Key Considerations for Selecting DC Isolator Switches in Solar Projects

Imagine a scenario where a residential solar system catches fire, but firefighters arrive to find they cannot safely cut off the power supply. This is not a hypothetical nightmare—recent incidents in Australia have exposed deadly gaps in photovoltaic (PV) safety, particularly involving substandard or improperly installed DC isolators. The Australian Clean Energy Council (CEC) has suspended certifications for several brands after rooftop fires linked to these devices. These incidents underscore that DC isolators are far more than simple switches; they serve as a PV system’s lifeline, capable of safely interrupting hazardous direct current (DC) during maintenance, emergencies, or fires.

A DC isolator, or DC disconnect switch, is a mechanical switching device designed to isolate the DC side of a solar system. Its primary function is to sever the connection between solar panels and inverters during servicing or crises, ensuring personnel safety. Unlike alternating current (AC) isolators, DC isolators face unique challenges:

• Arc suppression: DC lacks alternating current’s natural zero-crossing points, making arc extinguishment significantly harder.
• Higher specifications: Requires larger contact gaps and specialized materials to safely interrupt current.
• Safety risks: Substituting AC isolators for DC models may cause overheating, fires, or catastrophic failures.

DC isolation is non-negotiable for three critical reasons:

• Maintenance safety: Enables technicians to work on inverters or combiner boxes without live DC voltage.
• Emergency shutdown: Allows first responders to de-energize systems during fires or accidents.
• Regulatory compliance: Mandated by international standards (IEC, AS/NZS, UL) at module, string, or inverter input levels.

Without reliable DC isolators, rooftop PV systems pose severe fire and electrical hazards.

• Enclosed DC isolators: IP65-rated or higher housings protect against dust, moisture, and UV radiation. Ideal for rooftop arrays or inverter proximity.
• Rotary switches: Lever-operated designs for quick manual disconnection at inverter inputs.
• String isolators: Isolate individual PV strings for granular fault management.
• Array isolators: Disconnect large-scale commercial or utility PV arrays.
• Fused isolators: Combine circuit isolation with overcurrent protection.
• Non-fused isolators: Provide switching only, relying on external fuses.

With PV systems operating at 1000V–1500V DC, selection criteria include:

• Voltage rating: Must meet or exceed system voltage.
• Current capacity: Match array output (10A–800A).

High-quality isolators feature:

• UV-resistant enclosures for rooftop mounting
• Silver-alloy contacts for reliable arc quenching
• Thermally stable materials for prolonged sun exposure
• Compact designs with secure terminal connections

Advanced safety features include:

• Lockable "OFF" positions to prevent accidental re-energization
• Door interlock mechanisms to prevent access while energized

Best practices demand:

• Installation at both string and inverter levels by certified electricians
• Annual inspections for overheating, corrosion, or mechanical wear
• Priority given to products with IEC 60947-3 certification

DC isolators represent a foundational safety component in photovoltaic systems. Proper selection, installation, and maintenance significantly mitigate electrical risks while ensuring regulatory compliance. In solar energy, safety must always come first.