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AS/NZS 5033
DC Isolators — Not the Same as AC Isolators
One of the most common compliance errors in solar installations is using an AC-rated isolator for a DC circuit. DC arcs do not self-extinguish at current zero-crossings the way AC arcs do — they sustain indefinitely until mechanically interrupted. An AC isolator used in a DC circuit may arc internally when operated under load, destroying the contacts and potentially causing a fire.
DC Isolator Requirements
Parameter | Requirement | Reason |
|---|---|---|
Voltage rating (DC) | Must equal or exceed the maximum system voltage at minimum site temperature | Calculated VOC can be significantly higher than STC panel VOC |
Current rating | Must equal or exceed 1.25 × array ISC (or string ISC) | Safety margin for temperature effects on ISC |
DC arc interruption rating | Specifically DC-rated switching mechanism — usually rated in "utilisation category DC-21B or DC-22B" per IEC 60947-3 | AC-rated isolators cannot safely interrupt DC loads |
IP rating | IP55 minimum for outdoor or rooftop locations; IP66 for exposed coastal locations | Protection against dust and water ingress |
UV resistance | Enclosure must be UV-rated — non-UV-rated PVC becomes brittle and cracks within 2–5 years | Rooftop isolators are in direct sun 25+ years |
Lockable | Must have provision for padlock in the off position | AS 5033 lockout/tagout requirements |
Common Installation Errors
Installing the DC isolator in a location that requires going onto the roof to operate it — the isolator should be accessible from the ground or a ladder without roof access
Using a 250 V AC rated isolator for a 400–550 V DC string — the rating is inadequate by a factor of 2 or more
Installing plastic-bodied isolators in direct sun without UV protection — the plastic yellows and eventually shatters
Not labelling the isolator with the maximum system voltage — emergency responders need this information to assess the hazard
Engr. Jason Morales — Founder, SolarEnergyPH
