Grounding Ethernet Cable on a Ship?

wrschaeffer asks: "On a ship (a vessel at sea) where a floating ground exists, we are running shielded Ethernet cable to connect between 5 and 10 PC in our LAN. We are unsure on the grounding requirement. ABS (American Bureau of Shipping) has no requirements concerning grounding of Ethernet cable (just smoke and waterproof standards). We have many possibilities: ground one end; ground every end; ground every end and install a isolation transformer at the punch-down block; or don't ground any end. What should we do?"

Comments

Ground one end

[jcwren]

I would only ground one end, and the end at the hub, since that would be the most common point of the network.

But this raises the question: Why are you using shielded ethernet? Ethernet is differential, and thus pretty immune to common mode noise.

I've had 25m ethernet cables draped across running engines (test equipment) with HEI ignition, and not experienced any problems (I can't say that I checked the error counters very often, but in high speed data collections, I wasn't seeing any unexpected decreased throughput).

In addition, I live on a houstboat, and I have ethernet run in the same conduits as AC power carring 5 to 15 amps on 4 different circuits, and don't experience any problems (max cable length is probably 20 meteres)

Re:Ground one end

[Zarquon]

But this raises the question: Why are you using shielded ethernet? Ethernet is differential, and thus pretty immune to common mode noise.

Probably to reduce emissions.. Most boats have a lot of sensitive electronics.. and when you're under EMCON, you don't want anything avoidable to emit. Also, consider you'd want to make at least a minimal attempt to harden the equipment against any kind of EMP.

Had a friend that worked at a naval shipyard.. spent weeks trying to locate, identify, and eliminate undesired emissions from various (uncertified and certified) electronics.

Re:Ground one end

[Halvard]

One of the reasons for the 100 meter spec for distance of Ethernet runs is potential voltage. If you are on a ship with long runs of Ethernet, you should definately ground the shields using good straps to the hull or to studs that aren't covered in paint. I would ground both ends and perhaps along the middle. I'd also use something akin an APC ProtectNet or Tripp Lite DNET1. The longer the run, the higher the potential voltage on the line and ground these to the hull as well.

The shield, while it could carry a charge, is mostly about radiation -- into or out of the shielded cable. If you are running your Ethernet along power like the fool on the houseboat (inducting 60Hz into his Ethernet) you are asking for collisions, power spikes, etc. The potential charge on Ethernet conductors itself over longer runs probably will spell trouble all by itself without assistance from 60 cycle.

Big problems.

[TheSHAD0W]

I've heard of ground differentials as large as 90 volts on ships -- a serious problem, and one you wouldn't expect on a metal vessel surrounded by nice conductive salt water.

I would recommend you run some fairly heavy -- 12 gauge should do -- ground wires in between the points you will be running the ethernet, and use the cables to interconnect the equipment ground points. Once you've done this, there shouldn't be any dangerous potentials on your ethernet lines. While you're at it, I'd also suggest you occasionally check for currents running on those ground wires; if you hook an ammeter around the wire and see more than a few dozen milliamps, you've got a bigger problem than ethernet grounding.

Ground at the switch.

[Anonymous Coward]

Allen-Bradley's Web site has a handy little pdf document describing a bunch of industrial Ethernet wiring practices, which would likely work well on a ship. Page 3-5 looks like the definitive spot for grounding shielded Ethernet cable -- connect the shield at the switch end, disconnect the shield at the remote end, and give the switch a good ground.

Hope this helps.

Always, always ground one end

[Kento]

If you ground both ends you'll set up ground loops - from what other posters have said, that sounds even worse on a ship. Ground neither ends, and you don't get much benefit from using STP. Always, when running any kind of cable, ground _one_ end.

Bad grounding is a killer - literally.

[stienman]

Bad grounding kills. I'd rather work with an ungrounded system than a badly grounded system.

First of all, ethernet doesn't need a ground.

If you're already dedicated to it, however, you do need to look into it, because an ungrounded shield can be dangerous as well.

For ideas, do a web search on grounding rs-485 network cables. rs-485 is a differential signalling system similar to ethernet that is meant for long serial runs (kilometer or more) where grounding is a an issue.

In most cases, you need to find out what the voltage differential is, possible current flow, and run a suitable grounding wire along with the cable since the shield may not be adequate to the job, and may still leave quite a voltage differential. Sometimes, however, it's appropiate to allow a voltage differential and ground through a capacitor and/or resistor.

In other words, there's no single way to do it, and the way you do it will depend on many things, not the least of which is what you're doing with the network and where it's being used.

Good luck! Don't suck on the wires, especially when you know for sure that they aren't live (they are always 'live').

