String fuses

For the correct selection of fuses, the installer must have sufficient data. This data is important for calculating the reverse current resistance. A reverse current can be caused by a short circuit through one or more panels in a string of the PV array (e.g., insulation damage, short circuit in the panel, short circuit in the DC cabling). In the extreme case, the sum of the short-circuit currents of all unaffected strings flows to the faulty string instead of to the inverter. This reverse current can damage other panels present in the string (consequential damage) or lead to excessive heating.

For the calculation of the fuse value, the following data must be known:

  • Number of parallel strings per inverter (Ns)
  • Resistance of the panel to reverse current (IMOD MAX OCPR).
  • MPP current of the panel under STC [Standard Test Condition] (Isc MAX).

To ensure long life and reliable operation, the fuse value must meet two conditions:

  1. The value must be 1.7 times higher than the MPP current of the panel under STC.
  2. The value must be below the panel’s reverse current resistance.

Are string fuses required with your solar panel installation?

Based on the above data, it is possible to calculate whether string fuses are needed at your PV plant. The NEN1010:2020 has established the following formula for this purpose:
712.431.4 The panels of a PV generation unit with several parallel connected PV strings must be protected against the effects of return currents caused by faults in a PV string:
(a) in a PV generation unit with one PV strand (Ns) or two PV strands connected in parallel, no overcurrent protection device is required;
b) in a PV generation unit with Ns parallel connected strands (Ns is greater than 2), the maximum return current in a strand with a fault is equal to (Ns – 1) ISC MAX. Protection devices should be applied to protect each PV strand if: 1.35 IMOD_MAX_OCPR < (Ns -1) ISC MAX

If the formula shows that string fuses are required, provision 712.432.5 requires both poles to be protected.