Low-current earthing is standard practice in distribution systems at the middle end of the voltage range: the neutral point is not connected to earth at all or connected through a Petersen coil only. A single-phase-to-ground fault is accompanied by small fault currents; hence the protection system does not immediately trip and the system continues operating. This way, continuity is bettered, but risks are underestimated quite often.
The most direct effect is the voltage rise on the unfaulted phases. Phase voltage is lifted to line-to-line level. If insulation is already weak anywhere, operating at about 1.73 times normal phase voltage for a long period can lead to a second ground fault or a phase-to-phase fault. Many equipment failures are not sudden; insulation deteriorates gradually under overvoltage until breakdown.
If an intermittent arc occurs at the fault point, it is even worse. The arc dies and is re-established over and over again, creating high-frequency oscillations as well as arcing ground overvoltages with peaks of several times the nominal phase voltage. Such overvoltages propagate through the system and are capable of penetrating transformers, cable terminations, motor windings, and other devices. The arc also puts harmonics on the bus, disturbing VFDs, PLCs, precision devices and other sensitive loads.
A single-phase-to-ground fault also creates voltage unbalance and a clear rise in zero-sequence voltage. Even without a feeder trip, power quality has already degraded. For sensitive equipment, the unbalance and harmonics can cause false operation, shutdown, or data errors. Some processes cannot tolerate that condition.
There is also a safety risk. Step and touch potentials can exist near the fault, especially around overhead lines, cable trenches, and equipment enclosures. If the fault is in an area people can reach, the hazard remains as long as the system stays energized.
Fault location is another problem. Because the fault current is small, conventional overcurrent protection rarely operates. Zero-sequence current distribution depends on line capacitance, so feeder selection is prone to error. Manual trial switching is direct, but it briefly interrupts healthy feeders and disrupts continuous processes. Transient recording and zero-sequence directional judgment improve location accuracy.
A better approach is layered monitoring and faster fault location, not just searching after an outage. At the bus level, zero-sequence voltage, voltage unbalance, and harmonic content can indicate whether a ground fault exists. At the feeder level, zero-sequence current direction and transient waveform characteristics can identify the faulted line. Combining this with Petersen coil compensation or low-resistance grounding conversion keeps fault current and overvoltage within acceptable limits.
Power quality monitors can be used to continuously record three-phase voltage, zero-sequence voltage, current harmonics and transient waveforms in critical circuits. When the zero-sequence voltage exceeds a pre-set threshold or changes drastically, the device will automatically trigger waveform capture and records data before and after the event. Maintenance personnel can compare trends to identify sporadic grounding, deterioration of insulation or arcing. This type of device can be used in substations, pad-mounted transformer, local storage and remote upload to be deployed on critical feeders.
How to Never Ignore Phase-to-Ground Faults if the System can Still Operate for Low Current Grounding Systems If detected early enough, it may involve the simple replacement of an insulator or a cable termination repair. If it escalates into a phase-to-phase fault or equipment failure, the cost becomes significantly higher. Even worse, online monitoring does not replace protection; it gives early warning, and proof. Observe zero-sequence voltage, voltage unbalance and harmonic trends at the bus and on critical feeders if your network utilizes an ungrounded neutral or Petersen coil. Most single-phase grounding problems can then be found before they escalate.
