To use steel pipe for telephone cable (Plant engineering)

Discussion in 'Wireless & RF Design' started by glorimda, Jan 13, 2015.

  1. glorimda

    Thread Starter New Member

    Oct 12, 2011
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    Hello,
    I brought up with my problem that I'm not sure I can discuss about here. But I think this section assumably give better answer in principle wise.
    If you know better board than here, let me know.

    I'm working in plant engineering industry and working as an electrical engineer. I encountered some problem with my customers on using steel pipe with telephone cable.

    Telecom Cable - 50 Pair (refer to the file attached)
    Steel Pipe - PVC Coated steel pipe (pvc is coated and outside of steel conduit, so basically it's all insulated from the air)

    Pipe runs around 600m through underground. For cable pulling, there are kind of manhole between every 50m steel pipe and steel pipe stops at each side. so continuity of steel pipe stops every 50m.

    The thing is, customer wouldn't allow us to use steel pipe even though it is coated with pvc.
    They're saying steel material of pipe will have an effect on signal running through the telecom cable. But they couldn't show exact reason or principle for this and neither do I.
    I tried to find standard regarding this issue, but couldn't.
    Are there anybody who can help me or make me understand with principle or standard to persuade them if they are wrong.
    I'd appreciate it in advance.
     
  2. BR-549

    Well-Known Member

    Sep 22, 2013
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    Where are you at and how many customers?
     
  3. #12

    Expert

    Nov 30, 2010
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    Why fight it? Use plastic armor. It's cheaper.
    Just be careful not to tell the customer that the cable already has a conductive aluminum shield around all the signal wires.
     
    alfacliff likes this.
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Curious - you can buy steel tape reinforced / sheathed comms cable for direct underground burial? What's the difference?
     
    Last edited: Jan 13, 2015
  5. BR-549

    Well-Known Member

    Sep 22, 2013
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    If your customer is using the cable for conventional purposes, you have a customer service issue, not a technical issue.

    If it's just one customer.....adjust engineering to his specs and charge appropriately.

    Was this stated in the original specs or bid?

    If it was not......then you used standard practices for your first bid.

    If they want to deviate from standard practices.......that will cost more.

    Also......deviating from standard practices can involve permit delays.

    The permitting agency can verify your engineering principles.

    If it is more than one customer, you will need to find out who is mis-educating them.

    There are other variables. Depending on your situation.

    Environment, location, law, culture, etc..
     
  6. glorimda

    Thread Starter New Member

    Oct 12, 2011
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    We are dealing with middle asia custormers. They tend to follow Rusian specs.
    It's large plant project not like dealing with personnel.
     
  7. glorimda

    Thread Starter New Member

    Oct 12, 2011
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    This is not a personal business so it's not that simple purchasing other material(almost impossible).
    By the way, can you explain about aluminum shield more detailedly? Do you think that's gonna make any problem on telecom lines when using the steel pipe or by itself.
     
  8. glorimda

    Thread Starter New Member

    Oct 12, 2011
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    This is the concept of our project. I think your comment seems more reasonable though. Purchase of material has been already done so need to find out the way in our given choice.
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I was aiming to indicate that if a cable manufacturer wraps the signal conductors in what is effectively equivalent to a steel tube sheath, then this would be counter-intuitive to the signal degradation argument mounted by your client. In fact, it is often recommended to place buried signal cables in steel pipe both for added physical protection and improved immunity to external electrical interference.
    Of course, in the unlikely event that the very thing the client feared turned out to be case, you would face a costly exercise in remedying the problem.
     
    Last edited: Jan 13, 2015
  10. #12

    Expert

    Nov 30, 2010
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    The basic theory here is that surrounding the signal wires with a metal shield is bad. We disagree. A metal shield keeps out interference rather well. We point out the fact that the cable you are required to use already has the electrical equivalent of a steel armor pipe, except that the aluminum shield is much closer to the signal wires and will produce more "badness" than a steel pipe. The badness that I name is the capacitance caused by any 2 conductors being close to each other. This capacitance allows the signals to interact with the grounded shield and will reduce the high frequency abilities of the signal wires. When the shield metal is included in the cable, it is consistent, measurable, and predictable. Thus, it can be accommodated.

    In addition, the aluminum shield eliminates any effect the steel armor might have produced if the aluminum was not there. When you add a steel shield to a cable that already has a metal shield, the only effect is better mechanical protection.

    It might be true that the Feng Shui dragon does not like iron, but electrical engineers are rarely taught about this.
     
    Last edited: Jan 14, 2015
  11. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello,

    Well there is a big difference between aluminum and steel, and that is that steel is much more magnetically active than aluminum is. This makes a big difference when it is used more directly for the wiring itself because the skin effect is more pronounced and thus will cause much more signal attenuation.
    Used as a shield though i can only guess that they are worried that the internal signals will react magnetically with the steel and cause some sort of inductive effect, which would degrade the signal. It could be that it has little or no effect when used as a shield, but they are just worried that it will and why take the chance. The wires could be closer to the steel in some places than other places, so in some places they could be very close.
    Other shields are made of copper, which also reacts much less to a magnetic field than steel.
    When the steel is viewed as the core of a very long and narrow transformer core, it would be like wrapping the two (or more) wires around a magnetic core, which will probably act a little like a power line common mode filter, which attenuates high frequencies. Just how much this effect works out to be in practice would have to be tested by running a pair of wires in air and testing, then running the same wires through a steel pipe and testing again. You could then probably work out a formula if there was a measurable difference. Alternately, make up two sets of identical pairs of wires and run two in air and two in the steel pipe, then power them both at one end with a signal generator, then check on the receiver side for any difference, perhaps with a differential amplifier or a scope with a differential input (or something like that).
     
