(DG11) DC current in SPX 30 RF ground wire

[patja]

Hi Folks.

I have my Raymarine SPX 30 and P70s connected over a Garmin N2K backbone. I spliced the unique SeatalkNG connector provided with the SPX 30 to a Garmin spur cable, including the drain wire. They communicate fine.

The Garmin NMEA 2000 network backbone is powered by the Garmin power spur.  The SPX 30 SeatalkNG power switch is turned off. The Garmin power spur cable has a DC - and a drain wire, and the installation instructions they should both be connected to the negative bus.

The SPX 30 has an RF Ground connection, which is recommended to connect to a submerged ground plate, or the DC - battery terminal/primary bus.

Internal to the SPX 30, the RF Ground and the SeatalkNG drain terminals are connected (unplugged everything and verified with my DMM).

I am observing current flowing in this wire, which is troubling.  Turn a DC load such as cabin lights on, and I will see about 10% of the current come back over this drain wire, which is tiny.  For the time being I have disconnected the SPX 30 RF Ground.

I think what I have is a ground loop, and based on my reading about drain or shield wires, only one end should be connected to ground.  Connecting both ends to ground can result in a ground loop. I think that's what I have going on here, partially due to the spliced drain wire.

Should I disconnect the drain wire from my splice to the SPX30 Seatalk NG? That would leave its RF drain isolated from the N2K drain...separate drains, separately terminated at the DC - bus.

[Derek - Raymarine - Moderator]

Hi patja

The drain wire,often called the shield is for RF protection and as you state,should only be connected at one end. Generally the drain should not be connected to the -ve volts power wire and so this should be separated.

The Drain is connected to an isolated RF ground plate under the boat, BUT is only used if you have specific EMC issues for example or an SSB which then would require a specific groundplate such as a dynaplate.

The Drain and RF ground connections are provided to meet specific EMC requirements, for test purposes, but in general are not used in the vast majority of installations, since most boats do not experience any find of RF or EMC issues.

In your installation, please separate the Drain and battery -ve connections, disconnect the drain connection on the SPX too.

Regards
Derek