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braided grounding strap isolation

dbaflyer

Well Known Member
I am working to put one of these from my engine to firewall FOT stud. Does the braided strap have to be isolated from touching anything between the two mounting points? My path from engine mount point to FOT is somewhat difficult from having a completely clear path. Is it OK to touch at least the engine mount?
 
Shouldn't be a problem so long as the grounds are solid at each end. So long as there is no potential (voltage) between the points contacted (e.g. no resistance between them along the ground strap) there should be no issues. In fact, on the RV-10 Van's recommended ground strap connection points are between the engine and tabs provided on the engine mount for that purpose.
 
Just be aware of the potential for the strap to abrade whatever it's touching. I'd rather see a clear path with plenty of room around it. The engine moves a lot on startup/shutdown, plus the vibrations during normal operations.

You don't want to set up a situation where it will damage something like the mount.
 
There is no requirement to use a braided ground strap, and some reasons not to.

I use the same gauge welding cable (#2) to ground the engine that runs power to the starter motor. I ground the engine on the nice ground lug on the back of the starter - that is what needs the good ground in the first place.

Carl
 
I also used a #2 welding cable to the starter lug. And another one to the vacuum studs..
 
There is no requirement to use a braided ground strap, and some reasons not to.

I use the same gauge welding cable (#2) to ground the engine that runs power to the starter motor. I ground the engine on the nice ground lug on the back of the starter - that is what needs the good ground in the first place.

Carl
Mine is similar, from the vacuum pump pad to my grounding point on the firewall. I used a longish cable in a U-shape (plan view) to allow for engine twist.
 
There is a reason that ground straps exist. The connection from the frame to the engine is a high vibration connection and any cable connecting the two needs to have a large quantity of small gauge copper strands. This is what a grounding strap is. A welding cable is similar and good for this application. However, regular cables are not appropriate for this application and there is a decent possibility of the individual strands breaking and reducing the ampacity of the cable.

A ground strap does not need to be isolated from anything other than exposed positive connections and, as mentioned, anything that can cause fraying of the strap.

Larry
 
Also, grounding straps are flat with their wires weaved in many different angles to minimize signals being induced into the ground path. But this "normally" is not an issue in metal airplanes.

Ground_Braid_01.jpg


:cool:
 
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Best practice would be to take a piece of rubber tubing and slit it and then zip tie it around the engine mount where the ground strap touches to prevent any degradation of the ground strap...
 
Talking to a mechanic recently, he recommended two ground straps for redundancy. Is anyone else doing that?
 
Anything worth doing is worth over doing

I've got two ground straps. One to each of two engine studs. Probably overkill, but it seemed like a good idea at the time!
 
I have two ground straps.... one braided and the other made from welding cable. One to the firewall, one to the pass through ground lug to the field of grounds on the cold side of the firewall.

Easy to do..... If you only have one and it fails, you will have big problems
 
Talking to a mechanic recently, he recommended two ground straps for redundancy. Is anyone else doing that?

If the connections at both ends of the cable are sound, a single cable is plenty. If they are poorly done, a second cable installed in the same manner probably won?t help!

I use di-electric grease to protect all surfaces within the connections from moisture and corrosion due to dissimilar metal contact, and make sure that the fasteners are amply sized, properly torqued, and positioned in a way that does not invite loosening due to vibration.

Cable connections, both power and ground, should always be the first suspects when the starter seems sluggish.- Otis
 
2 grounds seems to be a good idea

I have two ground straps.... ..... If you only have one and it fails, you will have big problems
Yes, the electrons try to find a path, and it could fry something that you don't want fried if your single ground wire fails. Having two adds a few grams, but could save a lot of money and time if there is a failure.
 
Thanks for the replies, seems like a good idea. Just as soon as we're finished with power cuts and big snow.
 
Very important..

Mine is actually a double flat braided ground strap that shares the same lugs. I liberated it from an airliner component ground, but it is quite redundant in the event one would break. Keep in mind, it is also the ground path for your starter, and carries significant flow during start.
 
Talking to a mechanic recently, he recommended two ground straps for redundancy. Is anyone else doing that?

The braided fuel and oil pressure lines work as secondary and tertiary grounding paths.

No need for another strap.

Although this sounds frightening, they will transmit the load without fear of an explosion.
 
That might be true for normal electrical loads, but I'd be very leery of expecting them to carry starting loads. Not because of explosion risk, but the risk of weakening and eventually failing the tubing inside the braid due to heating the braid during starting events.

Automotive example: I once owned one of the 1st front wheel drive cars sold here in the USA. After a warranty repair at the dealership, the car became progressively and intermittently harder and harder to crank, as if it had a bad battery, though the battery was good and its connections clean and tight. This went on for literally years, until one day when I had the hood open and had someone try to start it. When they hit the starter, I saw smoke coming off the clutch actuating cable. The dealer had left the braided ground strap loose on one end, and the only ground from engine to the chassis was through the clutch cable. If it had been a hydraulic line instead of a Bowden style cable, I'm confident that the rubber inside would have been damaged and would have eventually failed.

Charlie
 
That might be true for normal electrical loads, but I'd be very leery of expecting them to carry starting loads. Not because of explosion risk, but the risk of weakening and eventually failing the tubing inside the braid due to heating the braid during starting events.

Automotive example: I once owned one of the 1st front wheel drive cars sold here in the USA. After a warranty repair at the dealership, the car became progressively and intermittently harder and harder to crank, as if it had a bad battery, though the battery was good and its connections clean and tight. This went on for literally years, until one day when I had the hood open and had someone try to start it. When they hit the starter, I saw smoke coming off the clutch actuating cable. The dealer had left the braided ground strap loose on one end, and the only ground from engine to the chassis was through the clutch cable. If it had been a hydraulic line instead of a Bowden style cable, I'm confident that the rubber inside would have been damaged and would have eventually failed.

