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Author Topic:   How to Wire Shield Conductor in Fiberglass Boats
Whaler_bob posted 04-09-2015 11:52 AM ET (US)   Profile for Whaler_bob  
I'm about to start setting up my NMEA-2000 network which will include the 2015 E-TEC 135 H.O., ICON PRO gauges, a chart plotter, a VHF radio, and an AM-FM radio. I took a look at the NMEA 2000 power connections--one for the ICON PRO gauges, another for the chartplotter, VHF radio, and AM-FM radio--they have a bare wire shield in addition to the red and black leads. Is it common practice to tie the shield wire to the black (negative) lead of the power injector leads?
jimh posted 04-10-2015 12:56 AM ET (US)     Profile for jimh  Send Email to jimh     
On any shielded cable, the best practice is to tie the shield to the chassis of the device it is connecting to. Often the really best practice is to tie the shield or drain wire of a shielded cable to the chassis at only one end. This does not apply for coaxial transmission lines.

A real shield conductor should not be carrying any current. It should just be a shield. It would normally be bonded to the device chassis, and the device chassis would be bonded to ground or a common bus.

In a fiberglass boat there is really no ground. The battery negative is the best alternative. The battery negative is usually bonded to the engine block of the outboard engine, and that is, in turn, connected to the sea via electrodes on the engine block and gear case.

By only connecting one end of a shield on a cable, you insure that no current will be flowing on the shield. Which end to use depends on the wiring practice in your particular wiring. Some systems are wired with the shields only connected at the source end, not the receiver end. Some are the opposite. And some connect both--this is probably the typical practice.

As for the wires you describe, they do not sound at all like anything to do with the NMEA-2000 network power wiring.

The ICON Pro gauges have their own power supply cable. It is not for the NMEA-2000 network.

Devices like a radio, a chart plotter, and broadcast AM-FM receivers also have power cables for their own power. Again, nothing to do with the NMEA-2000 network.

If the power cable for the NMEA-2000 network has a shield, you can connect it to the battery negative, I suppose.

Do all those devices you mention:

--ICON Pro RPM gauge

--chart plotter

--VHF Marine Band radio

--Broadcast AM-FM receiver

really have shielded power cables? That sounds very odd.

Whaler_bob posted 04-10-2015 10:44 AM ET (US)     Profile for Whaler_bob    
Thanks Jim. Only the Garmin NMEA 2000 power cable has the shielded lead. See:

jimh posted 04-10-2015 10:48 AM ET (US)     Profile for jimh  Send Email to jimh     
Consult the GARMIN literature to see how they recommend their NMEA-2000 power cable be wired. I'd follow their recommendation.
Whaler_bob posted 04-10-2015 11:16 AM ET (US)     Profile for Whaler_bob    
^ Unfortunately, Garmin's NMEA 2000 tech reference doc shows their own branded power connector with only two leads... no shield. 2250_TechnicalReferenceforGarminNMEA2000Products.pdf

I'll probably end up calling Garmin since I have to speak with their tech support on another matter anyway.

Thanks for trying Jim!

jimh posted 04-10-2015 11:17 AM ET (US)     Profile for jimh  Send Email to jimh     
Well, I am not done, yet. The manner in which the shield on a NMEA-2000 power cable should be connected is dictated by the NMEA organization. A PowerPoint presentation they have published at pdf

contains the following advice regarding what to do with the shield or drain conductor of a NMEA-2000 power cable:

Connect shield to RF ground only once

If your small boat has a radio-frequency ground system, you can connect the shield of the NMEA power cable to that system. See the illustration in the presentation with the heading POWER INSERTION and with the folio number "79" in the lower right.

If you have an older Boston Whaler boat, there may be an electrode installed on the transom for carrying the boat fuel system bonding conductors to the sea. That bronze electrode could be used as a RF ground point. The bonding circuit is usually kept isolated from the battery negative circuit.

The shielding is really only necessary if the network wiring will be in an environment with strong radio-frequency fields.

The presentation I have hyperlinked (above) is a good reference for installation practices of NMEA-2000 networks. I recommend reviewing it.

jimh posted 04-10-2015 11:23 AM ET (US)     Profile for jimh  Send Email to jimh     
It is rather odd that a company sells you cable with three conductors and only tells you how to connect two of them. GARMIN should amend their literature.
Whaler_bob posted 04-10-2015 11:29 AM ET (US)     Profile for Whaler_bob    
Just read the 42-page Garmin [literature and] found the info I needed in one sentence on page-5: "Be sure to ground the NMEA 2000 power cable. Connect the bare shield-drain wire to the same location as the ground (black) wire."
jimh posted 04-10-2015 01:36 PM ET (US)     Profile for jimh  Send Email to jimh     
That is contrary advice from that given by NMEA, but I am sure it will be fine. Most fiberglass small boats do not have radio-frequency ground systems. As I mentioned in my initial reply, the battery negative is the alternative.
Whaler_bob posted 04-10-2015 03:51 PM ET (US)     Profile for Whaler_bob    
Thanks for your help Jim!
jimh posted 04-11-2015 11:03 AM ET (US)     Profile for jimh  Send Email to jimh     
I was just browsing that NMEA-2000 presentation again. I notice that in many illustrations they only show two conductors in the power cable, a positive and negative, and the shield is omitted. I also just checked the NMEA-2000 power cable from another vendor (Lowrance) that I have here on my bench: it has no shield or drain conductor provided. I suspect that there are many, many installation of NMEA-2000 power cables that don't have a shield or drain conductor, and of the ones that do, most are not bonded to a radio-frequency ground system.
jimh posted 04-15-2015 09:58 AM ET (US)     Profile for jimh  Send Email to jimh     
If a boat really were to have a dedicated radio-frequency ground system, it probably would also have some sort of radio frequency transmitter that uses an unbalance, single-ended, series-fed vertical antenna that needs a counterpoise system. In that application, I would recommend against connecting the shield conductors of any sort of shielded cables, other than the transmitter's transmission line to the antenna, to the radio-frequency ground system. There will be radio-frequency antenna currents flowing on the radio-frequency ground system, and that sort of current flow is precisely the sort of current you want to PREVENT from flowing on the shield conductors of cables carrying power or signals to electronic devices. For just about any small boat, even if equipped with a VHF Marine Band radio, the best choice for where to bond the shield conductor is probably going to be the battery negative terminal.

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