How to Select a 1500 V DC Photovoltaic Fuse

Why PV fuse selection needs a dedicated method

A photovoltaic array behaves differently from a conventional AC source. Its operating current sits close to short-circuit current, output changes with irradiance and temperature, and parallel strings can feed current back into a faulted string. A fuse that looks adequate from nominal current alone can therefore operate unexpectedly or fail to isolate the fault as intended.

IEC 60269-6 defines supplementary requirements for fuse-links used to protect photovoltaic strings and arrays up to 1500 V DC. The gPV utilization category is designed around this application, including the low overcurrent levels that can occur in PV circuits.

Establish the real maximum DC voltage

Start with the module open-circuit voltage, the number of modules in series and the lowest expected cell temperature. Open-circuit voltage rises as temperature falls, so the maximum string voltage can be higher than the value seen during normal operation. The fuse-link and its holder must both be rated at or above the calculated maximum system voltage.

A useful first calculation is: maximum string voltage = modules in series × module Voc at STC × [1 + |Voc temperature coefficient| × (25 °C − minimum cell temperature)]. For example, 28 modules with Voc 49.5 V and a coefficient of −0.28%/°C at −10 °C give approximately 1,522 V. That already exceeds a 1,500 V design limit, so the series count or system design must be changed.

• Use the module manufacturer's temperature coefficient and project design conditions.
• Check DC polarity, creepage distance and the complete holder assembly—not the fuse-link alone.

Determine current without relying on a single multiplier

Use the module short-circuit current and the design rules required in the destination market, then check the result against cable ampacity, connector ratings and the permitted fuse value stated by the module manufacturer. Installation inside a sealed combiner box, high ambient temperature and adjacent loaded poles can reduce the fuse's usable continuous-current capability.

A common calculation factor from one code edition or market should not be copied to every project. The final rating needs to satisfy the applicable electrical code and the selected fuse manufacturer's published derating curves.

Verify reverse current and breaking capacity

When strings are paralleled, healthy strings can drive reverse current through the faulted string. Compare the worst-case contribution with the module's reverse-current withstand and maximum series-fuse rating. Also confirm that the fuse's DC breaking capacity is higher than the prospective short-circuit current at its installation point.

For an initial screen, the reverse-current contribution of n identical parallel strings can approach (n − 1) × Isc. Four strings with a 14 A Isc can therefore contribute about 42 A from the three healthy strings toward one faulted string. This is a screening value; the final study must include the actual topology, irradiance, conductors and module data.

Coordinate the fuse-link, holder and installation

A correct fuse-link can still overheat if it is installed in an undersized holder or terminated poorly. Match the holder's voltage and current ratings, conductor size, tightening torque, ingress protection and thermal performance. For outdoor or combiner-box use, evaluate enclosure temperature and ventilation at the expected solar load.

Information to prepare for a selection review

• Module model, Voc, Isc and maximum series-fuse rating
• Modules per string and number of parallel strings
• Minimum and maximum site temperatures
• System voltage, installation location and enclosure conditions
• Required standard, target market and preferred mounting format