Solar Inverter Low Isolation Resistance

While some smaller solar photovoltaic (PV) systems are low voltage and designed to operate under 60V, most modern residential PV systems are designed to operate up to 600V DC. Larger commercial installations can operate at up to 2000V DC! These circuits must be well protected to ensure the system operates safely and minimizes current to ground, even with such high voltages and complex wiring.

You may have come across error codes like “STATE 475” on a Fronius inverter, “LOW RISO” on an ABB/Power-One inverter, or other forms of “Low Isolation” or “Low Insulation Resistance” on other solar power systems. In this article, we discuss what low isolation resistance means, why inverters monitor it, and how to resolve a low isolation condition if one occurs.

What is low Isolation Resistance

Isolation resistance refers to the resistance between the PV array circuits (DC side) relative to other circuits, particularly the protective earth (PE). It is often notated as R_iso (Resistance of Isolation).

It can be thought of as a measure of how well the electrical components are insulated from each other and from the ground. If this insulation fails completely, there is no longer isolation, which means full current and an active short or ground-fault. For instance, this could occur if bare conductors from a damaged wire were to touch the metal frame of a solar module. These conditions are typically easy to detect, and modern inverters utilize residual current devices (RCDs) or ground fault circuit interrupters (GFCIs) that detect this excessive fault current and quickly shut down operation.

However, low isolation resistance is another condition that indicates a potential issue before a full ground fault develops. A low R_iso alert indicates that resistance between the array conductors and ground has dropped below a preset threshold designed to prevent excessive leakage currents that can damage equipment or pose a safety risk.

Instead of monitoring leakage current itself—and because of capacitance effects during operation—resistance is tested between the conductors and ground (PE) during the system startup process. Using Ohm’s law and the system max voltage, resistance indicates the amount of current possible. Therefore, we use a measurement of isolation resistance to ensure leakage currents will be kept to a safe level during operation.

Low Isolation Thresholds

In any PV system, when all circuits are functioning normally, the fault currents are typically minimal, but they are not non-existent. Because there is often some leakage measured in milliamps, especially in larger, high-voltage systems, a specific threshold must be determined that defines what amount is considered excessive. In a wide range of system designs, how much leakage should be considered unsafe? The answer has not been so straightforward.

There have been several standards developed that have attempted to address this over the years as technology has advanced. Not all inverter manufacturers refer to the same standards, so different behavior may be observed depending on the manufacturer, country of origin, and age of the equipment in the system.

The following are some of the most common standards used to define minimum Isolation Resistance thresholds:

IEC 62109-2

This standard refers to the International Electrotechnical Commission (IEC) section “Safety of power converters for use in photovoltaic power systems - Part 2: Particular requirements for inverters,” which is a standard that many manufacturers, national standards, and general practical guidelines use. It is a derived value of:

Riso=(Vmax_PV/30mA)

The 30 mA threshold is specific, and based on the principle of limiting leakage current to a safe value to prevent human shock. The most widely recognized safety standard that includes this threshold is IEC 60364-4-41, which covers protection against electric shock in low-voltage electrical installations.

The 30 mA threshold is significant because it is considered the maximum current level that the human body can tolerate without experiencing severe harm. At this current level, there is still a risk of injury, but it is much less likely compared to higher currents.

In the application of a solar inverter with a maximum array voltage where V_{max_PV} = 1000 V, this would allow a minimum insulation resistance threshold between the DC conductors and Earth of 30 kΩ.

1000VDC÷30mA=30 kΩMinimumRiso

IEC 60364-7-712:2017

This standard, “Low voltage electrical installations - Part 7-712: Requirements for special installations or locations - Solar photovoltaic (PV) power supply systems”, states that the following Table 712.4 can be used to measure the resistance from the DC circuits to earth before starting operation. The thresholds vary based on the PV array rating in kilowatts (kW):

This indicates that typical residentail systems could be 30 kΩ or lower.

DIN VDE 0126-1-1

The DIN VDE 0126-1-1 is a German standard that specifies requirements for the connection and operation of photovoltaic (PV) power generating units in parallel with the low-voltage public distribution network. It focuses on ensuring safety and reliability in the operation of PV systems. It is generally consistent with other international standards in requiring that insulation resistance be sufficiently high to prevent dangerous leakage currents.

• For PV arrays with a system voltage up to and including 500 V, the minimum insulation resistance to earth is R_iso = 500 kΩ.

• For PV arrays with a system voltage greater than 500 V, the minimum insulation resistance to earth is R_iso > 1kΩ / V. Example: 1000V * 1kΩ = 1 MΩ min

This is a much higher resistance value than the previous two, which reduces leakage current to a very small value. Although this standard is specific to Germany, many of the leading string inverter manufacturers are headquartered in Germany and Austria, making this one of the more common thresholds to see in the US and other countries around the world.

