Mobile network wiring

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 Question by Rich Vanderwerker posted 10 Jul 2005
 Mobile network wiring
I am currently wiring a "mobile command post" truck for our viallage and wish to install a wired LAN within the truck. I wish to use CAT5 or better, but am trying to take into consideration RF and electrical interfernce and vibration from the truck's movement. I was considering using a shielded twisted pair cable with stranded conductors instead of solid for better life in the vibration ridden enviornment. What cable do you suggest and am I on the right track here? Do I terminate all the shields together at the patch panel and can I punch down the stranded cable into the panel connectors as I would a solid conductor? thanks
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 Answer by Dmitri Abaimov posted 11 Jul 2005
Dear Rich,

I'd like to address your question, but I have to tell you up front that I do not have real-life experience in cabling vehicles. Therefore I can only throw some common-sense points that come to my mind having dealt with high speed cabling in commercial buildings for a decade.

First of all, I do not think stranded cable will perform too well in a vibration environment because in the punch-down contact you will have the outside layer of strands of the conductors being constantly rubbed against the body of the blade, and that would eventually break that layer and loosen the contact. The solid conductor, I can imagine, would withstand that better because the entire mass of the copper will be rubbing against the blade, and it's about 10-11 times thicker, so more difficult to break. Besides, solid conductors sit in the blade very tight, so there might not be too much of the rubbing at all since no (or very little) movement is possible.

On the grounding/shielding I should say that it will not be as easy as it sounds because there is no ground in a vehicle, just bonding to its body (unless itís semi-permanently setup at the site with a grounding conductor). Most vehicles I know of have PLUS terminal of the battery connected to the body, whereas most pieces of electronics not specifically designed for automotive use would expect a NEGATIVE. Just a thought, again, I donít have much experience in the area.

All in all, you are much more into realm of Industrial Cabling that does not have its own standard as yet, believe you or not. A TSB that would supplement existing TIA/EIA-568-B standard with additional industrial requirements is expected soon. There are different kinds of industrial ruggedized RJ45 connectors out there, too. Check them out on the Web, see it would be suitable for you. The emphasis though is more on elements protection (dust, water), not so much the vibration.

One other pointer I can give you is that UNSHIELDED (UTP) cabling has been successfully used to cable ships, cruise liners, specifically. I can imagine that, due to the size of the ship, EMI is not this much concentrated, but there is a great deal of RF equipment on a ship, obviously, from radars to radio communications, that seem to co-exist with UTP cabling.

Sincerely,
Dmitri Abaimov, RCDD

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