telecomm wiring next to high voltage

[anfeng]

Is it acceptable practice to route telecommunications lines adjacent to high-voltage distribution lines in an underground duct bank?

This particular situation involves a 7200 volt primary electric cable in 4” pvc conduit, serving up to 150 kVA of load. In the same trench, in another 4” pvc conduit, are a number of Cat 6 cables carrying data and phone. The conduits are separated by 4” to 6”, and run next to each other for 500’. Will the high voltage primary cause any interference on the data lines?

 

[anfeng]

Since no one has replied to my question, I can only assume that no one has experienced any problems with telecommunication wiring installed adjacent to high voltage distribution wiring. I assume that this situation would commonly occur in campus type areas. The most informative web site that I found while researching this was Field Management Services,

http://www.fms-corp.com/

They mostly talked about magnetic interference from electrical lines near computer video monitors.

Please let me know of any concerns I should be aware of. It just seems that the 60-hz hum would affect computer network wiring.

 

[eryp]

At the local utility where I work it is sometimes necessary for the telephone and data companies to run their facilities in the same trench as our 15kv power cables. I have not heard of any problems with 60hz interference from our power system.

Cat-6 cables have twisted pairs specifically for the purpose of minimizing electromagnetic interference.

Also, Cat-6 runs should be limited to 100 meters between devices. Beyond this distance the signal loss can be great enough to cause equipment malfunction.

If you have the money to spend, fiber would be the way to go as far as eliminating the distance and interference problems - if you don't mind spending a couple thousand for that 500 foot run.

 

[dpc]

Probably shouldn't be making design decisions based on a lack of response to a single question on an Internet forum.

More separation is always better. Shielding of both the high voltage and CAT 6 cables will also help. Steel conduit would help as well.

Personally, I would try to get more separation. But it may not cause any problems.

And you won't hear much hum coming from the feeder.

 

[anfeng]

Hey dcp,I wasn't making a decision, I was trying to stimulate some response, which apparently worked!

Yes eyrp, I agree that fiber would be the best way to go. But there may also be some analog lines for phone, intercom, and radio communication, which was where I was concerned with the 60hz hum.

 

[eryp]

as dpc said, shielding of the cables is a good idea as well as the steel conduit.

Speaking from experience, in a mixed use (residential and light retail) development project here, the electric, cable tv, and phone cables were all placed in the same trench. Our specifications stated that the communication facilities should be at least 12" above our electric cables but I know from seeing the project during construction that it did not happen. The phone and cable ducts were installed at the bottom of the trench right alongside the high voltage electric. Each utility had its cables inside their own HDPE Duct so no steel conduit was used. This method was used all over the development that had several thousand feet of trench. I spoke with the phone and cable reps this morning onsite and there have been no complains from any of their customers. There are already about 500 customers online over the past 2 years so it seems that everything is working well.

As I quoted dpc above, it can't hurt to use shielded cable and steel conduit for the communication lines. Those methods are proven to work in high interference environments.

 

[EEJaime]

It's quite common to utilize a common trench for running distribution voltages and telecom conduit in a campus setting. We also specify 12" of seperation between power and low voltage conduits. But even at 5-6" of seperation you should not experience a lot of interference. If you can specify either an overall shield on the comm cables or individually shielded pairs you will further decrease the chances of getting noise on your telecom cables. The older outside plant 50-100 pair Cat 1 and 2 telephone cables would pick up noise at times due to their not using tightly twisted pairs, but even that was minimal, and not too noticeable on voice lines.

Good luck.