MIL-B-5087B - Electrical Bonding
Many aspects of lightning protection revolve around electrical bonding requirements and keeping electrical joint resistances low. In MIL-B-5087 the 2.5 milliohm bonding requirement is enshrined. The 2.5 milliohm requirement for lightning protection is derived by limiting the maximum voltage in the aircraft at 500 volts and defining the maximum current at 200,000 amperes (500 volts / 200,000 amperes = 2.5 milliohms). Here is the relevant paragraph from MIL-B-5087B:
The following bonding requirements are designed to achieve protection against lightning discharge current carried between the extremities of an airborne vehicle without risk of damaging flight controls or producing sparking or voltages within the vehicle in excess of 500 volts. These requirements are based upon a lightning current waveform of 200,000 amperes peak, a width of 5 to 10 microseconds at the 90-percent point, not less than 20 microseconds width at the 50-percent point, and a rate of rise of at least 100,000 amperes per microsecond.
This document also contains very useful bonding examples that are still used across the aerospace industry.
You can download a copy of MIL-B-5087B here: MIL-B-5087B
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