Q & A: Series String OCPD Requirements for Grid-Direct Inverter Applications

When are overcurrent protective devices (fuses or circuit breakers) required in direct current circuits between the PV modules and grid-direct inverters?

Before answering this question directly, it is important to note that properly rated fuses and circuit breakers are equivalent in this application and are collectively referred to as overcurrent protective devices (OCPDs). This is true even though the required label on the back of certified and listed PV modules lists the “fuse” rating.

In most electrical systems, the NEC requires that every ungrounded circuit conductor be protected from overcurrents that might damage that conductor. OCPDs provide this function. All commercial scale systems with central inverters require an OCPD for each series string of modules, but some smaller grid-direct systems may not need OCPDs in the dc circuits between the inverter and PV modules.

In UL Standard 1703, Underwriters Laboratories has established that modules must have an external series OCPD if there are external sources of current that can damage the internal module conductors. In the case of an external or internal fault, the module can be damaged if reverse currents are present in excess of the value of the module’s maximum series fuse rating. However, if there are no sources of external currents that exceed this marked value, then no OCPD is needed to protect the internal module wiring.

PV modules are current-limited devices, and their worst-case, continuous outputs for code calculations are 1.25 times the rated short-circuit current (Isc). An exception to NEC Article 690.9(A) allows conductors, typically rated at 1.56 times Isc, to be used with no OCPD where there are no sources of external currents that might damage that conductor.

Backfeed considerations.

Many of the smaller grid-direct inverters (below approximately 10 kW) are designed so that they cannot backfeed currents from the utility into array faults. Established, abnormal operation tests for backfeed do not rule out backfeed during normal operation of the inverter. Currently there are no normal operation tests in UL Standard 1741 to validate the lack of backfeeding from the utility. As a result, the manufacturer should provide written certification that the inverter cannot backfeed from the utility into an array fault. Larger central, transformerless and bipolar inverters may require additional certification verifying that they cannot backfeed.

If the inverter can backfeed utility currents into the dc PV wiring, the NEC requires that an OCPD be installed in series with the output of all individual module strings to protect the cables and the modules from reverse currents from any backfed ac currents through an inverter. In large systems, fused combining boxes are typically mounted at the array. In these systems, assuming that the inverter is a potential source of overcurrents, an OCPD may also be needed at the inverter input. This OCPD will have a minimum rating based on the number of strings connected in parallel on that circuit and the short-circuit current of each string. It will be sized to allow maximum forward currents from the array (all strings of modules) to pass through without interruption while keeping the overcurrent device from operating at more than 80% of its rated value.

Guidelines for string inverters.

The following guidelines and examples relate only to grid-direct PV systems using inverters certified by the manufacturer to not allow backfed currents. The goal is to determine how many strings of modules can be connected in parallel and still meet NEC and UL requirements before an OCPD is needed on each string. Where required, one OCPD will protect all modules and conductors in a single series string of modules per Section 690.9(E).

Additionally, the NEC requires in Article 110.3(B) that the manufacturer’s instructions and labels be followed. These materials list the maximum value for the OCPD. Lesser values can be used, according to Article 690.8(A) and (B), as long as they meet the NEC requirement of 1.56 times the module Isc in order to protect the conductor associated with the module or string of modules. Note that in these examples we are not determining the rating of any required OCPD. We are merely making some calculations to indicate whether or not an OCPD is needed on each string of modules.

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