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ISE Columnist Don McCarty, OSP Expert

Electric Fence Interference

Feb. 1, 2019
I received a question from a field technician about electric fence interference. He shared that he had a case of trouble that included noise on the telephone circuit emanating from […]

I received a question from a field technician about electric fence interference. He shared that he had a case of trouble that included noise on the telephone circuit emanating from an electric fence in the area. He said that the line is free of DC type faults. He was wondering if it could be a grounding issue?

In some 50+ years in the telephone business, I have dealt with the effect of electric fences on voice grade circuits, bandwidth circuits, and even rarely, circuits where the customer is fiber fed. Once the cause of disrupted service is proven to be an electric fence, the time-sensitive field technician has his hands full with no idea where to start. Most electric fence problems require several trips to the area and many hours to identify, find, and fix. It rarely is fixed on the first visit.

An electric fence problem on voice grade service is usually identified with a telephone handset. The customer complains about "clicking" on the line and this is verified at the network interface and/or at the central office (CO) or remote with a field technician’s handset. The clicking may be heard on an off-hook or on an on-hook condition, or both. For bandwidth and fiber circuits it can be identified with an impulse noise test as Repetitive Electrical Impulse Noise (REIN).

Once it’s proven that the problem comes from an electric fence, the question is which electric fence and the location of that fence. I have been involved in cases where there is more than one electric fence along the cable route, and that one fence may be miles from the complaining customer or customers. I have also seen where the interference is from a fence along one cable that is causing noise on another cable fed by the same remote.

Finding and fixing electric fence interference depends on the cooperation of the owner of the fencer unit (the unit that controls the electricity running through). Some owners are helpful, and others are not. Some use their phone to check if the fencer is working. It saves a trip. Part 15 of the FCC rules require that the fencer operator correct the problem or cease operation of the fencer. I guarantee you that this is almost impossible to enforce.

Query other field technicians and the dispatch center. See if there are other complaints and if they have been rectified or not. A running log of electric fence problems is most helpful.

When dealing with bandwidth customers, the interference may be from one cable pair affecting other customers in the same Digital Subscriber Line Access Multiplexer (DSLAM). If fiber-to-the-home is affected there is an issue with the fencer ground that is interacting with the residential power ground at the residence. The electric fence interference is usually REIN. This noise sweeps the frequency band, so radio and television may also be affected.

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When dealing with repetitive electric fence interference, the first stop is at the hardware store for the purchase of an electric fence controller. They run from around $30.00 to $2,500. One around $70.00 will do. Also, purchase an electric fence tester. Get one around $30.00 that will indicate the voltage potential applied by the electric fence controller. Electric fence controllers put out from 3,000VDC to as high as 10,000VDC. Also, get a dozen or so insulators. If you walk the fence line you will need a few. And bring along a weed eater.

Test the cable pair from the CO or remote. Identify and fix any DC type faults such as crossed-battery, a short, ring or tip ground. Test the longitudinal balance and make sure that it is greater than 60dB. Place a grounded short on the cable pair and proceed to the Network Interface (NID).

Visually inspect the NID and make sure that it is grounded to the power ground electrode. Measure the station ground: it should measure 0 to 25 ohms. If not, remake or replace the station ground. To test the resistance of the station ground first measure the tip to ring resistance from the NID. Then measure the tip to ground and ring to ground resistance. Tip and ring to ground should show equal resistance and measure ½ tip to ring resistance. For example, if tip to ring resistance measured 100 ohms tip and ring to ground should be 50 ohms. If tip to ring measured 100 ohms and tip to ring measured 60 ohms station ground is 10 ohms acceptable. If the station ground is more than 25 ohms or is not tied to the power ground electrode, then that could be the root cause of electric fence interference.

Next, with a grounded short on the cable pair at the CO or remote, measure power influence. It should measure less than 80dBrnC. Measure circuit noise. It should measure less than 20dBrnC. If power influence is greater than 80dBrnC, then the root cause of electric fence interference could be a bonding and grounding issue. At this point I recommend a visual inspection of every pedestal and/or terminal. Each bond should be torqued to 40-inch pounds. Be sure to measure power influence again, and test again to see if the electric fence interference is reduced.

Most electric fence interference is the wire itself, but I like to look at the electric fence controller and the electric fence controller ground. With owner permission, disconnect his electric fence controller and connect yours. If the interference is gone, give it to him. You will save a lot of money and time.

Placement of the controller ground is critical. It should not be connected to the telephone or power ground. A separate ground rod should be placed at least 30 feet from the controller, and away from other ground systems such as the house electrical system, and underground power or phone lines. In dry soil or frozen ground, more than 1 ground rod may be needed. If so, the ground rods should be spaced at least 10 feet apart. Connect the ground rods in a series using joint clamps and insulated cable in one continuous length without joints or splices. Do not just wrap the wire around the wire: use a clamp.

Bad splices in the fence wire and gate hooks are 2 of the more common problems associated with electric fence interference. Vegetation can cause intermittent problems. Walk the fence line with your tone probe and a portable AM radio. Bad insulators can be readily identified for replacement. Sometimes the AM radio picks up bad insulators that the tone probe misses, and vice versa.

If the problem still isn’t solved, you may try a straight isolation transformer on an AC-powered fence controller. A straight isolation transformer is built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings. Isolation transformers block transmission of the DC component in signals from one circuit to the other but allow AC components in signals to pass.

As you can see, solving a repetitive electric fence interference is a convoluted time-consuming process. If you have any solutions that I have not covered, pass them on to me and I will pass them on to our readers.

Signing off
I’d particularly like to hear your stories about problems that are difficult to resolve, and to hear how you fixed it. Or, if you weren’t able to fix it, I may be able to dedicate a column to helping you find solutions. You don’t need to provide your name or the company name. Contact me: 831.818.3930 or [email protected].

About the Author

Don McCarty

Don McCarty is the OSP EXPERT columnist for ISE magazine, discussing the issues around provisioning, testing, and maintaining copper for all services from POTs to IPTV. Don is also president of and the lead trainer for McCarty Products, a technical training and products company training field technicians, cable maintenance, installation repair, and Central Office technicians and managers. For more information, email [email protected] or visit