The resistance of the ground point on an aircraft electrical single-wire circuit should be less than how many milliohms?

• A. 1 milliohm
• B. 3 milliohms
• C. 5 milliohms
• D. 10 milliohms

Answer: (B)

In an aircraft’s single-wire grounding scheme, the return path through the airframe must have very low resistance so that the voltage drop along that path remains negligible when current flows. The ground point serves as the reference and return for many circuits, so any resistance there directly affects the voltage seen by equipment and sensors.

Keeping the ground point resistance under a few milliohms minimizes voltage loss. For example, with 50 A of current, a 3 milliohm resistance would drop about 0.15 V, and at 100 A it would drop about 0.3 V. Those small drops help ensure proper operation, accurate readings, and reduced heat at the bonding point. If the resistance were higher, the greater voltage drop could cause erratic behavior or degraded performance.

Thus, the specification is to keep the ground point resistance less than 3 milliohms.