Electricity question: Grounding -- 12 volts DC -- Two circuits share a common ground which is "poor" (rusty, loose, etc.). One circuit is high current, say 100 amps (engine starter), while the other circuit is low current, say 10 amps (lights).
Which circuit (starter or lights) is more likely than the other to still properly work ? Both circuits will NOT be "on" at the same time. Each circuit is otherwise adequately wired for its function.

Question

Electricity question: Grounding -- 12 volts DC -- Two circuits share a common ground which is "poor" (rusty, loose, etc.).  One circuit is high current, say 100 amps (engine starter), while the other circuit is low current, say 10 amps (lights).
Which circuit (starter or lights) is more likely than the other to still properly work ?  Both circuits will NOT be "on" at the same time.  Each circuit is otherwise adequately wired for its function.

egenriether · Answer

I would say the low current one works best.  A bad ground means there is resistance between the ground wire and the actual ground (as you said).  This resistance is going to develop a bigger voltage (and heat up) with more current, much like a resistor.  As the current goes up, there is a bigger voltage drop across it, and less voltage across the circuit element (i.e. it forms a voltage divider). Since the starter draws 100A with 12V, its it has an effective impedance of about 0.12 ohms. The resistance of the ground in the starter circuit will be a bigger component of the voltage divider than in the lights circuit, whose resistance would be closer to 1 ohm.  Let's say the ground resistance is 1 ohm, then the voltage across the lights would be around 6V, but the starter would only get about 1.3V.  If this is a real world problem, it could be cause for concern.  I'd clean the ground lug with sandpaper or something.

anonymous · Answer

Thanks for such a quick and thoughtful response !  I was also looking at this via the Ohm's Law process -- as you have done -- but then I started considering an analogy to the arc welding scenario ... it is often difficult to "strike an arc" with a low amperage setting, such as <70 amps, if the ground clamp is not well attached to the part being welded.  However, turn up the "heat" to >110 amps, and the integrity of the ground connection is not as crucial -- striking an arc becomes easy to do.  Now, the relation of current/heat/amps to that of voltage/pressure/potential is inverse as one changes the heat settings.  I assume arc length also factors in somehow to this relationship.  Anyway, that's why I didn't rely solely on Ohm's Law to try to solve my question myself.  But perhaps my "arc welding logic" is not valid for my "grounding analysis".