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Bonding Impedance

R

rvmv

Active Member
I'm new to the electronics stuff. I ran across this in the install instructions for a transponder.

"Bonding impedance between aircraft ground and the unit chassis must be less than 2.5 milliohms."

Is this the resistance to current flow from the chassis to ground and can you measure it by using a multimeter on the ohm setting with one probe on the chassis and the other on the aircraft body where it attaches? Thanks
 
I think that practically this just means a good electrical connection, and a good electrical connection is made by creating a good physical connection. Measure it with an ohm meter, usually a multimeter set to ohms.
 
You're not going to measure that with a regular ohm meter. The simplest way to ensure a good connection is to just use a 'star washer' or internal tooth lock washer under the screw head and the nut.
 
rv7charlie said:
You're not going to measure that with a regular ohm meter. The simplest way to ensure a good connection is to just use a 'star washer' or internal tooth lock washer under the screw head and the nut.
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The above will work, however this method should only be used in non-structural applications (star washers will damage aluminum).
Info on proper grounding installation/assy can be found in 43.13 1B chapt 11.
 
Bob Nuckolls discusses resistance measurement extensively in The Aero Electric Connection (I believe everybody needs to read this book, cover to cover, before starting your aircraft electrical system design and installation.) In it he gives an explanation on how to construct a very sensitive meter using two VOLT/OHM meters wired together. I took the time to build the setup because my brand new high compression IO-360 with a SkyTec HD starter was not spinning over as quickly as I assumed it should. I found it useful in identifying/confirming even very small resistances (milli-ohms) in ground connections and even the approximately 10 foot long size 2 cable. At minimum, it gives you confidence that resistances are very low at connections, as they should be.
 
My take

Except for the starter circuit, and major power grounds, i think the electrical bonding measurements can be made with a simple ohm meter. As long as things measure less than 1/2 ohm , and the meter reaches that low level rather quickly, then the electrical bonding should be fine. For the high current connections, these measurements are more critical, so a simple ohm meter check may not be suffispcient. JMHO.
 
PilotjohnS said:
Except for the starter circuit, and major power grounds, i think the electrical bonding measurements can be made with a simple ohm meter. As long as things measure less than 1/2 ohm , and the meter reaches that low level rather quickly, then the electrical bonding should be fine. O.
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I very much disagree. If it?s a transmitter pulling 6 amps, then that half ohm means a 3 volt loss in voltage. But it?s worse than that: a half ohm means you made a poor connection somewhere. A few years from now, with air and moisture getting in there (because it is a poor connection) there?ll be oxidation, etc., and that poor connection will be a bad connection. Truth is, in a new airplane that is all shinny and clean, almost anything will work. But give it some time, and poor workmanship or technique will start to show up as strange whinning noises, etc. At that point, tracking them down is a nightmare. Best to do it right in the beginning. There are whole books on this. Personally I like to use a copper buss bar as a ?single point ground?, although in practice virtually no one has a real single point ground.
 
Not to speak for John, but I doubt that's what he meant. You just can't measure resistances lower than an ohm or two accurately with a regular ohm meter. But a 1/2 ohm reading on a joint that was made using common sense good practices will almost certainly be a good joint. I'm confident that not 1% of all the a/c ever built have had a resistance measured with a resistance bridge, but the *vast* majority still have good electrical connections.

And to the issue of damaging structure with a star washer: wasn't the damage already done by the hole? And how many transponders are mounted to spars, etc? ;-)
 
Walt said:
The above will work, however this method should only be used in non-structural applications (star washers will damage aluminum).
Info on proper grounding installation/assy can be found in 43.13 1B chapt 11.
Click to expand...

Hmmm....I do agree with you, however....have you installed a transponder antenna lately? Those all come with star washers...from the manufacturer with TSO certification for the unit and all.

https://www.aircraftspruce.com/catalog/avpages/av22.php?recfer=5242
 
Every procedure must have a specification. That?s true in any manufacturing process that purports to have a quality standard. Measuring to spec is another problem, so many mfrs adopt a ?correct by construction? process which is what AC43.13 is all about.

So, follow AC43.13 and build on!

As an aside, I find the kit aircraft business to be a bit sketchy in the quality assurance. Passing the QA function to the assembler/builder dramaticaly lowers kit costs but it requires a well informed and skilled customer base. Thanks VAF for helping with this.

Yes, I have mispunched parts from Vans (one was a spar!). I recognized the problems and worked with Vans for a resolution.
 
Yes

rv7charlie said:
Not to speak for John, but I doubt that's what he meant. You just can't measure resistances lower than an ohm or two accurately with a regular ohm meter. But a 1/2 ohm reading on a joint that was made using common sense good practices will almost certainly be a good joint. I'm confident that not 1% of all the a/c ever built have had a resistance measured with a resistance bridge, but the *vast* majority still have good electrical connections.

And to the issue of damaging structure with a star washer: wasn't the damage already done by the hole? And how many transponders are mounted to spars, etc? ;-)
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Yes this is what i meant. If you use the proper method for electrical bonding and happen to only have a simple ohm meter , the assembly might be OK. By no means is any electrical bonding method OK if the measurement is 1/2 ohm; This is too high. If the assembly method is questionable, then a precise measurement is required.
 
rv7charlie said:
And to the issue of damaging structure with a star washer: wasn't the damage already done by the hole? And how many transponders are mounted to spars, etc? ;-)
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Gouging the structure or skin of an aircraft with a locking type washer is unacceptable, I can't recall ever seeing an assy call out using a lock type washer without first installing a plain washer to protect the underlying material. And a properly drilled/deburred hole with a fastener installed per an installation guideline and/or standard practice, is not considered "damage".

Bottom line, lock washers are supposed to be used to prevent nuts from loosening, thats it.

I've also never seen a transponder mounted to a "spar", not sure what you are referring to?
 
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Walt said:
I've also never seen a transponder mounted to a "spar", not sure what you are referring to?
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rv7charlie said:
The op asked about installing a transponder. How far do you stretch the definition of 'structure'?
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Sorry I missed your sarcasm on the spar comment.

Does it matter if you install a star washer on a rack rail, probably not, but my point is there is no need for one and it's not considered good/standard practice IMO, obviously folks can do whatever they want but I tend to point them to standard/approved methods rather than home grown ones.
 
dpansier said:
I believe many see the reference to the use of "star washers" in this EAA video.

http://www.eaavideo.org/detail/video/1136841468001/antenna-installation?autoStart=true&q=Antenna
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Yes but notice the "doubler" (actually reinforcement plate as most doublers are riveted) which prevents damage to the skin from the star washer. The ground path is through the skin and doubler in this case, so if you don't ensure you have a good contact between the doubler and the skin, the star washer is useless, I think he may have forgot that important step.
 
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