National Electrical Code Tips: Article 690 -- Solar Photovoltaic Systems, Part
Part V of Article 690 provides requirements for grounding and bonding. Keep in mind that grounding is just that, a connection to the ground (earth) . If you stop and draw out the circuit with impedances, you can see very quickly that grounding on the load side does not provide a low-impedance path back to the source and does not put"grounded" items at the same electrical potential. The circuit you've drawn is a parallel circuit with many branches of varying impedances, so there will also be differences in voltage and current among the branches. But when a metal conductor forms the return path, the impedance is neglible and thus so are the differences in potential.
System grounding actually is grounding. That's why you have a ground electrode system at the service or separately derived source. Equipment grounding is not actually grounding, it is bonding even when we are talking about connecting to the Equipment Grounding Conductor (EGC).
- For system grounding, 2-wire PV systems muste have one functionally grounded conductor [690.41(A)].
- For system grounding, PV system circuits that exceed 30V or 8A shall be provided with ground fault protection [690.41(B)]. Further, it must meet the requirements of 690.41(B)(1) and (2). For example, it must detect ground faults in the PV system dc conductors, including any functional grounded conductors
- Ground-fault protectino must proved indication of ground faults at a readily accessible location [690.41(B)(3)]. To acheive this might require installing a remote indicating lamp. For example, the PV system equipment such as inverters, etc., is secured in a locked room. An indicator on a ground fault protection device in that room is not readily accessible. But an indicator light (or panel of such lights) mounted outside the room near its entrance would be.
- You'll find the requirements for equipment grounding and bonding in 690.43, except we know from a reading of the definitions in Article 100 and the application of Kirchoff's Law of Parallel circuits that load side equipment is not grounded. Grounding it serves no electrical purpose, wastes materials, wastes installation labor, and presents the possible introduction of undesired circulating currents into equipment. This subsection begins by saying the equipment must be connected to an EGC, which we know is actually an equipment bonding conductor, per 250.134 or 250.136.
- You can bond equipment to grounded metal supports [690.43(B)]. But again, review the definitions in Article 100 and ensure you are creating a low-impedance path back to the source. That's what's meant by the last sentence in 690.43(B).
- Size your equipment bonding conductors per 250.122 [690.45]. Some people believe that the larger the bonding conductor, the better. However, when you do the math you see that isn't really the case. If you want to improve the bonding quality, torque the bolted connections and install bonding jumpers across things such as raceway joints. It's not really the quality per se (reduced impedance) that you're improving, it's the reliability.
- So you are trying to apply 250.122, but there's no OCPD. What can you do? The solution is to use an "assumed" OCPD, the size that would be needed to protect those conductors per 690.9(B) [690.45].
- If you bond the heck out of your PV system and also provide the required system grounding but it's mounted on top of a building, can't lightning or other transients flash over between the two? Yes, that's why any structure supporting a PV system must have a grounding electrode system that meets the requirements of Part III of Article 250 [690.47(A)].
- If your PV system is not a solidly grounded one, the EGC for the output of the PV system can be the only connection to ground for the system [690.47(A)(1)].
- You can install additional grounding electrodes and attach them directly to the PV module frames or support structure. But each electrode must be sized per 250.66 [690.47(B)].
This concludes our series on Article 690, even though we did not cover the remaining three Parts, which are all very short:
- Part VI. Marking.
- Part VII. Connnection to Other Sources. In its entirety, it reads, "PV systems connected to other sources shall be installed in accordance with Parts I and II of Article 705 and Article 712.
- Part VIII. Energy Storage Systems.