LV Underground Faults
Low Voltage
Underground Faults
Buried low-voltage DC cables in utility-scale and ground-mount PV systems are subject to insulation failure from mechanical damage, moisture ingress, rodent activity, and UV degradation at exposed sections. Time-domain reflectometry (TDR) — commonly associated with cable fault location — is not compatible with the DC cables typically used between combiners and inverters in solar installations. TDR requires a shield and conductor, or a twisted pair configuration to function; the single-conductor or two-conductor unshielded DC wiring used in PV systems does not meet this requirement. For LV DC underground faults, the preferred approach combines the bridge method — which uses a precision resistance bridge to calculate fault distance from cable resistance ratios — with step voltage testing to confirm fault position and polarity. This section covers equipment selection, field methodology, and practical application of bridge and step voltage techniques for underground DC string runs, combiner box circuits, and buried collection cables.
Surge and thump methods — limitations in PV applications
A common fault location methodology used in utility distribution cable networks combines TDR prelocation with high-voltage surge pinpointing — a technique often referred to as “thumping.” While well-suited to the cable types and operating conditions found in utility infrastructure, this approach presents significant drawbacks when applied to the low-voltage DC cabling typical of solar PV installations.
TDR incompatibility. As noted above, TDR prelocation requires a shielded cable or twisted pair conductor arrangement to function. PV DC wiring from string combiner boxes to inverters is unshielded and typically single-conductor, making TDR prelocation ineffective in this application regardless of the instrument used.
Surge pinpointing and cable damage risk. High-voltage surge pinpointing works by applying repeated high-energy pulses to the cable to cause the fault to arc, generating an audible thump that a technician locates by walking the cable route with an acoustic detector. While effective on utility-grade distribution cable with robust insulation, this approach carries a meaningful risk of further degrading already-compromised PV cable insulation. The surge voltages required far exceed the operating voltage of typical DC PV string wiring, and repeated surging at a soft insulation fault can extend the damage zone, complicate repair, or create secondary fault sites in adjacent cable sections.
Power supply dependency. Instruments designed for the utility cable fault location market are typically mains-powered. Utility-scale ground-mount PV sites — particularly during commissioning or fault investigation on de-energised arrays — may not have convenient AC power access at the cable route, creating a practical logistics challenge that battery-powered bridge and step voltage instruments do not face.
The bridge and step voltage approach. For PV underground DC cable faults, a resistance bridge provides a non-destructive, cable-safe distance calculation that does not require a return current path through a shield — only a known loop resistance and a low-voltage test signal. Step voltage testing then pinpoints the fault location by detecting the voltage gradient at the surface above the fault without any high-voltage application to the cable. This combination is both safer for the cable and more practical in the field conditions typical of ground-mount solar sites.
Time-domain reflectometry requires either a shielded cable or a twisted pair conductor arrangement to propagate and reflect the test pulse correctly. PV DC string cables are single-conductor, unshielded, and not twisted — they do not provide the return current path that TDR depends on. Applying TDR to PV DC cable produces unreliable or no results regardless of the instrument used.
The resistance bridge method uses the known loop resistance of the cable to calculate fault distance. A low-voltage test signal is applied to the faulted cable with a known loop created via a return conductor. The ratio of resistance from the test point to the fault versus the total loop resistance gives the fault distance as a proportion of total cable length. It is non-destructive and works on unshielded single-conductor PV cable.
At a ground fault site, fault current flowing from the cable into the earth creates a voltage gradient at the surface above the fault. A technician walks the estimated fault zone probing the ground surface at regular intervals with two electrodes separated by a fixed distance. The voltage gradient peaks directly above the fault and drops on either side, pinpointing the dig site without excavation guesswork.
Mechanical damage at installation is the leading cause — cable cut or crushed by excavation equipment, bent below minimum bend radius, or trapped under a rock or concrete edge. Rodent gnawing is common in ground-mount sites with vegetation. Moisture ingress at conduit entry points degrades insulation over time. UV degradation occurs where cables exit the ground and are not adequately protected by conduit or UV-stable sheathing.
Confirm the fault electrically first using insulation resistance testing — a low Riso value on the affected cable with acceptable values on adjacent cables confirms the fault is in that specific cable rather than in a combiner, connector, or module. Then use the resistance bridge method to estimate distance, and step voltage testing to pinpoint the surface location. Only excavate once you have both a confirmed fault on the right cable and a pinpointed surface location.