Electrical ground cable to starter?

I'm not an electrical expert and might get myself massacred by the following question
Has anyone done a direct electrical ground cable/strap directly from the firewall grounding point to the starter/alternator? This, instead of running a ground strap from the firewall to the engine case. This will probably not add much more than 1.5 ft of fat wire and the respective weight. But won't it be a more predictable conductor than the engine case?

Depending where you attach the ground strap/cable you will have several aluminium to aluminum interfaces, some of them with sealants between them. So in worst case, only the bolts will conduct the current. Most of the bolts are steel with a conductibility that is approximately 7 times less than copper, i.e. you will need 7 times the cable's cross section in bolt's cross sections.

Example: 2AWG cable -> 0.0521 inch2 cross section, almost the same cross section as an AN4 bolt (0.049 inch"). So it would need at least 7 AN4 bolts to have the same conductivity as an 2 AWG copper cable.

So, in that point of view, the engine case is not a very predictible conductor. Any experience or opinions?

Electrically, there is no problem with doing what you are proposing.

But, but....

But thousands of planes have been flying for decades with the starter current ground path passing through the case. And cars.... and semi trucks... and bulldozers... and... ... and I did it anyway. Just seemed more true to theoretical. Battery negative directly to a bolt far forward on the case, near the starter, same size cable as the positive cable. Hasn't prevented dead batteries, though. If I were to do it again, I would use the same amount of copper to plumb a simple ground power jumper loop. Make the plane easy to live with even on days when things aren't going perfectly.

If it makes you feel better, ground the strap to the same case half that the starter is attached to (even numbered cylinders). The cross section area of the case is orders of magnitude greater than the area of the copper cable.


- Roger

RKellogg said:

The cross section area of the case is orders of magnitude greater than the area of the copper cable.

- Roger

Click to expand...

Exactly why it is used instead of a copper cable.

Your starter actually gets it's electrons from the negative side of the battery, to the case and on to the starter.

The flow IS from - to +..................

The standard practice (for certified and experimentals) is to run a ground strap from a bolt on the back of the engine to the airframe.

Why add complexity and a possible failure point?

Keep it simple and you will have fewer problems.

Two ground straps to the case are better than one........ A broken ground strap can ruin your day.

Ok. I know that was "tested" in thousands of planes... But in other threads here on Vansairforce people are caring about ground loops in their electrical systems. Isn't the engine case as a conductor prone to ground loops? The current can go different ways from the ground strap back on the engine to the alternator and starter - most probably with different resistances. Or does this not matter because it's within the engine and not within the aircraft structure? I was just wondering why this never posed some problems?

Other good point: Using two grounding straps.
I was told by an aircraft mechanic that I should better use two grounding straps attached at two different locations for safety reasons. Ok. But aren't we stepping in the ground loop trap again? If I want to be sure that I do not have a ground loop, I will have to attach the straps at the same locations. If I use the same bolt to clamp the terminals, I will loose some of the fail safe characteristics. So better use two adjacent bolts on the engine case and two bolts in my firewall grounding tab? Anyone who did this?
Other question for those using two grounding straps: How are the straps sized? Each of them 2AWG, if I use 2AWG wire for the starter? Or each of them the size that two of them equal 2AWG together?

The longer I reflect on those electrons flowing in my plane, the more confused I get

I prefer braided ground straps like this:



I use 3:
Bat to eng
Bat to firewall (thru forest of tabs ground plate)
Firewall to engine

ground loops

I'm not qualified as an expert on this, but... sometimes I listen to guys that are.

Ground loops are an issue with sensetive electronic equipment that is measuring a signal voltage difference relative to ground, such as avionics. If a large current is flowing in the groung conductor there will be some voltage difference along its length. Device A feels the difference between ground and some signal line as 2.5 volts, say. Device B, at the other end of the ground conductor, measures, perhaps 3.5 volts difference between the signal line and the ground reference conductor. As soon as the current that has been flowing through the ground conductor stops, both devices see 3.5 volts with respect to ground. The two devices become confused because of the difference in potential at the reference ground, and static or misfunction results. Electronics that measure or operate on very small voltave differences are most sensitive. This could be seen as fluctuations in fuel gages, temperature measurements, or pops and booms on audio equipment, ouch.

