In small-current grounded systems, single-phase-to-ground faults are the most common type. Unlike solidly grounded systems, these faults produce very little short-circuit current, so protective relays don't trip immediately. The line can keep operating with the fault for an hour or two. That ability to ride through the fault often gives users a false sense of security. But if you work in precision manufacturing, data centers, chip testing, or new energy equipment, this kind of fault demands serious attention.
Once a single phase goes to ground, the voltage on the two unfaulted phases rises to full line-to-line level-1.732 times the normal phase voltage. On a 10 kV system, phase voltage is just under 6 kV under normal conditions. After the fault, those two phases jump to nearly 10 kV. And this overvoltage doesn't fade; it stays until the fault is cleared. Internal insulation in equipment ages much faster under continuous high-voltage stress. A cable termination or motor winding that could run reliably for over ten years might fail within months.
Intermittent arcing ground faults make things worse. When the ground current is unstable, the arc repeatedly extinguishes and re-strikes. Each re-strike triggers high-frequency oscillations and generates switching overvoltages three to four times the normal voltage. Those overvoltage waves travel along the line with no fixed target-they simply hit wherever the insulation is weakest. It's not unusual to see a cable termination flash over to ground, and then have a motor winding fail several hundred meters away. That's the cumulative destructive effect of overvoltage.
For production lines full of power electronics, there's another layer of trouble. Voltage imbalance and harmonic distortion cause drives and servo controllers to misinterpret the grid condition. At the mild end, you get nuisance alarms and unplanned stops. At the severe end, the DC bus voltage goes out of range and power modules fail outright. These devices are inherently sensitive to power quality, and voltage swell plus waveform distortion is an unacceptable input condition for them.
Now, how to deal with it effectively. The priority is finding and isolating the faulted feeder quickly and accurately.
The real headache with small-current grounded systems isn't the magnitude of the fault-it's that you can't immediately tell which line is faulted. The traditional approach relies on manually tripping feeders one at a time, which heavily disrupts continuous production. Today you can solve this with zero-sequence current transformers paired with a feeder selection relay, or by deploying power quality monitoring terminals.
From the equipment we provide, the focus is on two things. First, high-precision zero-sequence current acquisition captures the faint transient component at the moment of ground fault. This is far more reliable than relying on steady-state criteria. Second, feeder selection is built directly into the power quality analyzer. Voltage swell, harmonic step changes, and zero-sequence current direction are all time-aligned on the same time axis. That yields much higher fault-location accuracy than a standalone selection relay.
Once the faulted feeder is identified, you can have a breaker clear it automatically, or set up staged alarms so the operations team can schedule a controlled shutdown instead of dropping an entire bus. For voltage-sensitive lines, you can add a voltage sag/swell correction device with energy storage to compensate for dips and swells within milliseconds-buying time to clear the fault.
