Best practices quiz: answers for question page 1

Answer page 1 (of 5)

In the questions and answers below, "transmission" refers to installations on lines >100 kV and "distribution" refers to installation on lines < 100 kV. NESC refers to the National Electrical Safety Code which governs all electric utility installations in the U.S. (and any other jurisdictions that adopt it).

You can work through the questions on all 5 pages, then look at the answers; take the questions one page at a time followed by their corresponding answer page; or hop back and forth by following the shortcuts at the end of every question or answer. Finally, if, like many people, you get sick of all the clicking, you can just go to one long page with all the questions and answers.

#1. (U.S.) Do you still meet NESC clearance requirements at your worst case wind storms and ice storms? Would your fiber survive these storms? (Shortcut back to question page 1)

First, the code requires that you meet clearance requirements. Second, you can be sued if you don't meet code and there's a problem. Third, recent extensive research on ice storm historical data has indicated that in many areas, your fiber network may likely experience at least one ice storm exceeding NESC loading during the life of your fiber network (we've got a tech note on this). Finally, heavily sagging fiber cables blocking roads get cut by cleanup crews if they're in the way.

#2. Did you find any damaged (step attenuation increases of 0.1 dB or more?) fibers after your installation was complete? (Shortcut back to question page 1)

It is reasonable to install hundreds of miles of ADSS or OPGW without one damaged fiber (let alone broken fibers). These cables contain either Kevlar (ADSS) or steel (OPGW) and are very strong -- theoretically, you could tow a trailer with this stuff and never damage a fiber.

We're defining "damaged" here as a step increase in attenuation of at least 0.1 dB.

While a step increase of just 0.1 dB is an insignificant amount of loss, it's an indication that something is pinching or stressing the fiber. That small step increase could easily become a break with a change in temperature or cable loading, depending on the cause of the step.

Furthermore, from our failure analysis consulting work, we've seen that certain types of installation damage can leave no external marks on the cable and yet still significantly damage long runs of the plastic (ADSS) or metal (OPGW) buffer tubes that protect the fibers inside the cable. Small changes in temperature and cable loading can cause the appearance (or disappearance) of attenuation steps all along these damaged stretches -- if you have an 'event' on one fiber, it may be a matter of time before you see more point defects on other fibers.

Some utilities and installation contractors have become used to a few damaged fibers during their installations and just shrug it off as "something that happens". That's absolutely wrong! Competent crews installing fiber cable under attentive supervision and following proper procedures routinely install fiber for years with no damaged fibers.

Depending on the circumstances behind a fiber fault, we may recommend an entire reel be replaced even if there's only a fault in one location.

#3. If you did, did you know how they were damaged? (Shortcut back to question page 1)

This question is perhaps more important than the previous question. As noted, faults don't "just happen". And one fault may just be the tip of the iceberg -- if a long stretch of cable is damaged, many more may just be waiting to appear with a change in the weather.

If an installation crew used improper installation techniques on one reel of cable, we've found they were likely to have done it in many other places. It's critical to figure out exactly the reason for any failure so you can determine where similar damage was done.

Installation crews aren't the only possible culprits -- for instance, designers can specify the wrong cable or hardware and hardware manufacturers can ship the wrong cable attachments.

#4. Are you sure they were damaged and not just an OTDR misinterpretation? (Shortcut back to question page 1)

We're big believers in the usefulness and accuracy of OTDRs (Optical Time Domain Reflectometers) -- two of us used to work for an OTDR manufacturer. We think any utility with fiber should own an OTDR (or mini-OTDR). Nevertheless, we've observed that the accuracy and power of modern OTDRs often exceeds the skill of the operator using it! Before "blowing the whistle" on fiber damage, the operator should make sure they've got their instrument set up correctly (we've got a tech note on proper OTDR setup). In particular, they need to ensure they have enough dynamic range and have selected the correct pulse width such that they're misinterpreting noise (static) in the display as a glitch in the fiber.

#5. Can you reconcile the differences between OTDR distances and actual locations (Shortcut back to question page 1)

OTDRs are very accurate in determining the distance to a fault on a fiber -- typically within a few feet. More accurate, in fact, than the machine that prints the foot markings on the cable jacket (which may be off by as much as 1%) and more accurate than most utilities' as-built records. Finally, the OTDR operator needs to enter the correct index of refraction value. It's reasonable to expect an OTDR operator to nail the fault location within a few feet, but it doesn't happen without proper training ahead of time and good record-keeping. Otherwise, even with a properly-working OTDR, it's common for technicians to be off by 1% to 2% on fault locations -- that's 300 to 600 feet on a 6 mile run!

#6. Later, did any fibers start displaying damage for reasons other than obvious external causes (tornadoes, etc.)? (Shortcut back to question page 1)

See the answer to the next question.

#7. If so, do you know why? (Shortcut back to question page 1)

Most installation damage shows at least one fault during final acceptance testing, but for the reasons explained above, it could hide until a change in temperature or cable loading flexes a damaged buffer tube and breaks or pinches a fiber. It's important to aggressively track down the cause even for a minor fault to ensure it's not likely to happen elsewhere.

Cable manufacturing quality for the manufacturers on our approved list is exceptionally high -- fiber faults that occur are usually going to be due to a different cause, but it's still important to check each reel before ruling out manufacturing errors as a cause.

#8. Do you test incoming cable on the reel? If so, have you ever spotted a defective fiber? Why do you test incoming cable? Do you keep all the results? (Shortcut back to question page 1)

As mentioned, cable quality is very, very high among our approved vendors. They ship tens of thousands of good ADSS reels without shipping a defective reel. All of the cable is 100% tested with OTDRs and documented at the factory before shipping. Incoming acceptance testing protects the cable purchaser from damage by the shipping company.

Some users don't test fiber cable on incoming inspection and we think that's a calculated risk to save time and money. We recommend taking this extra step and documenting the results; doing so protects the utility if a contractor subsequently damages the cable ("we didn't do that -- the trucker must have damaged it!")

If by some chance, you did receive a defective reel of cable on incoming inspection in the last 5 years, we'd be interested to hear about it and who the vendor was.

Finally, we should note that OTDRs are good at spotting point fiber defects on incoming reels. They are less accurate at measuring dB/km -- you may often find your measurement of dB/m varies from the factory's by 0.02 dB or so. Keep this in mind if you're trying to measure vendor performance against dB/km specifications.

#9. Do you know where all the long distance carriersÕ fiber cables serving your area cross your route and have you made provisions for potentially connecting to them in the future? (Shortcut back to question page 1)

See the answer to the next question

#10. Have you identified additional corridors carriers might use in the future and made provision to run taps to them in the future? (Shortcut back to question page 1)

Tying your network to long distance carriers' fiber may increase the potential revenues you can realize from dark fiber leases if you can then connect them to local businesses on your route and enable them to cut out the expense of going through the local carrier (usually Bell). It's important to identify where these carriers are crossing your route (or might in the future) and leave slack storage at those locations to facilitate splicing in taps to their fiber.

Go on to question page 2 (page 2's questions are quicker to answer)

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