IEC 61215-2:2021

This IEC standard, “Terrestrial photovoltaic (PV) modules - Design qualification and type approval - Part 2: Test procedures,” is specific to solar modules used in terrestrial solar arrays. Remember, insulation resistance in an array not only applies to the wires and junction boxes that connect to an array but also the internal wiring within the modules themselves. This standard specifies a minimum insulation resistance to ground, determined by the array area, with a threshold of 40 MΩ·m².

For example, if a PV module has an area of 1.5 square meters, the minimum insulation resistance required would be:

40MΩx1.5m²=60MΩ

This insulation resistance is measured under specific test conditions (STC), typically after exposure to humidity and a high voltage range of 500V to 1000V DC. While this standard sets a high value that is practical for testing PV modules themselves, it is not always suitable for inverters. The reason is that the insulation resistance value is relative to the size of the array. In a large array, the allowable leakage currents can conflict with those permitted by other standards, which could pose safety risks or impact the overall performance of the system.

Legacy Solar Industry best practices

The above shows us there are a number of standards used that approach the problem from different perspectives and apply differently to the separate components of a single system. In practice, what you will find is that many older photovoltaic inverters, especially those designed to adhere to German standards, use 1 MΩ as a low isolation resistance threshold. This value is very safe, but as arrays age, it can be restrictive, causing nuisance detections during high humidity or heavy rain events. In the case of a 1000 VDC inverter:

1000V/1MΩ=1milliamp(mA)

This is very safe, well below the 30 mA standard for shock safety, but in larger high-voltage systems, it can be considered excessive.

UL 1741

UL 1741 (Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources) primarily covers safety requirements for inverter equipment, including ground fault detection and interruption. While it doesn’t define a specific insulation resistance threshold like IEC 62109-2 or DIN VDE 0126-1-1, it does require that inverter systems:

• Include ground-fault detection and interruption (GFDI) mechanisms
• Prevent unintentional current paths between DC and ground
• Shut down operation under ground fault or isolation fault conditions
• Be tested for dielectric withstand (insulation resistance) under high-voltage conditions

In essence, UL 1741 requires inverters to detect and respond to insulation faults, but it does not prescribe a specific Riso value. Manufacturers may still adopt thresholds like 1 MΩ or 30 kΩ to comply with other global or regional standards. It’s worth mentioning UL 1741 as a general safety standard that mandates detection of faults such as Low Riso, but without providing specific thresholds.

How do these standards apply to my specific inverter?

SolarEdge Inverters

Every time the SolarEdge inverter enters operational mode and starts producing power, the resistance between ground and the DC current-carrying conductors is checked. The inverter displays an isolation error when it detects a total combined isolation resistance of less than 600 kΩ in single-phase inverters, or 1 MΩ in three-phase inverters. You can identify an isolation fault using either SetAPP or the inverter LCD display.

An isolation fault may disappear and recur after a short period (especially if it is caused by morning moisture); therefore, it is recommended to troubleshoot the fault as soon as it occurs before it disappears. Before troubleshooting on site, you can check the isolation value in the SolarEdge monitoring platform. If the value is borderline (within 10%) or below the limit (600 kΩ for single-phase inverters; 1 MΩ for three-phase inverters), troubleshoot the fault on site. If the value is at least 10% higher than the limit, it is better to wait until an isolation fault error recurs.

SolarEdge Knowledge Center: isolation fault troubleshooting guide

Enter the isolation status screen by pressing and holding down the LCD light button until the following message is displayed:

• To enter setup mode, release within 5 seconds.
• Short-press (one second) to scroll down to the Maintenance menu and long-press to enter the menu. The following screen is displayed:

• Short-press to scroll down to the Diagnostics menu and long-press to select Diagnostics Isolation Status. The following status screen is displayed:

R_iso is the value of the isolation resistance in kOhm. The asterisk (*) and the percentage value indicate the approximate location of the fault within the string, relative to DC+: 0% indicates the fault is at DC+ 100% indicates the fault is at DC–

Using the screen, identify the fault source area: Multiply the number of Power Optimizers in the string by the percentage value. The result is the module near which the fault occurred. For example, in a string with 15 modules and Power Optimizers and a percentage value of 55%:

15 x 55% = 8.25 This means that the fault is near module #8, counted from the DC+ side. If there are more than one string of optimizers, you may need to repeat this process with only one string connected at a time to dientify wich sting the low resistance is in.

