How to find isolation resistance faults in solar farms

Isolation resistance (Riso) faults are the most common DC faults in solar PV arrays. About 50% of all PV Riso faults go undetected. This article explains what causes ground faults, how Riso degrades over time, the financial impact of faults, and how to locate them efficiently in the field.

What is a PV ground fault?

According to the Photovoltaic Systems textbook published by NJATC, a solar PV ground fault occurs when current unintentionally flows through the grounding conductor. This happens when a current-carrying conductor in the PV array makes an unintended electrical connection with the equipment grounding conductor.

When a ground fault occurs, the electric current that should flow to the inverter or combiner box is instead diverted to the ground terminal. If the fault current exceeds a certain threshold, the inverter shuts down. This halts energy production until the fault is fully resolved.

The key grounding components in a PV system are:

  • Equipment Grounding Conductor (EGC): Provides a path for ground fault current and connects metal parts of the PV equipment to the grounded conductor.
  • Grounding Electrode Conductor (GEC): Connects system equipment to the grounding electrode.
  • Grounding Electrode: A metal rod driven into the soil, creating a direct earth connection for grounding.
Solar PV array grounding diagram showing equipment grounding conductor, grounding electrode and inverter connection
Figure 1. Illustration of a PV array connected to an inverter and the various conductors that make up the full PV circuit. Isolation resistance exists between the active conductors and earth.

Understanding isolation resistance levels in PV systems

When a PV system is first installed, its electrical isolation resistance (Riso) is typically above 40 MOhm, creating a strong barrier against current leakage. Over time, Riso can decrease significantly as insulation degrades. The following Riso levels define the operational state of a PV array:

Above 20 MOhm
Healthy PV array Riso is high and there are no leakage currents. No action required.
3 to 20 MOhm
Potentially degraded insulation Pay attention when buying or selling PV assets or before taking on O&M responsibilities. If Riso is on a downward path, considerable expenses will follow.
1 to 3 MOhm
Intermittent ground faults Ground faults appear as intermittent events. This state often leads to truck rolls in vain, as the fault disappears before the technician arrives. Lost revenue and service costs rise significantly.
Below 1 MOhm
Permanent power loss and fire risk Manifested ground faults and permanent power loss. One fault can lead to more faults, electrical arcing, heat dissipation and fire. Irreversible damage to components is possible.
Photovoltaic isolation resistance Riso schematic setup showing DC generator and system power components
Figure 2. The main setup for a DC power generator with isolation resistance Riso to ground, connected to the system power components and the electricity grid. The system power components are equipped with a ground leakage current monitor.

Why does isolation resistance decrease over time?

When installing a PV system, newly installed conductors have insulation in good condition. However, manufacturing defects and installation errors can still occur, making it essential to test all conductors before commissioning. Some studies suggest that early stage installation errors are becoming more prominent.

Over time, various factors contribute to insulation degradation. Many PV conductors, particularly those connected to modules and home runs, are installed outside of raceways or protective conduits. While manufacturers design these conductors for outdoor use, prolonged exposure to UV radiation, extreme temperatures and moisture all play a role in breaking down insulation.

Mechanical damage is equally common. Conductors may be affected by wildlife interference, abrasion against racking systems, movement from tracking arrays, and accumulation of dirt and debris. Any of these conditions can gradually wear away insulation and increase the likelihood of a ground fault.

In some cases, insulation resistance can drop suddenly, such as when a rodent chews through a cable. In most cases however, degradation happens gradually. This provides ample warning signs if Riso is monitored consistently and tested at regular intervals, typically annually.

The financial cost of ground faults in solar PV

A PV ground fault does not just reduce output from one panel or string. It causes the entire combiner box or inverter to shut down. To illustrate the financial impact, consider the following example:

Example: 1 combiner box with 20 strings of 24 panels each

Total panels: 20 x 24 = 480 panels

Energy per panel per day: 250 Wp x 5 hours x 0.75 = 0.94 kWh

Daily combiner box production: 0.94 kWh x 480 panels = 451 kWh

Energy price: 0.10 USD per kWh

Daily financial loss with ground fault: 0.10 x 451 = 45 USD

Monthly loss with ground fault: 45 x 30 = 1,350 USD

For utility-scale PV systems, a ground fault often means that 200 to 400 modules are not producing while the fault persists. Add service team costs, transportation and scaffolding, and the total cost of a single ground fault can far exceed the cost of the faulty component itself, which is typically a cheap connector or a length of cable.

Market analysis from Wood Mackenzie confirms that annual PV plant operations and maintenance costs exceeded 9 billion USD in 2024. Ground faults on the DC side are a significant driver of this figure.

Troubleshooting costs are rising

The safety requirements for PV arrays are covered by IEC 62446-1:2016, Photovoltaic (PV) systems: Requirements for testing, documentation and maintenance. Systematic troubleshooting is however not yet an integral part of the code of practice.

A significant cost driver in the field is the time spent locating faults physically. While monitoring software can often pinpoint issues on a circuit diagram, many systems lack physical labelling and mapping of rows and strings. Even when components are labelled, large modern systems take time to navigate. Finding the right string on a utility-scale site can alone represent hours of billable time.

Ground faults often result from material damage that is invisible to the naked eye, making troubleshooting costly when relying on standard PV testing methods. Conventional techniques do not enable early detection or precise fault localisation. Simple voltage measurements and voltage pulse testing are often tedious in operation, and in some cases the voltage pulse itself can ionise metal parts and thin conductors in the PV system, causing additional damage.

How to locate PV ground faults quickly

The most effective approach to PV ground fault troubleshooting is to locate faults early, before Riso falls below 1 MOhm and permanent power loss or fire risk sets in. Most known methods for testing Riso are not well suited to this. Equipment designed to measure Riso for safety compliance purposes is often repurposed for troubleshooting, but the internal resistance of voltage testers acts in parallel with Riso, making fault localisation more complex rather than simpler.

Impedance-based field diagnostics offer a more effective approach for troubleshooting. By using gentle measurement signals rather than high voltage pulses, it is possible to assess isolation resistance and locate faults at almost any level of Riso, with minimal risk to the PV equipment. This method requires very little system understanding and can be carried out as soon as the operator is familiar with the basic concerns of PV testing.

emazys Z300 PVT Measure Riso, Voc, Isc, Rs and polarity in a single 20-second test. Ground fault location included. Results sync automatically to the emazys cloud.
See the Z300 PVT