The starter circuit is a gross consumer of electrons. As you note, resistance anywhere in the loop reduces the voltage available at the starter motor. Even with large gage conductors there will be measurable voltage drops in the loop. It is not practical to create a loop that provides a voltage drop of , say, 0.1 volts. The cables would be huge. Starter motors operate in extreme overload conditions. Wittness warnings about not cranking for more than ten seconds, then cool for two minutes. So...it is inevitable that there will be a measurable voltage drop along the ground conductor. Don't expect it to provide a stable ground reverence while the starter is engaged.

When the starter is engaged, most of the avionics are offline, no?

Roger

You are going to have multiple ground paths, as do all of us, that ground loops don't seem to be an issue.

Paths on my plane:

Ground strap
Oil pressure line
Fuel pressure line
Throttle cable
Mixture cable
Carb heat cable
Fuel line
EGT & CHT probes
Other probes
EICommander ground
Etc.

That's all I can think of right now but I'm sure there are more.

Mmhh, you are right. There are multiple paths anyway. But the "not intended ones" should have higher resistance.

I have not seen that the issue was discussed as well in a previous thread: http://www.vansairforce.com/community/showthread.php?t=109567&highlight=starter+ground

... and I learned that there are cases around, where grounding through the engine case is a problem. There wase a case here in Switzerland with a Lancair Legacy. They measured a 2V drop through grounding of the engine case in the starter wire. After adding a ground wire directly to the starter the remaining voltage drop was 0.1-0.3V. Makes a difference when you want to crank your engine.

Will see in near future, what I will do...

Ground Straps

RKellogg said:

But thousands of planes have been flying for decades with the starter current ground path passing through the case. And cars.... and semi trucks... and bulldozers... and... ... and I did it anyway. Just seemed more true to theoretical. Battery negative directly to a bolt far forward on the case, near the starter, same size cable as the positive cable. Hasn't prevented dead batteries, though. If I were to do it again, I would use the same amount of copper to plumb a simple ground power jumper loop. Make the plane easy to live with even on days when things aren't going perfectly.

If it makes you feel better, ground the strap to the same case half that the starter is attached to (even numbered cylinders). The cross section area of the case is orders of magnitude greater than the area of the copper cable.


- Roger

Click to expand...

I had a problem once on my Super 8 with slow starter turnover, which made it hard for hot restarts, etc.. Thought it was a weak battery, but to trouble shoot I ran a long jumper cable from the battery (negative side) to the starter case. That "weak" battery spun that engine like a top. The weak link ended up being the ground strap at the engine case where it bolted on. I cleaned that up and it's been fine since. I still think that the size of my ground strap from the airframe to engine case limits the current to the starter.

A no so short story about ground straps and poor connections. The aircraft was a PA28-140, but this type of thing could happen on any aircraft. In the PA28-140 (Cherokee) the battery is under the rear seat, just aft of the main spar carry though. The battery is grounded to the spar, via a chunky flat ground strap.

The aircraft had been weak starting but not bad enough to be a concern.

The story starts with the smell of fuel in the cockpit when we arrived at the hangar and opened the cabin door. It quickly went away and was not noticed during flight. However, it kept getting worse. After being barely noticable "do I smell fuel" it got to a point of "we need to check this out". So looked at all of the obvious points, fuel valve, primer and connections. No obvious leaks. Next, the Cherokees had a SB on vent connections deteriorating and fuel running down the spar into the cabin. Pulled the tanks, noting noted. Now scratching our heads pretty hard and the smell keeps getting worse. I started a search on every fuel line with a white rag. Upon wiping the fuel line near the battery found a small blue fuel stain. No connections nearby. Hmmmm. With a strong light, and mirror started to examine every inch of line in the area. The ground strap passed about 3/8 to 1/2 half inch below the fuel line. With the mirror couldn't see anything. Repeated the rag wipe an hour later, found more stain. Removed the tube and saw a small black spot where the stain was found. Magnifying glass next. Couldn't believe, it, there was a very small hole from arcing. Wow!
With no fuel in the line, reinstalled and started the airplane. Yep, when the starter was released there is a small arc, ground strap to tube. Only visible in fairly dark conditions. Not sure at what point this would have caused a fire or explosion in the compartment.

Today, with diodes across the relay coils, this shouldn't happen, but there was obviously a resistance between the "grounded" fuel line and the ground strap.

We cleaned up the ground point and protected from corrosion. The engine started a bit better too.