Enphase micro-inverters

Microinverter systems are not quite as straight forward when it comes to troubleshooting insulation isolation. An IQ Envoy (the Enphase monitoring gateway) is required to identify the error state, but the good news is each inverter has its own seperate DC connection. Therefore, if the “DC Resistance Low – Power Off” message is received in Enlighten (The Enphase online monitoring platform), it should be possible to identify the affected serial number, and locate it on the serial number map.

Enphase states:

DC Resistance Low – Power Off Condition. For all IQ Series models, a solid red status LED when DC power has been cycled indicates the microinverter has detected a DC Resistance Low – Power Off event. The LED will remain red, and the fault will continue to be reported by the Envoy until the error has been cleared.

This may indicate defective module insulation, defective wiring or connectors, moisture ingress, or a similar problem. Although the cause may be temporary, this microinverter condition persists until the sensor is manually reset.

A sensor in the microinverter measures the resistance between the positive and negative PV inputs and ground. If either resistance drops below a threshold, the microinverter raises this condition. This may indicate issues due to moisture, overload, defective circuits, or live wires making contact with the ground wire, etc. Since the system identifies exactly which inverter is affected, only that specific module need be inspected.

While Enphase does not state the Low Insulation Resistance threshold, they do publish the minimum QCable insulation values:

QCable – Insulation resistance (20ºC): ≥ 20 MΩ/km

If a issue cannot be found in the connected module, one trick we have found is to swap the module with a nearby inverter. If the error condition follows, you know the probelm is with the module, if it does not, then most likely the inverter itself is defective.

SMA SunnyBoy Inverters

SMA has been a leader in manufacturing string inverters for many years, which has lead to a wide range of device topologies, as well as protocols for how these events are handled. In more recent SMA inverters, you may see event numbers such as 35, 3501, 3601, or 3701 indicate that the inverter has detected a ground fault in the PV array or the insulation resistance is too low.

For inverters without galvanic isolation (transformer-less), in accordance with DIN VDE 0126-1-1: As per DIN VDE 0126-1-1, the following applies: R_iso > 1 kΩ/V, but at least 500 kΩ.

So in the case of a 1000V system:

1000Vx1kΩ=1MΩ

SMA inverters with transformers can also measure the insulation resistance. As other regulations apply for galvanic isolation, they do not refuse the grid connection, but only display a warning message.

Additionaly, SMA states:

Together with the German professional association, SMA Solar Technology AG developed a formula different from DIN VDE 0126-1-1 for the insulation resistance. The new R_iso threshold is inversely proportional to the power of the inverter and corresponds to the R_iso threshold of 40 MΩ·m² required by DIN EN 61646 and DIN IEC 61215 with a module efficiency of 5%:

R_iso = 2000 kΩ × (kW / P_{DC_inverter})

This power-dependent R_iso, which deviates from the standard VDE 0126-1-1, was accepted by the professional association, as it fulfills the underlying standard protection target. However, the value has to be at least 200 kΩ, as smaller insulation resistances can cause dangerous residual current. Particularly with PV plants over 10 kW, and with additional insulation problems in the cables and plug connectors, disturbances may occur from time to time — even with the new regulation.

Power-One / ABB/ Fimer Aurora Inverters

Power-One was once one of the leading string inverter manufacturers with the popular Aurora inverter. It was later sold to ABB, and then again to Fimer. On these inverters, you may see a red GFI LED and the error message “Riso Low.” Below the message, it will display the insulation resistance reading:

In certain devices, you may see an E18 error, which also indicates leakage current detection, while an E19 error is due to the sensor itself failing.

The first thing you can try is power cycling the inverter. To do so, turn off both the AC and DC power sources and allow it to discharge for a few minutes before turning it back on. When the inverter restarts, it will perform a new insulation resistance test. If the error returns, you will need to isolate the strings and begin the troubleshooting process to identify the low resistance string. In addition, check for condensation buildup in the inverter enclosure. While these issues need to be ruled out first as the cause, we have found that certain failures in this particular inverter can lead to the message being displayed permanently, requiring replacement of the unit.

The standard Power-One Riso threshold is 1MΩ. For some models, it may be possible to adjust the Riso threshold using the Aurora Manager Lite software to as low as 400kΩ. Contact Fimer technical support for details.

Sungrow

Sungrow states that a “Low Insulation Resistance Error” (039 for Grid-Connected and 302 for Hybrid Inverters) indicates that there could be an Earth Fault in the PV array. This type of fault can appear at different times and frequencies and could be caused by different reasons.

