Best practices quiz: answers for question page 4

Answer page 4 (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).

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#31. (ADSS) What's the maximum line angle you use for tangents? Suspensions? (Shortcut back to question page 4)

In designing for a 40 year life with little or no maintenance, it pays to be conservative in selecting attachment hardware.

The maximum line angle Fiber Planners recommends for tangents is 12 degrees, vertical or horizontal -- considerably more conservative than what the hardware suppliers specify. This limit is based on seeing others experience' using tangents for larger angles then having them chafe through hardware over time as cable moves back and forth to balance changes in cable loading.

The inside surface of a tangent has grit to increase the fiction so the cable doesn't easily slip until it has several hundred pounds of uneven tension to make it move. That friction and abrasion could damage the cable jacket and over time, damage the cable. (See the next question for an answer to avoid this condition)

Also, installers sometimes drill the attaching bolt into wood poles at something other than a 90 degree angle -- either by mistake or to avoid some obstruction. As a result, unless it's floated with a shackle (see the next question) the tangent may already effectively have a 10 to 15 degree on it before even considering the line angle on the pole.

Related to line angle limits are span length limitations when using tangents. Conservative designers usually avoid tangents on spans over 400 feet even if the vendor claims it can be used for longer spans. Above 400 feet, we recommend using Armor Grip Suspensions (AGS) and we limit their use to angles 25 degrees and under; for larger angles we use dead-ends.

#32. (ADSS) Do you "float" your tangents and suspensions with a shackle? (Shortcut back to question page 4)

If you want a 40 year life with little or no maintenance, then Fiber Planners strongly recommends the use of an anchor shackle with each tangent and suspension to "float" the hardware. The anchor shackle allows the hardware to shift temporarily an inch or two to equalize uneven tensions at the pole, due to wind or ice. Once the external force is gone, the anchor shackle can shift back to balance the pole.

If the "fixed" tangent design is used, the cable physically moves from one side of the pole, through the hardware and into the longer span to equalize the load. Once the temporary weather condition is gone, someone must physically go to each piece of hardware and manually shift the cable back through the hardware. This is unrealistic when designing for a 40 year life with little or no maintenance, as we do. In most cases, the extra sag will stay in the longer span for years until it interferes with other cables or something on the ground.

Letting tangents and suspensions float also compensates for any bolt holes not drilled perpendicular to the pole (see the previous question.)

#33. (ADSS) WhatÕs the ratio of dead-ends to tangents in your system? Dead-ends to suspensions? (Shortcut back to question page 4)

The ratio of dead-ends to tangents is not as critical as realizing that it takes many more dead-end structures to design an ADSS system than there are utility-labeled dead-end structures on your system. Fiber dead-ends are needed at storage loops, highway crossings, railroad crossings, windy valley crossings, vertical inclines and many more system features, even if these are not considered dead-end structures for the conductors. Experience and judgment are needed since the hardware manufacturer's instructions don't explain all the field conditions and their requirements.

If a tangent or suspension is installed where a dead-end is required, the cable will take the abuse of the friction or tension until failure starts. The bolt holding it to the structure may shear or the cable jacket may be rubbed enough to wear a hole in the material leading to later Kevlar® failure and a loss of the cable's tensile strength. Finally the cable will just fail and fall to the ground.

#34. (Distribution - U.S.) Do you always comply with the NESC requirements for clearances and for maintaining a 40" safety zone? (Shortcut back to question page 4)

The NESC is complex and has a number of exceptions to the 40" safety zone for certain power utility equipment like streetlights. But except for limited situations where 30" may be permissible, 40" is the required spacing between any ADSS in the supply region and the highest communications cable. As long as that limit is met, the NESC gives considerable flexibility as to where to place the ADSS in the supply region -- above the neutral, below the neutral, on an extension arm, attached to a crossarm, etc. Also, a designer can sometimes shift around other power utility items on the pole such as streetlights to get enough room for a good attachment location.

Even then, it can be quite challenging to determine an NESC-compliant attachment point on a "fully loaded" pole (crowded safety zone, transformer, downguys, streetlight, etc.) that installers can actually install without damaging the cable. Also, there's the requirement for 4" bolt spacing. The designer still has to understand power distribution design and construction well enough to come up with designs that are not too dangerous for linemen to install and don't hamper future line maintenance, such as transformer change-outs. Inexperienced ADSS designers may end up with more pole replacements until they've designed a number of routes.

