Protection and isolation equipment allows a solar system to be operated and maintained safely while limiting damage from faults and surges. Product ratings must be based on maximum voltage, current, fault contribution, environment and switching duty—not only the inverter kW.
Key takeaways
- DC switching is more demanding because arcs do not cross zero naturally.
- DCDB functions can include combining, isolation, fusing and surge protection.
- ACDB equipment protects and isolates the inverter AC connection.
- Breaking capacity and enclosure suitability must be verified.
DC isolator selection
A DC isolator provides a means of switching the PV circuit under the conditions for which it is rated. It must be certified for DC, maximum system voltage, current, pole configuration and utilization category. Incorrect polarity, wiring or product substitution can create dangerous sustained arcs.
- Use a PV-rated DC switch-disconnector.
- Check maximum voltage at low temperature.
- Apply current and environmental derating.
- Follow the specified pole connection diagram.
DC distribution or combiner boxes
A DCDB may combine strings and house fuses, isolators, monitoring and SPDs. Whether string fuses are required depends on parallel-source current and module maximum series-fuse rating. Internal busbars, clearances, heat dissipation, cable entries and enclosure rating must suit the outdoor DC duty.
- Calculate reverse-current exposure for parallel strings.
- Coordinate fuse current and voltage ratings.
- Use PV-rated SPDs and terminals.
- Provide durable labels and safe access.
AC distribution boxes
The ACDB connects the inverter output to the facility or utility point through breakers, isolation and surge protection as required. Breaker current, poles, trip characteristics and breaking capacity must be coordinated with inverter output, cable ampacity and prospective short-circuit current.
- Select current rating from actual inverter output.
- Verify fault breaking capacity at the location.
- Coordinate earth-leakage protection with the inverter.
- Use a suitable enclosure and segregation.
Coordination and documentation
Protection devices should operate selectively where practical and should not create nuisance trips or leave cables unprotected. Drawings should identify device ratings, cable sizes, earthing, isolation points and labels. Manufacturer data and applicable standards control the final selection.
- Coordinate breaker, fuse and cable ratings.
- Check SPD backup protection.
- Verify enclosure thermal performance.
- Include ratings in the single-line diagram.
Frequently asked questions
Is a DC isolator the same as an AC MCB?
No. DC interruption has different arc behaviour and requires a correctly rated DC device.
Does every small system need a separate DCDB?
The required functions depend on inverter-integrated protection, string arrangement, code and project design.
How is breaker kA selected?
The device breaking capacity must exceed the prospective short-circuit current at its installation point with the required margin and rating basis.
Turn guidance into a practical system plan
Use Spectra’s engineering calculators for a preliminary estimate, then request technical verification before purchase or installation.