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OSP Magazine JUNE 2008

I recently taught a noise mitigation class. Or I should say that’s what I was told I would be teaching. But as is often the case, most of the class was surprised to find out how often the problem is bonding and grounding and not distribution power problems. In fact, I would bet that 80% of your noise issues are really missed bonding and grounding problems.

It’s unfortunate that many technicians see bonding and grounding as unfulfilling work. I don’t think it is at all: it requires determination, strong skills, a big picture understanding of POTs to Noise, and often a creative approach. Regardless, whether you like or hate bonding and grounding, it has to be done, and having good equipment and a methodical process will help you solve many of the “noise” problems that are really bonding and grounding issues.

Bonding and Grounding Processes
To begin, there are strict requirements for bonding and grounding telephone cables and telephone service interacting with the distribution power system. There are two parts to the process: 1) protection, and 2) mitigating higher than acceptable AC interference to all circuits that use paired copper conductors.

Lightning Protection
Direct lightning strokes to plant will cause widespread damage because of the magnitude of the currents involved, sometimes in excess of 100,000 amps. Lightning surges may also reach the conductors and shields by currents from wire facilities not enclosed in a metal shield such as a drop wire.

To prevent failure of the conductor insulation, all conductive elements must be of the same potential. That is why ground rods are necessary in all buried pedestals. To prevent the cable conductors to be blown open all conducting elements of a cable have to be at the same potential. The shields of all aerial cable sections should be bonded together and to connecting underground or buried cable shields and to the central office ground.

All of the strands should be electrically continuous and the cable shields bonded to the strand at all terminals at the right intervals. Any of the old ready access terminals should be replaced with the newer terminals and any degraded wire work should be rebuilt.

Phase Power Protection
The practices that apply to lightning also apply to phase power protection. They provide low impedance paths to ground which will aid in rapid de-energizing a phase power line in the event of a contact.

NOTE: The National Electrical Safety Code (NESC) also requires this type of grounding.

Protection of telephone cable in all areas normally should consist of complete bonding and grounding cable shields at central offices and remotes. The shields and other conductive members of the plant entering a central office or remote should be bonded to each other and to the central office or remote ground.

Shield continuity (bonding) must be maintained in order to provide a path to ground for lightning and power currents and to provide an effective noise shield. This will also distribute ground return current over a large cross sectional area.

The distribution plant should be bonded a minimum of 4 times a mile. Where long runs without splices or terminals are involved, at least 1 ground per mile should be provided even if the cable sheath has to be opened solely for this purpose.

If more than 1 cable is attached to the same pole, the shields of the various cables should be bonded together. This should take place at crossing poles, at the beginning and ending of multi-cable runs, and at approximately 1,500-foot intervals in long multi-cable runs.

Crossings between aerial telephone cables and electric distribution lines should be made on jointly used or jointly occupied poles. At joint pole crossings with Multi Grounded Neutral (MGN) type power lines, the cable strand should be connected to the MGN with a vertical pole ground wire.

When dealing with joint use or joint occupancy connections should be made where the joint use or joint occupancy arrangement begins or ends and bonds should be made to the MGN every 1/4 mile (quarter mile).

Application of an effective grounding system is recommended for all locations. Both the National Electrical Code (NEC) and the National Electrical Safety Code (NESC) quote a 25 ohm resistance to ground for grounding systems. They also say that by obtaining at least 4 grounds per mile that, except at equipment sites, obtaining 25 ohms at each ground is not necessary because multiple grounds obtain overall low impedance to ground.

Identifying and Repairing Open Bonds and Unacceptable Grounds
A current meter is all-important in identifying open bonds and unacceptable grounds. All ground return current wants to get back to the distribution power substation ground. Clamp-on current meters will indicate to the field technician where open shields and unacceptable grounding exists.

In many instances the cable bond has to be taken apart to identify and repair open cable shields. It may take weeks of arduous work before AC interference is mitigated. Continuous bonding will mitigate those interfering frequencies above 1 KHz, but bonding does not solve all low-frequency problems. Identifying associated distribution power problems that stem from faulty distribution power is next in order. Keep in mind that there are some distribution-power-related problems that can be solved only with an induction-neutralizing transformer.

Before jumping to an assumption that the problem belongs to the power provider, make sure you’ve covered your bases. Most of the problems you will come across are bonding and ground issues, so try to develop a solid process and follow it through with each case of trouble.

Signing Off
Thank you for the emails and calls. Your job is a tough one, and I appreciate that you will take the time to provide tips or problems that might benefit others. Contact me at dmccarty@mccartyinc.com or call me on my cell at 831.818.3930. If I’m not teaching a class, I’m likely to answer your call - even at dinner, much to my wife’s chagrin.

About the Author - Don McCarty

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Check out our website (www.mccartyinc.com). In addition to providing information about our extensive selection of courses, we are adding an equipment review section and a section on troubleshooting, where we will discuss some interesting cases of trouble. If you have an interesting case, send it along (names withheld by request). As always, I appreciate hearing from you with questions or comments on any topics that you think are of general interest to outside plant technicians and managers. McCarty Associates 18281 NE Rainbow Lane Newberg, OR 97132 Telephone: 503.538.2747 dmccarty@mccartyinc.com

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