Re: [Amel Yacht Owners] Re: Two questions

Ryan Meador

The drain and shield are connected together, and should be regarded as a single conductor.  The drain wire is a lot easier to attach to than the shield.

Maretron has this to say:
The NMEA 2000® network should be grounded at ONE location. Grounding at more than one location may produce ground loops, which can cause problems with communications on the network. In addition to the ground wire, connect the drain or SHIELD wire at the supply ground location and NO other place.

I take this to mean both ground and drain should be connected to the output of the 12V converter that is powering the system, which is the way it is set up on my boat.  This also matches general electrical engineering practice.

Part of the confusion I think is the use of the term "ground" for too many things.  Every circuit has a chosen point that is the zero-volt reference, and this is called ground.  The shield must be connected to the reference point of the power and signal wires to be effective.  But on an Amel, we also have the 24VDC ground and the AC ground, which are all distinct.  We even often call the DC ground by the slightly-inaccurate name "DC negative" to emphasize that it isn't connected to the AC ground.

Ryan and Kelly
SM 233 Iteration
Boston, MA, USA

On Fri, Jun 1, 2018 at 4:11 AM, divanz620@... [amelyachtowners] <amelyachtowners@...> wrote:

In the NMEA 2000 scheme there is a DRAIN wire and a SHIELD.

The recommendation is that both should be connected to GROUND at ONE POINT ONLY.
Problems have been caused it seems by earth loops where the drain and / or shield have been grounded at more than one point.
On most boats the ground connection is connected to battery -ve which maybe the cause of these earth loop problems as they now effectively have multiple ground connections.
Amel boats do not have a connection between battery -ve and ground.
So on our boats, my instinct tells me that the drain and ground should be connected to ships ground to eliminate interference problems from RF....from VHF or SSB for example 
However if the NMEA 2000 system has issues as you have said, then disconnecting that ground connection is not going to harm anything....and if there are issues it may be worth disconnecting the drain or ground from ships ground to see if either makes any difference. In any event you won't damage any equipment by doing that.
Just what I think...I may be has been known !
Elyse SM437
Malumu Bay, Beqa

Join to automatically receive all group messages.