Photovoltaic arrays are typically installed on rooftops, near power transmission lines, constructed of aluminum frames, and must be free from objects that shade them. Optimum exposure to sunlight also means increased vulnerability during electrical storms. Studies indicate that lightning is the number one cause of catastrophic failures in solar electric systems and components.
But is lightning protection important?
Lightning can strike anywhere at any time without warning. In most cases a “direct hit” is rare, in fact, the more common “near strike” hit (to the ground or within several hundred feet) can send thousands of volts into wiring systems if the proper precautions are not taken.
The NEC (National Electrical Codebook) requires that all exposed metal surfaces be grounded regardless of system voltage.
- Single Point Ground- A ground wire that connects to a ground rod or ground wire under the electric meter. If you have a panel array that is more than 50 feet from the rest of the system, it should have it's own frame/mount ground (not electrical ground).
- Ring Ground- A #2 AWG bare wire buried a minimum depth of 30" in the soil encircling a structure.
- Halo Ground- Consists of bare or insulated wire and runs around the ceiling of a structure and bonded to the corners of a buried ring ground. It acts as a shield for reducing RFI (radio frequency interference) from electromagnetic fields.
- Ufer Ground- Metal bars that are encased in concrete and buried a few feet under ground to be used when terrain or other barriers prevent single point grounding.
- Isolated Ground- An isolated ground is a separate, insulated safety ground wire that connects an equipment cabinet to the nearest ac distribution neutral-ground bond. It is used to maintain isolation from building conduit, which can conduct high frequency noise during an electrical storm.
PV systems have DC and AC circuits and both must be properly grounded. If the PV array system is mounted to the roof NEC 690.5 requires a GFP device be included. Grounding is essential and using the proper PV hardware is as important as using it correctly.
Since the primary focus of NEC requirements is electrical safety not lightning protection it is important to note NEC requirements can be extended.
Arrestors and Capacitors
In general, surge arrestors go across live wires with another wire going to ground so if the voltage goes above a certain level, it begins to conduct, shorting the higher voltage to ground.
Arrestors usually do not react fast enough to work alone. Surge capacitors act extremely fast and catch those high voltage spikes on the AC line for the surge arrestor. For the best defense in lightning protection combine a DC surge arrestor on the array side and a surge arrestor and capacitor on the AC side.