    Last edited: Jan 14, 2015
  12. #12

    Expert

    Nov 30, 2010
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    @MrAl
    In the interest of giving glorimda useful information, I disagree with you on these points:

    "steel is much more magnetically active than aluminum is."

    The required cable already has an aluminum shield. If the voltage difference between the aluminum shield and the steel shield is zero, how much effect is caused by the magnetic permeability of the steel? This scenario resembles winding the core of a power transformer with RG-59 (shielded co-ax).

    "I can only guess that they are worried,...but they are just worried."

    I find this to be a less than compelling reason to investigate and try to measure what you guess they might be thinking. Communication would probably be a lot more efficient method.

    "When the steel is viewed as the core of a very long and narrow transformer core,...Just how much this effect works out to be in practice would have to be tested by running a pair of wires in air and testing, then running the same wires through a steel pipe and testing again. Alternately, make up two sets of identical pairs of wires and run two in air and two in the steel pipe."

    There is no option on this job to run the installed wires in free air, and there is no option to use unshielded signal wires. Given these requirements, what might be accomplished by doing this measurement?

    You are not required to respond to me, but I think you could be helpful to (me and) glorimda if you did.

    Thank you for considering my request.
    Number Twelve
     
  13. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I'm also puzzled as to how skin effect relates to cable sheath shielding effects.

    The very long transformer core concept can probably be better related to common mode signals - though I would be surprised at the significance of the level of attenuation that might be inferred at telephone cable bandwidths. One does find ferrite core "clamps" or common mode signal traps in use in RF, data / comms and audio systems.

    There is a lot of speculation going on which seems of little interest to the OP who appears to have moved on.
     
    Last edited: Jan 14, 2015
  14. BR-549

    Well-Known Member

    Sep 22, 2013
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    Maybe the customer is worried about detection or something. Maybe the steel can be remotely detected easier than the requested materials. It's hard to tell now days. I once had a women tell me that every time she turned on the microwave.....it pulled her bra off.

    That's NS.
     
  15. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    I dont mind responding at all. The only times i have difficulty responding to a reply is when the 'alerts' dont work right. Maybe i should tell someone about this too because a lot of times i sign in and i dont see the alert for a thread that i had participated in and had at least one more reply, but there is no alert. This means sometimes i have to search the whole forum to see if anyone replied to one of my posts which could be in several threads on any given day :)

    Ok back on topic, it seems that either you did not understand my intent or i did not make it clear. I'll try to clear some of these points up.

    I have a great deal of trouble understanding how you could disagree with:
    "steel is much more magnetically active than aluminum is."

    It doesnt matter what applications it is in, that sentence is definitely and certainly true. Anyone with a small magnet in hand knows that you can pick up a ferrous object but cant pick up an aluminum object. That's because the iron reacts much more magnetically than the aluminum. True, the AL reacts a little, but it is not very reactive magnetically.
    And your points of application seems to lie in the reasoning that the AL already shields the wire(s) so the steel cant have any effect, but in fact the AL can only shield the wire(s) electrically not magnetically. In other words, if the wire(s) can generate a net magnetic field, it will reach the steel and so it could have a negative effect. Then i went on to suggest a test set to prove or disprove that the steel really does in real actual life have a measurable effect, but you seem to have rejected that too, which doesnt make any sense to me either because why would you reject a test to find out for sure. Maybe you can elaborate a little here too.

    Next, i mentioned that perhaps the company may have been just plain old 'worried' that the steel MIGHT have an effect. In the context of not having any sure factual scientific data on hand, they might not want to risk having to dig up the whole thing again and start over. You have to weigh in the financial context too, where right here in this forum it does not cost either one of us any money at all to say "YES' or "NO", and be wrong. For the company under consideration, it could cost the entire profit margin or even more to redo the project if a problem does come up. So they choose to go with something that they know works because it has worked in the past rather than some new way of doing it and risk everything. If you had to give up your only computer if something went wrong you'd think twice too. It is also possible that they even have data available already that tells them not to do it that way, and we dont have that information yet.

    As to the test, we could find out for ourselves at least for sure if we tested both methods ourselves. I for one am not willing to do this kind of test at this time, but someone else here may feel like experimenting a little.

    I hope i have cleared up my position on this matter at least a little but if something still does not seem clear or you dont agree dont hesitate to respond again and i'll check it out.
     
  16. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi tnk,

    Yes, speculation because that's all that we can do right now. We dont have any real data to work with. That's one reason i suggested the test set.

    The skin effect i mentioned in my previous post was just to remind us that in steel WHEN USED AS A CONDUCTOR the effect is much more pronounced than when using copper or aluminum. That means that the material itself is affected by current more than copper or aluminum which is the more typical conductor, and that is because it is more magnetically active. In other words, it is another property of steel that reminds is that steel is much different than other materials used in electrical work.
     
  17. BR-549

    Well-Known Member

    Sep 22, 2013
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    "And your points of application seems to lie in the reasoning that the AL already shields the wire(s) so the steel cant have any effect, but in fact the AL can only shield the wire(s) electrically not magnetically. In other words, if the wire(s) can generate a net magnetic field, it will reach the steel and so it could have a negative effect."

    I haven't heard this before.......it is very curious.
     
  18. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Just do it how they want it, cash the check and move on
     
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