During a recent summer thunderstorm, I was looking out the living room window when a blinding white light flashed accompanied by a deafening, crackling bang. I didn’t need a power quality monitor to know what that was, nor did I have to count the seconds between flash and boom and divide by five to figure out how far away the lightning strike was. I paused and listened for the secondary sound that often follows such events. Sure enough, about two minutes later, the siren whine of the responding fire engines confirmed that lightning had done it again.
With millions of lightning strikes per year in the United States causing nearly 6,000 structure fires and more than $400 million in damage (according to NFPA statistics), this scenario is far from unique. What was unique but not unexpected in my experience was that the lightning hit the solar panels on the roof of the house and blew an 8-foot hole in the ceiling of a bedroom. Fortunately, it was mid-afternoon and no one was in that room. The resulting fire from the lightning strike was quickly extinguished. Nearly every piece of electronic equipment in the house that was operating at the time was damaged. With standby mode in many such devices, that effectively means they are operating all the time.
By necessity, solar panels are in exposed areas. Rooftop-mounted panels on residential dwellings put them at the highest point in the near vicinity. Hence, it is not surprising that lightning strikes are a leading cause of downtime in photovoltaic (PV) systems. Both NFPA 780, Standard for the Installation of Lightning Protection Systems, and UL 96A, Standard for Installation Requirements for Lightning Protection Systems, address this.
Before reviewing those standards, let’s see how lightning operates. While it may not always follow the rules in the standards, it does follow the rules of physics.
The same rules that we use for power quality—Ohm’s and Kirchhoff’s laws—apply to lightning. As a unipolar impulsive transient over 1 million volts and current possibly exceeding 10 kilo-amperes in mere microseconds, there are some special circumstances to consider. Using a digital voltmeter (DVM) in the “ohms” setting to check the ground impedance may work for electrical safety purposes but will not tell what the high-frequency impedance is between the grid frames and the grounding electrode. The impedance at those frequencies and rate-of-rise of the impulse are much higher than at DC, which is what most DVMs test at. If lightning directly hits part of the PV cells, the energy is going into the DC side of the system and will likely be coupled into the AC side at the inverters without proper surge protection, destroying much in its path. Once on the AC side, the transient’s energy is into the house wiring and the connected devices. Many “surge protection” power strips do not have the capacity to mitigate this much energy. They are usually more effective when the energy is induced or coupled into the distribution system and flows down through the house service wiring at a much lower voltage and current level.
A significant differential voltage can be created between the wires that ground the panels and the grounding electrode, particularly if wiring rules (e.g., no sharp bends) aren’t followed. Use of a separate grounding electrode for the panel frames can exasperate this potential. The “ground” is no longer an equipotential ground, and all devices connected in the building system may have an extraordinarily high ground voltage compared to the line or neutral conductors. This is a lethal combination for electronic loads. Proper grounding and bonding is essential.
The newly introduced requirements in Chapter 12 of NFPA 780 aren’t extensive, but they can minimize the damage to the PV system and the structure. Though not often used anymore on residential dwellings, creating a zone of protection using air terminal or other strike termination devices can help to dissipate the potential that attracts lightning and provide a dissipation path should a strike occur. In National Electrical Code Article 250 on Grounding and Bonding, and Article 690 on PV Systems—particularly 690.41 through 690.50—specify Code-compliant wire sizes, materials and techniques.
The lightning-protection components, including surge-protection devices (SPD) specifically designed for PV systems, should be sized, located, installed and used in accordance with UL 96A. These include any communication lines associated with the system. An on-site inspection by an approved organization can provide assurance that the installation is in compliance with the appropriate standards.
So why did the bedroom ceiling collapse? In milliseconds, lightning’s energy can superheat moisture in any structural materials. This creates a sudden steam explosion for which drywall is no match. No matter who you look at it, lightning is impressive and should be treated with the utmost respect and protection.