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OSP Engineering
More About Cable Locating
Last year I wrote a column on locating fiber cables. Colin Ward from Norscan offered some comments that I think might be helpful to you.
Hi, Don,
I recently came across your 5 Questions About Fiber Optic Bonding, Grounding, and Locating article (https://isemag.com/2016/09/5-questions-about-fiber-optic-bonding-grounding-and-locating/), and it’s sparked some interesting discussion around the office. I wanted to share our thoughts with you.
The line that stuck out the most to us was this one:
Assuming that fiber cable locating is the only reason to ground the sheath or locate wire, why not just do it during cable locates, and then remove the ground when the locate is completed?
We had a couple of thoughts regarding this point. First off, we believe there are other reasons to ground your sheath or wire — namely to protect your cable from dangerous transients such as lightning strikes. When lightning strikes, it searches for the best path to ground and that can very easily be the cable armor. This can cause tremendous damage to the cable. Over the years, we’ve seen many cables end up looking like this after a lightning strike. (See Figure 1.)
Figure 1.
By not re-grounding the outside plant, we force transients to escape through the sheath and pierce the protective layers. Hard grounding delivers the transient to ground and therefore limits the potential for damage.
This leads us to our next point regarding locating. While we believe it’s important to keep your sheath grounded, doing so does limit the distance the locate signal can travel. Just like a transient strike, the locate signal can only travel as far as the next ground point. This translates into a lot of extra effort on the part of the locator, as he will have to repeatedly setup his portable transmitter after each grounding point.
In response to creating greater efficiency when locating cable, we developed a system that keeps the cable sheath grounded at all times when not locating, and lifts the ground when locating to allow the signal to pass through. This ensures the cable is protected from dangerous transient surges while also enabling the locate signal to travel far distances. We also have a system that eliminates the need to physically access the outside plant with a portable transmitter when locating. If you would like more information, you can contact me, Colin Ward, Norscan Instruments: [email protected].
Thanks,
Colin
Norscan’s Touchless Locating system offers one answer to efficient fiber locating while protecting your shield or locate conductor from phase power or lightning. I’ve also heard of Vivax with their FLS-2 system, and they offer a similar product for long distance fiber optic cable locating and monitoring.
I still feel that, assuming that fiber cable locating is the only reason to ground the sheath or locate wire, why not just do it during cable locates, and then remove the ground when the locate is completed? I believe that un-bonded metallic shields and wires are not susceptible to lightning for this reason.
In my 50+ years in the business, untold copper and fiber cables, copper and fiber drops, have been plowed in, trenched in, and placed, by horizontal boring. I do not know of one instance where an ungrounded cable shield was struck by lightning before it was tied to a bonding and grounding system.
In the same time factor a myriad of improperly bonded and grounded cables have been destroyed by lightning including the end user’s equipment. Once those cables are grounded you better have done a proper job of bonding and grounding. Improper bonding and grounding also creates a safety factor.
NOTE: If your company’s safety practice states bond and ground your fiber system, go bond and ground.
To properly locate buried fiber cables with a portable cable locator, an independent ground must be placed on the metallic shield or locate conductor at the far end, and an independent ground must be placed at the transmitter. When measuring the resistance between the 2 grounds with an ohm meter the resistance needs to be less than 1,000 ohms.
The transmitter must be set to a frequency less than 1Khz. At any higher frequency, the signal will capacitively couple to other utilities in the area, resulting in a miss-locate.
Signing off
Thanks, Colin, for your input. Cable locating is a critical practice that requires both the right tools and the training, and experience. If you have thoughts on this or any outside plant issues, please call 831.818.3930 or email [email protected].