Charlie
Different issue and different type of connection for a clutch cable vs. a tightened AN connection.

Don't get me wrong, I wouldn't want to use those lines as a primary ground but as a backup, they will be fine.
 
It wasn't an issue of tight or not; it was an issue of cross sectional area and type of metal (conductivity) of the conductor. What amounted to a small gauge steel cable became a heater, with starter current flowing through it.
 
What about a secondary ground from one of the starter mounting lugs directly to the negative battery terminal to reduce chances of poor connections slowing the starter?
 
What about a secondary ground from one of the starter mounting lugs directly to the negative battery terminal to reduce chances of poor connections slowing the starter?

I think that one of the morals of charlie's story is that you can overcome the need with proper maintenance. You have can have three ground cables, but if they are all loose on one end, you'll still have a problem.

Larry
 
I think that one of the morals of charlie's story is that you can overcome the need with proper maintenance. You have can have three ground cables, but if they are all loose on one end, you'll still have a problem.

Larry

Bingo. You only need one, as long as it works. Make sure it works.
 
Question for post #12 GASMAN " bad things will happen"

For my education. I thought it was good practice to bring ground functions back to the ground buss to avoid the chassis as a ground, example Pmags and sensor functions. It seems to me that the only thing that would suffer is starter performance if the 'big' ground conductor were degrading, and you get a warning from cranking performance.
So, not challenging two grounds if preferred, but rather looking for best practice input on FWF grounding other than the starter. Good timing, I am wiring my 2nd RV.
 
The problem can arise in the situation I described earlier. If the heavy duty ground is lost, and you have a few light-gauge ground wires (mag shields, sensor shields, etc) connecting the engine to the airframe, you can smoke the insulation in just a few seconds of cranking action. In this situation, none is better than inadequate. :)
 
Ahh, Thanks Charlie

Yes that will happen, so the conclusion is, a second heavy ground is Protection' for ALL grounding. This is a catch 22. A poorly maintained third ground cable leaves the same problem. Isolating grounded components ( example sensors that tread into manifolds and engine block) is challenging to impossible.

So, for electric gurus, how do you confirm, proactively, the quality of a high current conductor/connection ?
 
Airframe grounds work fine for low-noise or resistive loads, but should be avoided for radios and audio work. I brought my battery ground to a copper bar on the firewall with another copper bar on the other side of the firewall, bolted together sandwiching the firewall between them and hung the battery ground on one large lug there. Another #2 welding cable from the engine ground also went to that copper bar for starting current ground, no secondary. The copper bars gave me an excellent ground point both forward and aft of the firewall and I used airframe grounds for items that were further away.

You really want to avoid ground loops for radio or audio circuits, but airframe grounds will work fine for most other work.
 
For my education. I thought it was good practice to bring ground functions back to the ground buss to avoid the chassis as a ground, example Pmags and sensor functions. It seems to me that the only thing that would suffer is starter performance if the 'big' ground conductor were degrading, and you get a warning from cranking performance.
So, not challenging two grounds if preferred, but rather looking for best practice input on FWF grounding other than the starter. Good timing, I am wiring my 2nd RV.

Chassis grounding is not evil. I can't think of a single auto that doesn't use the engine block as a ground for the starter and most then have a ground strap to the frame/chassis and from the chassis to the battery. There are benefits to home runs to a common ground block, but they don't really apply to starter circuits.

Nothing wrong with backups, but that doesn't mean a frame ground is bad for a starter circuit, minus the failure mode issues outlined by charlie. A failed connection is a failed connection, whether that cable goes to the engine or the ground block. A failed connection on either will create the issues outlined by charlie.

Larry
 
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Yes that will happen, so the conclusion is, a second heavy ground is Protection' for ALL grounding. This is a catch 22. A poorly maintained third ground cable leaves the same problem. Isolating grounded components ( example sensors that tread into manifolds and engine block) is challenging to impossible.

So, for electric gurus, how do you confirm, proactively, the quality of a high current conductor/connection ?

I see this as two different questions, one about workmanship; the other more, well, philosophical.

I would say that you confirm quality of cable/connection the same way that you confirm the rest of your workmanship: Use known-good practices and check your work. And during any maintenance, keep some kind of checklist of what came off, so you can check off items as you replace them.

From a more 'philosophical' angle (more of a risk/benefit analysis, actually), I only have one 'fat' ground cable from engine to battery/airframe on my stock Lyc powered -4, & see no need to add a second. However, I'm installing an engine on my -7 that will be completely electrically dependent. I'll almost certainly install a 2nd ground path on that plane; more for 'belt&suspenders' insurance of continued engine operation than worries about smoking wires during startup. Properly installed, there's no need to worry about the ground path, but it's relatively cheap in labor, materials, and weight, to create a redundant system. That's the 'philosophical' part: deciding when and how far to go, when creating plan Bs. Most of us demand dual ignition, but accept a single carb to supply fuel. But when we move to electronic injection, we tend to demand dual controllers. But we accept single injectors on each cylinder. Etc Etc.

Charlie
 
Different issue and different type of connection for a clutch cable vs. a tightened AN connection.

Don't get me wrong, I wouldn't want to use those lines as a primary ground but as a backup, they will be fine.

I wouldn?t count on a braided hose at all. The resistance of stainless steel is 40+ times that of copper for the same length and cross section.
 
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