RF Interference between AC Power and Communications cables Question

[ProfessionalNewbie]

I'm new to this forum and hopefully this is the right place.

My company has a job where we are placing 2 conduits (one for AC Power and the other for Communications) into a trench. Our plans originally specified that the conduits be made from steel encased in PVC but the contractor only used PVC conduits. Instead of having to remove the conduits, we're looking for a possible alternate solution if possible.

1) I believe the spacing between the 2 conduits was suppose to be a minimum of 12 inches in the plans but unfortunately they were placed within a few inches from each other and hence the concern with RF interference. The contractor suggested pouring concrete over the 2 conduits inside the trench but I'm not sure if this will prevent interference. This is a subject I'm not familiar at all with.

2) Do we need to separate the conduits to the 12 inches minimum if we are going to use the PVC conduits and encase them in concrete?

3) If the conduits have to be removed, are steel conduits a good solution for canceling interference?

4) Is it helpful to put in shielded communication cables in my situation?

Thank you.

 

[zbang]

A lot of it depends on what signals the comms cables are carrying and the cable types. If they're POTS or balanced high-level signals, not as much of a problem. Unbalanced/single-line comms.... recipe for disaster, even in metal conduit. Shielding will be helpful, in general, unless distributed capacitance gets you.

If they're all on fiber, interference isn't a problem . Or, if the system can be converted to fiber (at the expense of the contractor who installed the wrong thing?), you should be good.

 

[LarryFine]

Welcome to the forum.

1) I believe the spacing between the 2 conduits was suppose to be a minimum of 12 inches in the plans but unfortunately they were placed within a few inches from each other and hence the concern with RF interference. The contractor suggested pouring concrete over the 2 conduits inside the trench but I'm not sure if this will prevent interference. This is a subject I'm not familiar at all with.

I don't know whether concrete would make a difference. The potential for interference depends on the characteristics of what's in each conduit, especially the RF. If it's shielded, interference should be minimal.

2) Do we need to separate the conduits to the 12 inches minimum if we are going to use the PVC conduits and encase them in concrete?

If you can still get to them, I'd suggest best separation, and don't worry about anything else.

3) If the conduits have to be removed, are steel conduits a good solution for canceling interference?

Again, it depends on the nature of the cabling and the nature of the interference in concern.

4) Is it helpful to put in shielded communication cables in my situation?

Again, it depends. When you mentioned "RF interference," I envisioned shielded coaxial cables.

Is this ethernet cabling, i.e., UTP? It's generally not subject to 60Hz interference.

What is the distance, etc?

 

[tom baker]

What is the voltage on the power?
Fiber is best solution, and is less expensive than copper. You can buy preterminated ready to pull cables.

 

[gadfly56]

I'm new to this forum and hopefully this is the right place.

My company has a job where we are placing 2 conduits (one for AC Power and the other for Communications) into a trench. Our plans originally specified that the conduits be made from steel encased in PVC but the contractor only used PVC conduits. Instead of having to remove the conduits, we're looking for a possible alternate solution if possible.

1) I believe the spacing between the 2 conduits was suppose to be a minimum of 12 inches in the plans but unfortunately they were placed within a few inches from each other and hence the concern with RF interference. The contractor suggested pouring concrete over the 2 conduits inside the trench but I'm not sure if this will prevent interference. This is a subject I'm not familiar at all with.

2) Do we need to separate the conduits to the 12 inches minimum if we are going to use the PVC conduits and encase them in concrete?

3) If the conduits have to be removed, are steel conduits a good solution for canceling interference?

4) Is it helpful to put in shielded communication cables in my situation?

Thank you.

You're being awfully nice to that contractor. At a minimum, he has to bear the cost of whatever solution you, or your boss, not he, determines is needed. He also owes you the installed cost difference between the steel encased PVC that was spec'd and the plain PVC he delivered. Also, if there was a clear 12" separation, edge to edge in the drawing, he has reduced trenching costs for not meeting that spec as well and owes a give-back for that, too. In fact, because of the high PITA factor involved, in your shoes I'd go over the installation, drawings, and specs with a fine-toothed comb and see what else he owes.

Thinking about it, if there is a performance spec anywhere that your company is responsible for meeting, that could be in jeopardy due to the contractor's failure to install to the drawings and specs. If you allow him to skate on the installation, any failure is going to fall squarely on your company, including potential liquidated damages. If there isn't a time crunch, tell the contractor to pull it out and do it right. Next time he'll pay closer attention to the drawings and specs.

 

[ptonsparky]

The contractor screwed up. If he can get to the conduit yet, he can still fix it.

 

[zbang]

Fiber is best solution, and is less expensive than copper. You can buy preterminated ready to pull cables.

