bends around a narrow diameter cable are a potential issue for both added loss and cable reliability as are the different rates of thermal expansion.
You don’t need to take my world for it on the loss. https://amphenol-ns.com/Blog/post/bend-radius-protecting-fib...
As far as the reliability goes, I wasn’t sure so I looked it up, see Table 1 on page 6 below. There’s an example of cable failure for a typical FTTH network. If you see this, then you’ll note that the number of turns and the radius of those bends is important in predicting the reliability of the cable plant. It’s worth noting that the cable plant failure mechanisms are dominated by breakage due to other things and not mechanical fatigue.
As long as the bend radius is >10mm the failure rate should be <0.5ppm per turn. For a 38mm diameter cable the failure rate <0.1ppm/ turn.
https://www.corning.com/media/worldwide/coc/documents/Fiber/...
Of course, that aside, cables generally don't need repair that much / often if they aren't in areas with extreme weather. And if they are, medium-power lines can go underground (as can the fiber optics).
They don’t mention the length penalty that comes from wrapping around the cable or any impact of loss to fiber itself. Given the helical pattern, I’d imagine that they are adding substantial length to any distance they need to achieve. Generally you have about 20dB of loss before you need an amplifier or a terminal node. A typical number for field installed fiber is 0.25dB/km. So there’s some distance penalty needed here. I’m not clear on how you’d add amplifiers to these routes or if they are simple short hops.
Generally regular SMF-28 has a bend radius of 25mm. This is partially for loss reasons as bends below this radius induce loss and partially because bending fiber below that radius creates micro cracks in the fiber. The micro cracks can spread with time and cause the fiber to break. Given the rather large CTE mismatch between the power cable and the fiber cable, the 2 will expand at different rates with temperature, putting additional stress on the fiber, potentially leading to failure.
It’s possible to have a less bend sensitive fiber but it’s generally used more in specialty applications and not as transmission fiber. So it’ll be more expensive per km.
Along the lines of mechanical failure, it would seem that there would be the potential for abrasion between the power cable and the fiber cable jacket.
Finally, it’s a little unclear how repairs would be made. Doesn’t seem very safe to have to splice the fiber on a live cable like this.