Designing Effective Lightning Protection for Residential Rooftop Solar Systems
How to reduce the risk of PV systems being struck by lightning
Residential photovoltaic (PV) systems are generally installed on the rooftop of residential buildings. However, the large metal surface area, height from the ground, and exposed location increase rooftop solar systems’ risk of being struck by lightning in thunderstorms.
If the solar system does not have adequate lightning protection & grounding, the probability of being struck by lightning is extremely high. It can cause fire, electrical safety issues, and loss of property and human life.
The PV industry has matured, and system design and construction have become more standardized. However, doubts about the requirements for lightning protection persist.
This article addresses the requirements for designing and constructing the systems, including components, brackets, inverters, electricity distribution boxes, and grounding cables.
Why is lightning damage still a common occurrence? The problems that are easily overlooked during installation and ways to make the system protection more effective are discussed below.
General Lightning Protection Solution
For areas with relatively less lightning frequency, simple grounding methods are commonly used without installing additional lightning rods. If a system is installed on a flat roof, it usually tends to ground via the inverter cover or connect to the building’s existing lightning protection system. Such lightning protection is potentially inefficient for areas with high lightning probability.
Enhanced Lightning Protection Solution
Before considering adequate lightning protection of a PV system, we need to understand the common types of lightning strikes.
For residential PV systems, the type I and type II lightning strikes – direct lightning and induced lightning – are the most common. If the property is in a lightning-prone area or if there is a presence of other high metal objects on the rooftop like solar thermal heaters, water tanks, and satellite antenna, the probability of these two lightning strikes will be higher. It is, therefore, necessary to enhance and strengthen the lightning protection during the design stage.
Metal equipment on the rooftop
To enhance the effectiveness of lightning protection for residential systems, the following points, in addition to usual lightning protection measures, should be considered:
- If the system has a large capacity or is located in an area with high lightning strikes, installing a lightning rod beside the solar array is necessary.
- If there are other metal facilities on the roof, the DC side of the PV system should be kept as far from these structures as possible. Make sure the PV system mounting does not come into contact with these structures. It is also recommended that a lightning rod is installed on the roof.
- Reduce the general PV system cabling cross-area to decrease the strength of an induced lightning strike.
- It is recommended to implement a separate lightning protection solution for the PV system and avoid merely connecting to the building’s original lightning protection system.
This is a solar project in Fenghua, China. According to statistics, the area’s annual average lightning days of more than 75 times defines it as a lightning-prone area. The on-site residential buildings have a large flat roof area. The total installed capacity is 28 kW, with a GCI-25K-5G Solis inverter. The lightning protection system scheme shown in the figure below is what was implemented.
In addition to the building lightning protection for the solar modules, brackets, inverters, and electricity distribution boxes, the lightning protection system of the project adds the following safety features based on the plan above:
- For the most vulnerable parts of the system, like the inverter, ensure both the electrical (internal) grounding and cover (external) grounding are effectively connected
- Added four 1.5m length and Φ10mm round steel lightning rods at the four corners of the array and welded them onto the building’s lightning belt to reduce the system’s direct lightning strike’s impact.
- Separated grounding of residential PV system: Choose a location where the ground is thick and humid enough and dig a 1.5m-deep hole, then use Φ8 round steel (40*4mm flat steel can also be used or Φ10 round steel) to make the separated grounding. The grounding resistor is 1.68Ω and meets the requirement of less than 2Ω.
The lightning protection and grounding of residential PV systems cannot be ignored. We must consider the scientific lightning protection and grounding technology in the system installation process and strengthen the system’s lightning protection according to the site conditions. In short, spend time on your PV system lightning protection because meticulous care and attention at the design and installation stage can avoid many problems later. Safe and stable lightning protection and grounding construction can avoid system damage, and even personal and property losses.
This article is sponsored by Ginlong Technologies, solar inverter manufacturers.