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)

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.

Grounding, ethernet, etc.

[AlphaOne]

First things first... lots of people are saying that ethernet doesn't require a ground. That's WRONG... a ground is always required, especially on transmission lines like ethernet cables. However, CAT5 is a balanced transmission line and shielding isn't all that important unless you're running the cable through an environment likely to disrupt this balance.

Second, the type of boat you're on makes a big difference. If you're on a metal hulled boat (or a boat with a metal mast or other convenient absolute ground), you're ground doesn't float... the water is ground. Salt water is especially good as a ground reference.

If you're in a fiberglass boat, the whole vessel is floating above ground. See if you can find a metal object (preferably brass or other non-oxidizing metal) always submerged and use it as your ship's ground.

Either way, the easy answer is don't worry about grounding the shield.

If you run into problems with data corruption (or want to prevent such problems), you should ground BOTH sides of the shield. On long cable runs (longer than a half-wave at your operating frequency) you should ground only ONE side. The hub-side is the best to ground as it is already the central network point... so making it the central grounding point makes sense as well.

The BEST solution is to use something called telescoping grounds. In a telescoping ground, both ends are grounded, but to different shields. Both shields are independent and isolated from one another and overlap for the length of the cable. This prevents common-mode currents on both components and avoids ground-loops at the same time. Professional audio cables use this technique often.

Another option you may not have considered is wireless. With a wireless network, the only ground that's important is that of the base station (for efficiency's sake). The client devices have their own ground reference and its relationship to the base station isn't important.

At any rate, good luck!