Electrical Question - Grounding The Auto Pilot

[Joe Mullee]

I'm almost done installing my auto pilot and have two questions right now. The instructions state that the course computer should be grounded to a negative terminal "of the battery". Is it acceptable to use the engines ground for this?

Also, it says to use "flat tinned copper braid, 30 A rating (1/4 inch) or greater". Is there an equivalent sized AWG size that I should be using?


Thanks for any help.

Joe Mullee

 

[sailingdog]

Joe—

IMHO, you really should run it back to a ground post or bus strip. Running it back to the engine can cause ground loops and the associated corrosion problems. As for how heavy a wire you need, it depends on how long the run is.

 

[kloudie1]

Joe, that is some ground wire!.. I think you can get the flat copper braid from an electrical supply place or, of course, on the internet. My guess is about ten feet of wire to get from the rudder area over to the batteries? They must have had some computer glitches caused by grounding to be calling for an extrordinary ground like that. Could be that they know something about the drive motor pulses that make them want the flat stuff.. An equivalent wire would be in the 8 gauge range but flat and round have different current and frequency carrying characteristics.. The round wire would probably work, but may not be optimal. I dunno! Good luck with that one..

 

[RAD]

I ran a # 6 when I did my installation and this ground is in addition to the negative 12 volt supply wire, I figured its for lighting/surge protection? and I agree go to a negative distribution buss or where ever your bonding terminates

 

[Orion4290]

SPX-5

I recently installed the new auto pilot. Raymarine says that if the flat braided wire is not avilable use #10 round wire about 1/4 inch in diameter. They also said to run it directly to the negative battery terminal if you are unable to connect it to the ground on the keel. Since they made the unit I would follow their instructions.

 

[sailingdog]

10 AWG wire is not nearly a 1/4" in diameter. 1/0 AWG wire is. Big difference.

I recently installed the new auto pilot. Raymarine says that if the flat braided wire is not avilable use #10 round wire about 1/4 inch in diameter. They also said to run it directly to the negative battery terminal if you are unable to connect it to the ground on the keel. Since they made the unit I would follow their instructions.

 

[kloudie1]

Ground Wire

I dunno, Dog.. Ya reckon they are talking O.D.. ??
Ya right, Rad.. It must be for lightning protection, especially if they are calling for a keel ground.

 

[RAD]

1/4 " is equivalent to #4 stranded

 

[paulj]

I just installed an S-1 and Raymarine supplied the wire fitting for the ground wire to attach to the computer ground terminal....so I used the largest YELLOW wire I could fit into the wire fitting and crimped it on and ran the other end to the batteries common negitive ground.

My thinking is the ground wire should not be bigger then the attachment point to the PC board.

paulj :troll:

 

[Stu Jackson]

...especially if they are calling for a keel ground.

The original question did not say keel, it said engine. Most people don't ground to their keels.

The concept is to get a "clean" ground and using your negative distribution post (NDP) or bus is preferred over direct to the engine. Just size the Ancor tinned wiring for the required 30A "load" (remember the circuit distance is out and back so you shouldn't see much difference in wire size even if you double the straight length you measure). I'd forget the dimension part stuff - seems weird to me.

 

[paulj]

OK so now you forced me to think.....
When you have wire shield grounds from various components hooked up to a PC board you need to ground these to a common ground off the board.
One of the things you do while trouble shooting PC boards is check to see if all the components attached need to be grounded and how.....Then check power in Then check signal out.
The PC board uses a RELAY for handling the 12volts and the big Amps for the motor and a BIG FUSE for protecting the motor to and from the battery in case of shorts or whatever.
So I would not size the ground wire for 30 Amps.

paulj :troll:

 

[Witzend]

The separate ground would be for shielding and sinking a lighting strike. I would bet it works fine with the ground jumpered to the negative term on the P.C. It's going there anyway. Most commercial equipment ties signal ground to chassis ground. On mil-spec equipment they are separate and can cause ground loops if connected. I would run it to the bus ground, not to the negative of the Bat. Using a big gauge wire is ridicules. It is not sinking any current normally. If it has it is because of a short or a lighting strike, think about the gauge of the wire used in the shields that are attached, those are going to fry first anyway.

 

[paulj]

Well usually ground loops are an after-the-fact type of problem in which the end-user blames the installer, the installer blames the manufacturer, and actually nobody is at fault. Neither the manufacturer nor the installer can usually predict where a loop will occur if it occurs. Only after the system is installed can it be determined if a problem will exist.
Power in signal out.......


Good Night John Boy

paulj :troll:

 

[Witzend]

Houston, we have a problem!
cheers,
T

 

[Dan Johnson]

Hey Joe...