And if you go that route, get single-mode fiber; last I checked it was no more expensive the multi-mode and you get better distance and higher speeds

 

[gadfly56]

And if you go that route, get single-mode fiber; last I checked it was no more expensive the multi-mode and you get better distance and higher speeds

Based on the products used for fire alarm panel communications, the equipment is 5X as expensive. Single mode is an advantage for long-haul - and by that I mean miles - not from the street to your demarc, or even for 5,000 feet or so inside a small campus setting.

 

[Hv&Lv]

And if you go that route, get single-mode fiber; last I checked it was no more expensive the multi-mode and you get better distance and higher speeds

Unless your going 3/4 of a mile or more I wouldn’t worry about that.
while the cable may be comparable, the ends aren’t. Some of the single mode vs multimode equipment can be two or three times as high.

 

[Jraef]

Encasing them in concrete does nothing for RF interference. If it did, radios would not work in concrete buildings. He is grasping at straws with that one. Your approval of pouring concrete on top of the problem however “cements” it in as your problem.

What you asked for is commonly called “Robroy” conduit for the originator of the PVC coated steel conduit. He “cheaped out” and maybe assumed you wouldn’t know the difference; Robroy conduit is likely 4X the cost of PVC conduit, plus extra labor time to install it in the first place. If you gave him specs to follow and he ignored them to save himself money, that is a breach of contract and he needs to remedy the situation to your satisfaction. But do NOT let him encase it in concrete.

Shielded cable for the comms would be important either way. Shielding the power cables in the other conduit will help if you can’t get him to replace the conduit. On the comm cables, you would ground only one end of the shields, on the power cables, ground both ends to create a sort of “Faraday cage” around the conductors.

 

[LarryFine]

But do NOT let him encase it in conduit.

May we presume you mean concrete?

 

[zbang]

Some of the single mode vs multimode equipment can be two or three times as high.

You're shopping in a very wrong place. A SM 1G ethernet SFP module might cost $10 more than the compatible MM SFP (ubiquity SFPs are $17 and $22 from UBQ); they're practically disposable. Even 10G ethernet SFPs aren't more than an under-$100 difference, including cisco-branded ones. (Everyone I know stopped installing MM because there wasn't any real cost savings, even with equipment factored in and for <1000' runs.)

If you're using equipment that doesn't have changeable optics, that's a different problem.

 

[hbiss]

My company has a job where we are placing 2 conduits (one for AC Power and the other for Communications) into a trench. Our plans originally specified that the conduits be made from steel encased in PVC but the contractor only used PVC conduits. Instead of having to remove the conduits, we're looking for a possible alternate solution if possible.

Soooo, eleven replies and everybody is just throwing ideas out there.

To the OP:
The way this forum works is you ask a question and if we have a question you come back and answer it.

Post #2 asked you what the "communications" is. If you don't provide that information we are wasting our time.

-Hal

 

[Hv&Lv]

Soooo, eleven replies and everybody is just throwing ideas out there.

To the OP:
The way this forum works is you ask a question and if we have a question you come back and answer it.

Post #2 asked you what the "communications" is. If you don't provide that information we are wasting our time.

-Hal

Really?

You should know by now the way this forum works is a question is asked, a couple of answers are given or other suggestions are made.
From there we diverge off into as many different tangents as possible...

 

[rnatalie]

If you're talking Ethernet (modern data wiring), there's no distance recommendation from power even without conduits. The thing can lay on top of unshielded power wire for miles without the coupled hum affecting it.

 

[zbang]

If you're talking Ethernet (modern data wiring), there's no distance recommendation from power even without conduits. The thing can lay on top of unshielded power wire for miles without the coupled hum affecting it.

...because underneath everything, it's a transformer-isolated balanced line... Of course the Ethernet part will stop behaving when you stretch the 330' spec limit.

Gotta love balanced lines . (Obligatory digression- would an ungrounded delta feed be considered a balanced line? I think it meets the description.)

 

[synchro]

(Obligatory digression- would an ungrounded delta feed be considered a balanced line? I think it meets the description.)

If the feeder conductors are symmetric, then yes I think it would be a balanced line fed by a balanced source.

 

[Jraef]

May we presume you mean concrete?

Yes. Lol.

 

[rnatalie]

...because underneath everything, it's a transformer-isolated balanced line... Of course the Ethernet part will stop behaving when you stretch the 330' spec limit.

Yeah, I should have mentioned that it's only good to 100 meters.
In addition to being balanced, there's more things that help: the twist in the twisted pair and the modulation being used. All this helps defend against interference from low frequency hum.

 

[LarryFine]

Twisted-pair works by assuring that both wires pick up the interfering signal (called common-mode noise), so it can be ignored by the differential amplifier at the receiving end (called common-mode noise-rejection).

The difference signal is passed through, while any signal common to both wires is rejected, or filtered out. The greater the number of twists per inch, the higher the frequency of interference you can assure will be ignored.