-Adam

Re:Bad grounding is a killer - literally.

[.milfox]

On our boat we use Fiber. =) Problem solved. (Military vessel)

Personally, I wouldn't ground the cables, to reduce the chance of them being hit during a lightning strike (on a small civilian boat)

It realy depends on what sort of vessel.

Good advice.

[Futurepower(R)]

Good advice. Run heavy ground wires between the points. Then use shielded Cat-5 with the shield grounded at one end. You don't want the shield to carry current; it is only to prevent common mode pickup.

"Common mode" is capacitive or inductive coupling to both wires of a pair at the same time. Common mode voltages can overload the Ethernet common mode voltage range. The common mode range is a few volts, so you also need the ground wires to insure that the range is not exceeded.

use fiber optic

[wotevah]

Grounding both ends of the cable would definitely be a serious problem. The lower the wire gauge is, the worse the effect is going to be (you are creating a low-resistance connection between two points of different potential, this will generate significant current draw through the wire).

I would suggest to go fiber, it's immune to this kind of problems and I have seen fiber optic cards for decent prices in stores. Or use fiber for long distance connections and regular wired ethernet for LANs.

If you must do copper, ask someone who has done a copper-based university or large building WAN. They will likely have had to solve the same problems, as the same grounding issues can appear between equipment powered from AC sourced different phase lines. I don't recall exactly but grounding at only one end and leaving the other in the air might have been the solution.

DoD grounding recommendations

[Anonymous Coward]

The applicable US DoD standard is MIL-STD-1310G - "Shipboard Bonding, Grounding, and Other Techniques for Electromagnetic Compatibility and Safety". I'm not sure if that document specifically addresses Ethernet, but one of the illustrations should describe how to ground some differential twisted-pair data network. You can get MIL-STD documents from DAPS.

go optical and/or don't do ethernet

[Anonymous Coward]

ethernet is not really intended for industrial situations. the signal levels are too low, too susceptible to induced noise; the connectors (RJ-45) are not seaworthy; on a ship you ALWAYS use stranded pretinned wire, which is not what cat5 cable is (solid, untinned). the salt and vibration will kill the cat5 cable and connectors.

first: if you can, put in optical fiber. that way you will avoid the ground loop problems entirely.

indeed, first time most folks saw optical fiber was in the raised floor computer room, separating the mainframe chassis and peripherals.

second: electric grounding on boats gets really complicated really quickly. you have several conflicting requirements for "grounding": HF radio counterpoise, electrical noise reduction, shore-side AC ground, perhaps onboard generator AC ground, onboard DC ground, lightning strike diversion, metal conductor bonding (hand rails and engines etc.) ... and electrolysis - don't let stray currents in the water eat your propellors!!!

go to a marine book store like red tide or armchair sailor or west marine and get some books on marine electrics. or hire a real marine electrician, it's money well spent.

Don't ground aboard a ship.

[Anonymous Coward]

Grounding? Not until you know everything there's to know about galvanic corrossion.

YOU COULD SINK THE SHIP IN HOURS.

Sorry to shout, but current flowing through the hull is about as bad as sailing over reefs or bumping into mines. Check with experts before connecting anything.

Noise vs. Electrical

[ONOIML8]

You've had a lot of input here, some of which seems good to me. But I think you're looking for input on something that isn't YOUR problem.

Grounding the network cables would be your problem if you were fighting noise. Then grounding one end would be the place to start.

But if you're worried about a difference in ground potential causing you problems, anything to do with network cable is NOT the solution. That is something that had better be dealt with by a marine electrician. Not any electrician, one who knows ships and knows them well.

Here's why: Network cable is small signal, low current stuff. If you have a difference in ground potentials you could be talking about high voltage and/or current. You don't want this on your network, it will fry everything. Have those problems solved first by an electrician and know that the enviroment you'll be working in is friendly for your network.....AND FOR YOU!

As an AC pointed out, you can do harm to the ship by changing the electrical properties of it. When those changes start to happen it will likely also do damage to your network.

If it were me I would:

1) Consult the marine electrician most familiar with the ship. Once you have his ok for safety and blessing with your project then...

2) Check with the radio operator/technician if you have one. He's got equipment that works much like your network. There are things he may know which will save you heartache and speed up your progress.

3) Install the network, tie all shields to the hub. Then "ground" the hub to whatever the electrician tells you to. Remember that you're only doing this to prevent noise on the network, it has NOTHING to do with ships ground...and you don't want it to.

4) Ask the electrician and radio op to check your work when you are done. Log this. Then if there is ever a problem you can show the captain the log and he'll know you did your homework. Cover your ass always.

5) Have a nice day.