Grounding -Canbus - Van's Common Harness - Confusion

[design4p]

Hi everyone,

I am getting confused with the idea of Grounding.

I am building a RV14 using the Van's Common Harness and a suite of Garmin LRU's ie:

My CanBus route is:GSA28 - GMU11 - GEA24 - GAD27 - GSU25 - GDU460 (Left) - GMC507 - GMA245 - G5 - GDU460 (Right) - GSA28.

Garmin recommend to ground at each backshell and to only have one location for grounding. Van's Common Harness ground the Pitch and Roll servos in the fuselage and wing and links the ground with other grounded wires. Also recommended is all Audio to be grounded to backshells and then my starter key/switch recommends using shielded wires which are grounded in close location to the switch.

I am trying to see some logic in so many ground recommendations - Any supportive assistance would be most welcome.

John

[Ed_Wischmeyer]

From my point of view, grounding is a black art that I don't pretend to understand.

But here's the key idea: for direct current (DC), current seeks the path of least resistance. But for AC (i.e., noise), current seeks the path of least inductance, not the path of least resistance. So if you just hook up a bunch of ground wires together as would make sense for DC, you can create a real mess for noise (AC).

There's lots of articles and such on all this, and good luck!

[RV6_flyer]

Originally Posted by Ed_Wischmeyer

From my point of view, grounding is a black art that I don't pretend to understand.

--- snip ---

There's lots of articles and such on all this, and good luck!

Before I retired, I worked as an engineer in the aerospace business doing lots of testing including EMI.

Shield grounds would typically be used or not used based on frequency of signal in the wire. Testing determined if you would or would not ground.

I agree with Ed that grounding is somewhat of a "Black Art."

When it comes to grounding, I tend to follow designer's or manufacturer's recommendations. IF I get static or undesirable operation when I test, I may then try elimating a ground or adding a ground.

[bkervaski]

Black Art, maybe, but there are some common conventions for your build, don't get off into the ditch with information overload, and don't unnecessarily complicate your harness, it will be hard to maintain later.

For example, in my all Garmin panel with the harness designed by SteinAir, all the avionics on my 14 are grounded to a "forest of tabs" located opposite side of the firewall to the engine ground and I haven't had any issues whatsoever. Really clean, easy to maintain, and reliable setup.

The only things that aren't grounded there are due to location not warranting an extra wire run, in which case they are grounded to the fuselage. Servos, Pitot controller, etc.

[Mark Browning]

Garmin have some great you tube videos for 3GX wiring

https://www.youtube.com/watch?v=KjeTWw4k1nw

Stein air also have some help full tips

https://www.youtube.com/watch?v=uhRCUAYoSXg

[mfleming]

Originally Posted by Mark Browning

Garmin have some great you tube videos for 3GX wiring

https://www.youtube.com/watch?v=KjeTWw4k1nw.....SNIP.....

While the basics of these videos are informative, the grounding technique depicted are outdated.

The current Garmin method is to ground at each backshell, not looping the grounds together as shown.

[wjb]

There are two types of grounding here getting mixed here, I think.

1) The canbus grounds are shield grounds and don't carry any real current; they work as a shield (hence the name) to keep external interfering electric fields from mucking up the signals on your wires (BTW, the Garmin canbus differential twisted pair signaling is quite resistant to external interference as it is). All these shields should be terminated on both ends to the connector backshells to maintain good shield effectiveness. Garmin makes GREAT metal backshells, with ties points for shield termination ... I bought a few extra install kits just for the backshells for non Garmin HW.

2) Power return grounds are the other. +12V at 1, 2, 5+ amps goes in to power the device, and this current needs to return to the battery/alternator. In this case, all devices should have a dedicated return line to a "single point" ground. Practically speaking, this is a "forest of tabs" or similar grounding block located very near the negative terminal of the battery, usually directly tied to fuse connect point. I've seen some installs that ground to the fuselage wherever they are, and force the current to return via the skin, etc. Not a good plan, since the fuse is not designed to be a high quality electrical conductor. Intermittent or otherwise sketchy connectivity will breed all kinds of gremlins.

Think of the electricity as water .. you don't just want to dump into the fuse, you want specific plumbing to route to the drain.

Forest 'o tabs; the battery is just on the other side of the firewall. One ground return for each box, light, motor, electro-thingy.