**Fault Analysis for Combined PV Circuits**

Where are all the sources of current in a PV system? It is a rare PV system where a fault on a conductor does not draw current from sources on both ends of the conductor. Therefore, just as your parents taught you to look both ways before crossing the road, you must look for possible sources of current from both directions on a conductor: the normal current and the reverse current. Reverse current, also called backfeed current, is the current that can flow through a conductor to a fault in the opposite direction of the normal current flow; this is usually the current that the conductor needs to be protected against.

**Single source circuit.** In a PV system consisting of a single source circuit, where the conductor is connected to the PV string on one end and to the inverter on the other, a fault on the conductor has two possible sources of current, as shown in Diagram 1. One source is the maximum PV source-circuit current, which NEC Article 690.8(A)(1) defines as the shortcircuit current multiplied by 125%. Therefore the fault current (IFAULT) at Point A in Diagram 1 is calculated as follows:

IFAULT = ISC x 1.25

= 8.19 x 1.25

= 10.2

The conductor at Point A is already sized to carry the maximum circuit current. The other potential source is the inverter backfeed current, or I BACKFEED at Point B. Most transformer-based inverters have zero backfeed current, which is assumed in Diagrams 1–3.

Note, however, that some transformerless inverters may have backfeed current. If IBACKFEED is greater than the conductor rating, then either an overcurrent protection device (OCPD) is required at the inverter connection or the conductor has to be sized for the backfeed current. If IBACKFEED exceeds the module’s series fuse rating, then overcurrent protection is required regardless of how the conductors are sized. In this case, the OCPD is not required in order to meet Article 110.3(B) of the NEC, which requires that equipment is “installed and used in accordance with any instructions included in the listing or labeling.”

**Two source circuits. **If two PV source circuits are combined at a common point and a fault occurs in a currentcarrying conductor, as shown in Diagram 2 (above), then the fault current carried by the conductor to the fault from the PV string is once again the maximum circuit current (IMAX) at Point A. At Point B, the available fault current through the common connection point is the sum of the current from points C and D:

IFAULT = IMAX + IBACKFEED

where IMAX is equal to 1.25 x ISC. Assuming that IBACKFEED is zero, as it most often is, the conductor at Point B has to carry only IMAX for which it is already rated, and no OCPD is required.

**Three source circuits. **If three PV source circuits are combined at a common point and a fault occurs in any of the strings, as shown in Diagram 3, then the fault current from the PV modules on the faulted string at Point A is the maximum circuit current (IMAX) as before. In this case, however, the fault current traveling in reverse from the common connection point is two times the maximum circuit current plus the inverter backfeed current—up to 15 A, at which point the fuse would open.

If there were no fuses, the fault current could be larger still:

IFAULT = (2 x IMAX) + IBACKFEED

= (2 x 10.2 A) + 0 A

= 20.4 A

As more strings are added, the difference between the fused and unfused currents increases. If the fault current is greater than the rated current of the conductor, then the designer has two choices: increase the size of the conductor so that it is rated to carry the fault current, or place an OCPD in each of the PV input circuits at the common connection point to limit the fault current that can travel back over the faulted conductor. Because the first option quickly becomes cost prohibitive, and PV modules require series fuse protection, system designers commonly rely on dc combiners to provide a convenient place to locate OCPDs in ungrounded current-carrying conductors in PV circuits. This allows for the use of smaller conductors while protecting PV modules from fault current in excess of the module’s fault-current rating.

With PV source circuits, it is always necessary to protect strings of modules according to the products’ series fuse rating. This ensures the integrity of the product in the event of a fault and mitigates fire hazard. In the absence of inverter backfeed, as shown in the previous examples, fault current in series strings is equal to the number of parallel-connected source circuits minus 1 multiplied by 125% of the short-circuit current:

IFAULT = (n – 1) x 1.25 x ISC

For more information on this topic, refer to the latest version of John Wiles’ Sandia Report, “Photovoltaic Power Systems and the National Electrical Code: Suggested Practices” (see Resources).