Electrical bonding is the practice of connecting equipment cases or structure to ground to prevent electric shock. This provides a return path for any stray voltage that the case may be exposed to via component failure (e.g. wire shorted against case). The reason why proper electrical bonding techniques are so important is that it protects those who may come in contact with the equipment. Bonding is more than just physically attaching equipment to ground or structure; there are a number of necessary factors that affect the proper grounding of equipment.
Not all components and systems can be treated the same when it comes to electrical bonding. There are six bonding classifications, each with their own set of recommended standard practices:
- Class A: Antenna
- Class C: Current Path Return
- Class H: Shock Hazard
- Class L: Lightning Protection
- Class R: RF Potentials
- Class S: Static Charge
The following is a brief summary of recommendations taken from MIL-B-5087B:
Class A: Antenna
- Radiating elements should be installed with a ground plane of negligible impedance and should not detract from the desired radiation patterns.
Class C: Current Return Path
- The total impedance in a circuit from wires, cables, ground, and connections shall not exceed the limits shown in the following table.
Class H: Shock Hazard
- Conduit carrying electrical wires should have a low resistance bond of less than 0.1 Ohms to structure.
Class L: Lightning Protection
- Soldered connections should not be used on jumpers or EWIS component that are required to carry lightning currents.
- Conductors should be equal to or larger than 12AWG wire.
- All sealing techniques and compounds must be verified as to not negatively impact the bonding resistance
Class R: RF potentials
- All conducting items longer than 12” or more within 1’ of an unshielded transmitting antenna shall be bonded to structure.
Class S: Static Charge
- Considerations are necessary for both electrical and non-electrical equipment.
- External vehicle components or those that are subject to frictional charging must have a mechanically secure connection to structure.
Preparation and Testing
The available fault current (maximum electrical current at the fault location) dictates the acceptable resistance level of the bonded surface. Damaged, corroded, or uncleansed connection points can create hotspots during failure event, possible ignition of flammable materials, and/or limit the fault current such that circuit protection devices fail to open. Proper preparation of the bonding surface means that it must be cleaned to remove all grease, paint, lacquer, or any impurities that may inhibit a good electrical connection. In some cases, it is recommended that a chemical cleaner be used to prepare the surface. Selection of such a cleaner should be based on the type of surface being cleaned (refer to MIL-C-5541 for further information).
The critical point here is that a standard handheld multimeter is unacceptable for measuring the resistance across bonded surfaces. Often these meters have an accuracy of ±0.1 Ohms which, if you look at the previous figure, is far too large a margin of error. Further, contaminates between the bonded surfaces may show a low resistance value when placed under a 9VDC measurement from a multimeter, but will significantly increase in resistance under fault current conditions.
Proper testing of equipment bonding requires that specialized testers be used. These testers may apply a high current across the bonded surface for impact assessment, or may be designed to have a more sensitive resistance measurement. For equipment carrying higher loads, testing with high current is recommended.
Equipment in areas with explosive fuels and gases must be properly bonded. The following figure shows the thresholds for maximum bonding resistance and the bonding limit for preventing ignition of explosive vapors; exceeding these values is not acceptable.
For more information about bonding, read the latest information available in SAE document AIR5128, the cancelled military version of this document, MIL-B-5087B at the assist website, or the NASA paper on “Electrical Bonding: A Survey of Requirements, Methods, and Specifications”. These documents provide good information on additional bonding considerations, proper techniques for connections, and references for additional reading.