A 0.4 Ohm Bond Reading That Should Never Have Been Recorded
During an avionics upgrade, a mechanic must show that a new remote transceiver rack is bonded to primary structure within 2.5 milliohms. Using a handheld DMM on the 200 ohm range with its original test leads, he measures 0.4 ohm between a rack mounting bolt and a nearby structural rib and writes up a bonding failure. The bond braid is 6 in. long, attached at one end with a star washer under a bolt on a painted surface. The rack is otherwise correctly installed. The DMM's calibration sticker is eight months old and current, and the mechanic proposes running a new 8 AWG ground wire to fix what he calls the high resistance.
1. Is the recorded measurement valid evidence of a bonding failure? Explain what the instrument and method can and cannot resolve.
Read the worked answer
No - the measurement cannot support any conclusion, pass or fail.
- A two-wire DMM measurement includes lead resistance, typically 0.2 to 0.4 ohm. The reading of 0.4 ohm is essentially the leads plus probe contact.
- On the 200 ohm range the display resolution is 0.1 ohm. The acceptance limit is 0.0025 ohm - 40 times smaller than one display count. The instrument is physically incapable of resolving the spec.
- The DMM's ohms source pushes microamps, which will not break down the thin oxide and primer films that matter at bonding currents.
The meter being in calibration is irrelevant; calibration does not add resolution. The proposed 8 AWG wire is also the wrong remedy - bonding resistance is set by surface preparation and the braid, not by adding a parallel conductor.
2. Describe the correct measurement, including instrument, setup, and safety.
Read the worked answer
- Use a dedicated four-terminal (Kelvin) bond meter with the test current the specification or OEM data requires (commonly around 1 A). Four-terminal measurement cancels lead resistance.
- Zero the meter with the leads shorted on the same range before measuring, and verify with a known reference if the meter provides one.
- Probe on bare metal at both ends of the bond path, at the actual bond points - not on paint, primer, or anodize.
- Verify the installation itself: star washer present and correct, fastener torqued to spec, braid undamaged, no corrosion, correct braid length and routing.
- Aircraft unpowered. In fuel tank or vapor zones, use only intrinsically safe test equipment.
- Record the actual milliohm value, instrument ID, and calibration date.
Do not use an insulation tester (megger) for this check - it measures the wrong quantity at the wrong current.
3. Re-measured properly, the bond reads 4 milliohms. What are your corrective actions, in order?
Read the worked answer
The bond fails at 4 mOhm against a 2.5 mOhm limit. Work in order of least invasive first:
- Surface preparation - remove paint/primer to bare metal at both bond points within the allowable finish scheme, reinstall with the specified star washer, torque to spec.
- Inspect the braid - corrosion, broken strands, or a braid that is too long or poorly routed. Replace if damaged; shorten if the installation allows.
- Re-measure with the same four-terminal method and record the value.
- If it still fails, escalate to engineering-approved corrective data before adding parallel conductors or drilling new bond points - a structural change made to pass a test can create its own problems.
- Document the original invalid measurement, the re-test, and the corrective action so the records show why the initial write-up was closed.