High Voltage Transmission Lines Over a CO/Hut

I recently received an email from Aaron, a manager of an independent telephone company, discussing the effect of high voltage lines on telephone equipment. He stated that a power transmission company is proposing to build a transmission line adjacent to one of his existing telephone buildings. From the proposal drawings it appears the lines will be within 100’ of the building, and could possibly go directly over the building. It is an unmanned building, but they have copper and fiber equipment and distribution at this location.

He asked me if I foresee problems from electromagnetic interference and, if so, is there anything they can do or request the transmission company do to alleviate any potential problems.

This is out of my area of expertise, but I emailed two known experts in the field, Russ Gundrum of Telecom Problem Solvers LLC, and Percy Pool, PE Consultant, to see if possibly they could shed light on it.

Comments From Russ
Hi, Aaron, and thanks for the copy Don!

I addressed this type of situation years back when I was the ICEP Engineer for SWBT here in Houston, but it doesn’t surprise me that the practice still goes on.

Obviously, these large high-voltage transmission line easements, fee strips, right-of-way, or whatever you want to call them, are sometimes considered convenient, easy-access places for not only our cables, but for remote terminals or other electronic equipment to be placed. I remember when we might not have necessarily had them in the easement, but we were right adjacent to them. The ones that concerned me the most were located next to the tower structure footings because of ground potential rise (GPR) issues if a fault to ground were to occur at that point.

However, with the situation you’re faced with, you may be able to prevent it from happening, or at least get them to minimize any potential problems that could occur, and I think you’re right to be concerned about this! Because you can have EMI issues depending on the proximity of the line, its size and type of construction. For example, if it’s of a horizontal construction instead of a vertical placement of the phase conductors, then you could be more under the unbalanced magnetic field of one of the phase wires that’s not being cancelled out by the other two. There’s usually a pretty good separation between these conductors if you’re talking about 345 KV and above lines.

And once they have this easement in, who knows what could be placed in the future, such as a High Voltage Direct Current type system, and what kind of EMI issues that could pose, or other types of electrical protection problems.

I prepared a practice for our OSP Engineers back in 1977. You can find it online here:
https://www.researchgate.net/publication/325070926_Russ_Gundrum’s_1977_SWBT_Houston_Area_Engineering_Policy

My memo doesn’t really address your main concern of the line’s possible impact on the electronics in the CO/hut, but it could certainly affect any of your copper cables that could be paralleling it, so hopefully that will provide some useful information for you. Unfortunately, a lot of those Bell System references I provided are no longer available, and there’s not much good information that’s available out there.

But I’m glad Don included Percy on this, because he is very familiar with the two IEEE inductive coordination standards 776 and 1137 that are currently in the process of being updated, and he’ll know if any of them specifically address this situation. There are also some IEEE electrical protection standards 367 and 487 that are soon to be slated for the same update process. However, they’re more concerned with the environment at a substation or generating plant, but you may find some of the information helpful, and maybe Percy will know if there’s anything in one of those documents that points to what you’re faced with.

My issue over 40 years ago was that the BSPs didn’t address the issues I pointed out in my memo, which was the reason I wrote it. I knew the guy at WECO that wrote the 876 practices on electrical protection, and he agreed they didn’t address what I discussed at the time, but that might have hopefully changed over the years since I made such a big deal about it.

Let me know if I can be of any other help to you in this process, Aaron, and good luck in your dealings with the transmission line company!
Russ Gundrum, MBA PMP SSGB
Principal Consultant

Comments From Percy
First, I would recommend against any power line (regardless of the voltage) to be routed directly over a telco building. Stuff happens and one, or more, of the power conductors could make contact with the building, creating a ground potential rise (GPR) condition that could cause physical damage to whatever is inside the building. Also, surges, whether caused by malfunctions or by lightning, can also cause damages to the telco equipment. True, power companies typically have sophisticated relay systems that disconnect power to the conductors (de-energizing the line) but even a brief GPR condition may be problematic.

Second recommendation would be to have the power company route the power line as far as possible from your building and certainly not less than about 1,000 feet (my rule of thumb). I know this is easier said than done but with constructive dialog between the parties it can be achieved.

An inductive coordination study between the electric utility and the telecommunications company would be very beneficial.

Electromagnetic interference (EMI) depends on the voltage and current on the power line (now and future) as well as the distance between the power line and your equipment. Also, you have to take into account the susceptibility of your equipment to EMI.

There are ways to calculate the expected EMI once you know all the variables (for the present situation as well as future changes) but it may be cumbersome (there may be software programs to do the calculations — I don’t know). For the complex calculations you would need to know the line-to-ground fault current at the location, the percentage of fault current expected to enter the earth and the expected GPR, the total fault current, the resistance of the telco building electrode system with respect to remote earth (using the resistance profile method), the longitudinal induction, the mutual impedance (both between the power conductors and the telco cables as well as between the power conductors and the telco building, and the total voltage (GPR plus longitudinal induction). You need to know the susceptibility of the equipment (typically given in joules) obtained either from the manufacturer(s) or from laboratory tests (such as UL, ETL etc.). If this value is exceeded then you will have a problem. The problem may not be only actual equipment damage but may be signal degradation or even latent damage (not obvious for a while).

A way to minimize the impact of EMI on your building equipment may be establishing a Faraday cage in the building. This may be expensive, and may not eliminate the problem.

Let me know if you need additional information.
Percy Pool, PE Consultant
[email protected]

Signing off
I don’t always have the answers to your questions but I often know of other experts in the field so please ask me about issues that are confusing you or that you think others would like to know about. And always contact me if you disagree. I’m not perfect! Email [email protected] or call 831.818.3930.

Russ Gundrum is the Principal Consultant at Telecom Problem Solvers, LLC. He has more than 40 years of experience in the telecom industry and is an acknowledged authority on wireline interference and electrical protection problems. Author of forthcoming book Solving Problems in Computer Networks in the IoT Age. For more information, please email [email protected] or visit https://www.linkedin.com/in/russgundrum.