Lots of good information here. I'm planning to run a piece of stranded 10AWG from that ground lug to the battery negative. The run will probably be less than 4 feet from where I'm planning to hang the computer box.

Checking the specs for our S-1, the drive is rated as 5 amps continuous. The larger fuse in the computer box is only 15 amps. I can't see the need for a "braided copper cable capable of handling 30 amps"--wait a minute--this thing is a Raymarine UK product--you don't think that they may have employed former Lucas Electrics engineers when BMC and other English car manufacturers went belly-up do you?

Seriously, what this ground lug is for, and other have eluded to, is a "Logic Ground". Looking at the motherboard itself, there are a lot of surface mounted IC's and other goodies. Highly digital environment--hence the need for a solid ground for the logic to have reference to as the actual voltage differentials between "High and Low" or "Off and On" are very small. It's not until you look at the circuit board itself right at that ground lug that you notice the etching "RF Ground"--AHAH!

A solid ground makes the "low" or "off" truely zero voltage. In my case, power is being fed by a relatively short run of dedicated 14AWG Ancor, which should provide sufficient power (it has been for the 4000 the S-1 is replacing).

As far as lightning protection, FERGITDABOUT IT! The induction field caused by a strike or near miss will probably scramble this thing's brains in an instant.

 

[Ron20324]

I'm almost done installing my auto pilot and have two questions right now. The instructions state that the course computer should be grounded to a negative terminal "of the battery". Is it acceptable to use the engines ground for this? Also, it says to use "flat tinned copper braid, 30 A rating (1/4 inch) or greater". Is there an equivalent sized AWG size that I should be using?"

Seems to me, if they want flat tinned copper braid, they are not saying this will be a "current carrying" conductor (and it's not fused), but it's use is that of a static strap to protect all those $$$ IC chips. You didn't mention where this is connected at the computer end. A chassis ground, i'm sure.

 

[Bill Roosa]

RF grounds vs DC grounds

If the PC board says RF ground then the keel is the place you want the braided (or flat copper ribbon). Thing RF ground for a SSB radio! Not electrical ground for DC.
I'd bet that the ground is RF because the PC board is running high speed digital signals that regular round ground can't see due to the frequency of the signals.
You should (if my if is correct) have an RF ground AND a DC ground. RF to keel and DC to Batt negative or buss.
I have never seen this kind of setup however. But the PC board etching is not there for shits and grins. Even Lucas would not tank insanity to that level.

 

[Stu Jackson]

I know we're mostly guys here, but can we please read the directions?

The instructions state that the course computer should be grounded to a negative terminal "of the battery". Is it acceptable to use the engines ground for this?

Also, it says to use "flat tinned copper braid, 30 A rating (1/4 inch) or greater". Is there an equivalent sized AWG size that I should be using?

That's what Joe says the book says. "(-) terminal "of the battery"." Is in quotes.

Could it be that we're only guessing about why they mention "flat tinned copper braid,..." again in Joe's quote.

Anybody have some specifics, since I didn't catch Joe's model # or manufacturer. How are we gonna check the direction/manual if we can't know what to download?

Hey, Joe! Are ya out dere?

 

[Don12364]

Joe,
Much good advice already.
I suggest a simple test.
Whilst alongside and with all other wiring complete just switch the pilot on and see if it works.
If it does then the ground connection specified does not form part of the power supply wiring to the pilot and is only a means of shielding the pilot's case.

From a designers viewpoint he is wishing to do his best to protect the pilot from outside influences. If the pilot is on sale in Europe then we have a specification requirement for "Electromagnetic Compatibility". I expect similar in the USA.
What this all means is that the item of equipment should neither cause interference problems with other equipment, nor should it suffer from interference caused by others.
To a certain extent protection from external interference is an unknown quantity so the 'best' grounding possible is desirable and the conventional way is to use a flat braid to, what is hoped to be, a 'good' ground point - and 1/4" wide copper braid is deemed to be satisfactory for all but the worst interference.

In fact just about any heavy gauge wire will suffice unless your system is being blasted by high power radar pulses - and if it is then even braid would probably not be enough.

My advice is to either buy a bit of braid from Radioshack or use some heavy gauge yellow/green wire and forget about it.
A Google search showed this site selling cut lengths of 3/8" tinned braid (best for marine) at $0.45/foot.
http://www.dxengineering.com/products.asp?ID=175.

 

[damauney]

Raymarin x-5 Ground Wire Size

I was just installing one today and apparently the manual has been updated. The manual now specifies that the ground go to the boat common ground and alternatively the battery ground. Also, the ground wire size in lieu of the 1/4 flat braid is a 4mm dia stranded wire which would be a #12 stranded wire. Hope this helps.

Good Winds
DaveM