Sungrow provides this table for checking the DC resistance to ground, which describes that for systems over 120 V, a value of 1 MΩ is used:

Sungrow introduced a newer firmware to improve the sensitivity of the insulation resistance. Therefore, it is recommended to upgrade the firmware for the inverter to prevent unnecessary 039/302 faults. The firmware can be upgraded both locally and remotely. Please refer to the documents below

• Local Firmware Upgrade PDF

• Remote Firmware Upgrade PDF

Fronius Inverters

Fronius has used a few different insulation resistance thresholds across their various devices, depending on their intended application. Some older devices had higher thresholds, and some of the more modern models can be adjusted in the settings. Fronius has published this comment:

From a safety perspective, Fronius decided to set the threshold levels of R_iso to a value tolerating no higher fault currents than 10 mA, resulting in a value of R_iso = 100 kΩ (= 1000 V / 10 mA). This level is set to achieve best results regarding safety and ensure that there are no issues regarding the minimum allowed module insulation resistances.

Presets and device limit values of the R_iso thresholds for each inverter series:

With some Fronius models, it is possible to adjust the low isolation resistance threshold value. Fronius states:

“If the low insulation value in these PV systems is not caused by a ground fault but by the system design, it might be possible to lower the default insulation threshold to an appropriate level for the system. It has to be clarified with all parties involved (system owner, authorities, manufacturers, installer,…) whether or not it is allowed to adjust these thresholds (and by how much).”

ADJUSTING THE INSULATION RESISTANCE (RISO) THRESHOLD LEVEL

The measured Riso value can be read on the inverter display under the menu item INFO. If it is determined as necessary to lower the DC insulation threshold, then this is possible via the inverter display using the "BASIC" menu:

• Press the 3rd button from the left 5 times.
• Enter the code 22742 and press enter (4th button from the left) to confirm.
• Select the sub-menu “Insulation Settings” and press enter.
• Select the sub-menu “Error Threshold” and press enter.
• Adjust the threshold accordingly and press enter to confirm.
• You can now exit the service menu (press the 3rd button).

This adjustment should only be made by Fronius professionals and if the conditions are well understood that are leading to the low R_iso. This vaule should not be adjusted to prevent warnings as a means to resolve the underlying issue.

How To Locate the Low Riso Cause?

Now that we understand that “Low Riso” indicates low resistance between the positive or negative conductors and ground, how do we fix it? In a true ground fault, tracking down the issue can be more straightforward. Let’s say you have a string of 10 modules that are each exactly 40 V. The full string, measuring from positive to negative, would give you the expected 400 V. If we measured from positive to ground in a normal system, we should see 0 V. Now let’s say there is a ground fault, and instead we see 80 V from positive to ground and 320 V from negative to ground. This tells us the fault exists between the second and third module.

You can read more about how to track down ground-faults in this article: https://paragon.solar/blog/solar-ground-fault-how-to-identify-locate-and-repair

With that said, low isolation resistance is not always so clear. There may not even be a single point in the system causing the leakage. It is common to get “Low Riso” errors after a heavy rain or wet snow, where moisture is intruding into areas it normally would not. Often, after a couple of dry days, the system can return to service on its own. Other times, it could be the modules themselves breaking down, and while each one contributes only a small amount of leakage, the sum of the entire array can be enough to exceed the threshold. We have also found long underground array circuits sitting in standing water to be the cause. Said another way, it could be any conductor in the array, including the junction boxes and internal module wiring.

It all starts with isolating each circuit and testing in free air to narrow down to the lowest-resistance conductors, and may even lead to testing modules individually. Ultimately, it comes down to having the right tools. For example, you may need a “Megger,” which is a brand of insulation resistance testers that can test insulation resistance of conductors that are de-energized—for example, testing the disconnected source circuits in conduit between the array junction box and inverter.

These devices don’t always work on energized circuits. For testing the array, you may need a tester made for solar, such as the Seaward PV150

Unfortunately, there is no simple answer. It comes down to having the right equipment, understanding the current condition of the array, isolating each circuit, and working through the system to identify the issue.

Conclusion

Ground leakage in solar arrays can be a potentially hazardous condition for people and equipment. Since it is not feasible to measure the leakage current directly, we measure the resistance between normally current-carrying conductors and ground. The allowable threshold is determined by the standard the inverter complies with. While some devices have lower limits, many of the most common string inverters require over 1 MΩ. When the inverter starts each morning, it will measure the insulation resistance of the array, and even if a true ground fault is not detected, if the resistance falls below this value you will receive a “Low Riso” warning, preventing inverter operation until the issue is resolved.

Depending on the size of the system, this can sometimes be labor-intensive to track down. But by using the correct equipment and isolating each circuit, these issues can normally be identified and resolved — though sometimes a Low Riso error could be due to the modules themselves.