Poor Ground
This is the condition where the negative return path has a higher resistance than expected, and it lowers the voltage at the device end as the load increases. Its symptoms are unstable operation, indicators dimming under load, a heating connection terminal and faults that cannot be reproduced. A voltage measurement taken with no load usually does not reveal this fault.
A poor ground looks fine while the circuit is open: the ohmmeter reads a few ohms and the voltage is normal. The moment current flows, however, voltage drops across that resistance and the device negative rises relative to the chassis. The device no longer sees the whole supply voltage.
- Causes: a lug bolted onto a painted surface, a loosened bolt, a corroded contact, an undersized chassis cable, a return path running through a hinge or a bearing.
- Diagnosis: measure between the device negative and the battery negative terminal while the load is running. Typical interpretation thresholds: below 0.2 V is good, 0.2–0.5 V is borderline, above 0.5 V is a fault.
- Side effects: the signal reference shifts between two devices sharing the same ground; analogue inputs read incorrectly and the communication line is corrupted.
The permanent solution is to run the device negative directly to the battery negative terminal or to a reliable main ground bar with adequate cross-section, instead of routing it through the chassis.
Context in which this term is used: Installation, redundancy and field continuity