Bonding Impedance

[rvmv]

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

[Peter Costick]

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.

[rv7charlie]

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.

[Walt]

Originally Posted by rv7charlie

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.

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.

[dwrichey]

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.

[PilotjohnS]

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.

[BobTurner]

Originally Posted by PilotjohnS

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.

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.

[rv7charlie]

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? ;-)

[Timberwolf]

Originally Posted by Walt

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.

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/catal...hp?recfer=5242

[vlittle]

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.