Blocking diodes and fuses in low-voltage PV systems

Instructions and labels supplied with listed PV modules and the requirements of the National Electrical Code (NEC) dictate that a series fuse shall be used to protect the module against backfeed currents. Few of the hundreds of thousands of low-voltage (12, 24, and 48-volt) stand-alone photovoltaic (PV) power systems use series fuses on each module or string of modules. Tests and simulations at the Southwest Technology Development Institute (TDI) and at Sandia National Laboratories (SNL) have established that the absence of these fuses can pose significant fire and safety hazards even on 12-volt PV systems. If the system has sufficient backfeed voltage and current, it is possible that a ground fault in the wiring or inside a module can result in the destruction of a PV module.