A zinc-rich primer may contain conductive pigment, but an assembled protective bonding joint needs a verified current path. Surface preparation, contact pressure and corrosion protection must preserve the bond. NASA’s aerospace bonding guidance treats these as joint-design issues; it supplies no transformer acceptance limit. [1]
Mark the actual contact area, bolt and washer arrangement, adjacent coated region and repair boundary. Identify the approved metal-contact or coating-compatible connection design. Do not remove paint or alter a grounding detail without that design.
Separate the zones and evidence
| Zone | Primary function | Evidence |
|---|---|---|
| Electrical contact | Intended low-impedance current path | Qualified joint construction and measurement |
| Fastener interface | Contact pressure and restraint | Actual hardware and assembly procedure |
| Adjacent paint | Corrosion barrier | Coverage and compatible edge treatment |
| Exposed transition | Balance contact access and protection | Drawing-defined sealing/coating detail |
Measure the path you intend to use
Contact evidence should refer to the assembled connection, not a separate primer coupon. Measurement location and lead contact matter, especially at low resistance. Use the specified bonding test method and account for parallel paths that can conceal a poor individual joint.
A low initial reading does not by itself establish retention after vibration, thermal cycling or corrosion exposure. The joint’s qualification should cover the relevant environment and electrical duty.
For a resistive path, P = I²R and a constant-current pulse deposits I²Rt. Use the assembled path resistance and the actual current duration. [2]
Calculate the contact-loss consequence
Assumptions: a 100 A RMS fault pulse lasts 0.20 s, and the joint’s allocated heat input is 10 J. A 1 mΩ contact deposits 100² × 0.001 × 0.20 = 2 J; a degraded 10 mΩ contact deposits 20 J and exceeds the allocation by 10 J. This model gives R ≤ 5 mΩ for that pulse. Hold the degraded joint; verify fault current, thermal allocation and retained contact performance before adopting the limit. Pigment content establishes none of these inputs.
Keep corrosion protection in the drawing
Verify the bonding contact and the adjacent coating separately. A zinc-rich primer formulation does not automatically supply the needed electrical path, and a sound contact can still leave a corrosion detail unresolved.
References
- NASA, NASA-STD-4003A with Change 1, Electrical Bonding for NASA Launch Vehicles, Spacecraft, Payloads, and Flight Equipment, 2013, change 2016, sections 5.1, 5.2 and 5.5.
- U.S. Department of Energy, DOE-HDBK-1011/3-92, Electrical Science, Volume 3, June 1992.