If the power utility uses fiber cable lashed to a steel messenger, it must stay in the communications region. If there's not that 40" space between this new lashed cable and the supply region, the utility can't attach to the pole without replacing it first. (Our understanding of the NESC is that the 30" exception is only available for supply region lines encroaching on the safety zone from above, not for communications cables encroaching on it from below.) Extension arms are not allowed. Pole replacements are more common in this situation than even with a totally inexperienced ADSS designer.

Our clients' experience is that pole replacement including moving over everything on the existing pole ranges from $500 (new simple pole) to $5000 (fully-loaded pole: telco and cable TV cables, transformer, multiple power circuits, etc.) Unfortunately, it's the more loaded poles that usually require replacement.

#35. (Distribution) What per cent of poles do you typically have to replace on ADSS projects in order to maintain NESC clearances? (Shortcut back to question page 4)

Our own experience is a pole replacement rate under 1% to 2% on most projects.

#36. (Distribution) Do you run ADSS on any routes where you have a history of insulator contamination problems? If so, did you consider dry-band arcing in your design? (Shortcut back to question page 4)

In North America, dry band arcing (DBA) is not normally a consideration on lines under 100 kV. In areas of heavy pollution, failures have occurred on lower voltage lines in this country in a few rare cases. If your existing distribution plant has insulator contamination problems in certain areas, you should consider analyzing the DBA potential more closely as you would for transmission lines. If warranted, you may want to install a tracking-resistant cable.

Outside North America, reports of failure in lower voltage environments are more common. We have a tech note that goes into electric field effects in detail.

#37. (Transmission) Do you sag your OPGW and ADSS with a dynamometer, sight lines or a stop watch? If an outside contractor installs your cable, does someone from your utility check the sags? (Shortcut back to question page 4)

ADSS can't be sagged with a stopwatch. Depending on the particular designs, stopwatch sagging may be possible with most OPGW.

ADSS on transmission lines should always be sight-sagged using sight lines provided by the designer.

Dynamometers are useful installation tools but are subject to certain limitations: their accuracy in estimating tension on the actual section being sagged deteriorates if there are many spans between the dynamometer and the span being sagged. More importantly, ask your installer when he last sent his dynamometer to a certified calibration shop ("a certified what?"). We recently worked on a failure analysis study where 40 miles of ADSS was systematically installed using a dynamometer (but no sight sagging) at tensions 20% to 50% less than specified.

#38. (Transmission) Do you use OPGW or ADSS on your transmission lines? How did you pick which to use? (Shortcut back to question page 4)

Both OPGW and ADSS have their place on utility transmission systems. Nevertheless, many utilities are either "OPGW bigots" or "ADSS zealots" because they're comfortable with one product and have either heard bad things or had a screwed up installation with the other product. Either way it's a mistake. For more information on the pros and cons of each, technology advisory subscribers can refer to our tech note on the subject.

#39. (Transmission - OPGW only) Have you had any OPGW failures due to lightning? If so, how many? How did you pick the fault-current rating for your cable? (Shortcut back to question page 4)

OPGW, like traditional groundwire, occasionally suffers lighting damage to one or more wires in the outer layer; like traditional groundwire, these spots are repaired using armor rods. OPGW and regular groundwire are, after all, intentionally designed to be struck by lightning and to safely conduct the strikes to ground.

Recently, there's been an undercurrent of negative rumors about outright OPGW failures (not just damage) due to lighting. Yet actual reports are hard to pin down (if it's happened to you, we'd like to know more -- please contact us). We're only aware of 3 utilities where this occurred in the last decade but one of those utilities experienced multiple failures. Interestingly, we haven't heard of any in Florida, the state with both the highest incidence of lightning and the most installed OPGW. Were these failures a result of poor system design or bad luck?

In any event, even with these failures, OPGW's reliability track record remains outstanding.

#40. (Transmission - ADSS only) Do you use corona coils? If so, do your inspectors check to ensure that they've all been put on? Do you use vibration dampers? Again do you check that all get installed? (Shortcut back to question page 4)

ADSS installed in high voltage environments can be damaged by corona effects occurring at the ends of the attach hardware. Corona coils prevent this.

Corona coils can be hard to install if bucket trucks aren't in use and the dead-ends are long, since the installer has to lean out far in the air to shove them over the end of the hardware. Vibration dampers also require similar skill to put on if the dead-ends are long. As a result, occasionally a damper or corona coil may be accidentally left off. System acceptance inspection should include a careful check of every span that neither has been left off.

Towers at the bottom of hills should be inspected after heavy ice storms to ensure melting ice has not slid down and knocked dampers off the line.

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