Why Do PV Inverters Report Low Insulation Resistance in the Morning?

A better connection system is needed.

Low insulation resistance is a common fault in photovoltaic and energy storage systems, especially in floating solar projects, humid regions and sites where DC cables are installed in underground trenches.

A typical pattern is that the inverter or PCS reports an insulation fault in the early morning or after rain, but returns to normal after sunlight and temperature increase.

Why Does Insulation Resistance Drop?

Moisture from rain, dew and condensation can reduce the insulation resistance between DC conductors and ground. However, moisture is often only the trigger—not the root cause.

Common underlying problems include:

Damaged or ageing PV cable insulation;
Cable jackets scratched by sharp tray edges;
Loose, poorly crimped or mismatched PV connectors;
Water ingress into connectors or junction boxes;
Cables damaged by rodents, UV exposure or mechanical stress;
Water accumulation around battery-to-PCS cables.

In one field observation, the indicated PV-to-ground insulation value increased from approximately 8.3 kΩ at 6:30 a.m. to 59 kΩ three hours later—a difference of about 7.1 times as the environment became drier.

This shows why a system may perform normally in dry conditions but trigger alarms during rain or morning condensation.

How Does the Inverter Detect the Fault?

PV inverters monitor the insulation condition between PV positive, PV negative and protective earth. If the calculated resistance falls below the equipment threshold, the inverter may stop operating and issue an alarm.

The detection method and alarm threshold vary by inverter model. Therefore, troubleshooting should follow the equipment manufacturer’s instructions.

How to Locate the Problem

First, review when the alarm occurs. If it appears mainly during rain or early morning and disappears after the system dries, moisture is likely exposing a weak insulation point.

Next, qualified technicians can isolate and test the PV strings or DC branches individually. If connecting one particular string triggers the alarm, the problem is likely located in that string’s cable, connectors, junction boxes or modules.

Pay particular attention to:

PV connectors with loose assembly, water ingress or overheating;
Cable entry points and sharp cable-tray edges;
Exposed cables showing ageing or mechanical damage;
Module backsheets and junction boxes;
Battery-to-PCS cables installed in wet trenches.

Repeated alarms should not simply be ignored, even if they disappear automatically. They may be an early warning of insulation deterioration.

The Greater Risk: Leakage and DC Arcing

Poor insulation can allow current to leak to ground. Damaged cables or loose connections may also cause localized heating and DC arcing, increasing the risk of equipment damage or fire.

AFCI functions can help detect arc faults, but they cannot replace reliable components and proper installation. Prevention still depends on certified cables, compatible connectors, correct crimping, waterproof sealing and adequate mechanical protection.

Reducing DC-Side Insulation Risks

NSPV provides TÜV-certified PV cables, PV connectors and customized preassembled DC wire harnesses for solar and energy storage projects.

Factory-controlled cable cutting, connector matching and crimping can reduce onsite assembly variation and help EPC companies improve the reliability of PV connections, particularly in humid, floating and large-scale PV applications.

Looking for a reliable PV connection solution? Contact NSPV to discuss your PV cable, connector and customized wire-harness requirements.