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AS/NZS 5033
Ground Faults in DC Systems — A Hidden Danger
A ground fault occurs when a DC conductor comes into contact with the earthed structure of the array or building. In a standard residential solar system, this can happen when cable insulation is damaged by UV, rodent activity, or mechanical chafing on roof fixings. Unlike AC faults, a DC ground fault does not necessarily trip a breaker — it can persist for months, creating a fire risk at the fault point and a shock hazard to anyone touching the array frame.
Earthed vs Unearthed DC Systems
System Type | Description | Ground Fault Protection Required |
|---|---|---|
Transformerless (unearthed DC) | DC circuit floats relative to earth — most common residential inverter type | Ground Fault Detection (GFD) built into inverter — mandatory |
Isolated (transformer-coupled) | Transformer galvanically isolates DC from AC/earth | Insulation monitoring device (IMD) required on the DC side |
Earthed negative DC | DC negative bonded to earth — rare in Australia | Current measurement on earth conductor + series overcurrent |
How Transformerless Inverter GFD Works
Modern transformerless inverters continuously measure the leakage current flowing between the DC circuit and the AC earth reference. When leakage exceeds a threshold (typically 300 mA for residential), the inverter:
Opens the internal DC disconnect
Opens the AC relay (disconnects from grid)
Displays a ground fault alarm code
Refuses to restart until the fault is cleared and the unit is manually reset
The installer's job at commissioning is to verify that the GFD is functional — typically by checking that the inverter reports a ground fault alarm when a test resistance is momentarily placed between a DC conductor and earth.
Finding a Ground Fault
When an inverter trips on ground fault, the search process using a megohmmeter:
Isolate all strings at the combiner box
Test each string in turn — positive to earth, then negative to earth (with all other strings isolated)
The faulted string will show low insulation resistance (<1 MΩ for a new array; <100 kΩ indicates a serious fault)
Within the faulted string, disconnect panels one at a time to locate the faulted panel or connector
Engr. Jason Morales — Founder, SolarEnergyPH
