The UK is committed to reaching net zero by and coupled with volatile energy prices, home and business owners are looking to invest in low carbon technology (LCT) with an aim to reduce energy costs and CO2 emissions.
With competitive price and timely delivery, SUNUA sincerely hope to be your supplier and partner.
Retrofitting direct current (DC) solar photovoltaic (PV) systems, especially cable routes, can be challenging to fit around existing services, be aesthetically pleasing and meet the requirements of BS :+A2:+A3:.
In the UK, unearthed DC solar PV systems are adopted and use double or reinforced insulation as the protective measure. As such, DC solar cables designed to BS EN are already familiar to many, but new innovations are hitting the market, designed to aid designers, and these raise challenging questions in relation to safety.
This article will be formed of two parts. Part 1, published in the March issue of Wiring Matters, explores the possibility of using steel wire armoured (SWA) cable as an installation method for unearthed DC solar PV systems, for instance, permitting the connection of the PV modules to an inverter.
Part 2 will be published in a future issue of Wiring Matters and will consider whether or not the proliferation of safety measures built into modern inverters and associated electronic equipment might permit the use of new wiring systems and installation methods.
The IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd Edition describes a PV module as a current limiting device with the short-circuit current of a PV array being not much greater than its operating current. Lack of significant fault current means a PV array requires a different approach when designing suitable fault protection.
In relation to the installation of cables, the current limiting nature of PV circuits means that additional protective measures are required to provide for fire and shock protection and Section 5.10.3 of the IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd Edition states:
All wiring systems shall have cables selected and erected to minimize the risk of earth faults and short circuits. All wiring systems shall be installed in accordance with Chapter 52 of BS to meet the requirements of Section 412 for double or reinforced insulation.
Solar systems utilize inverters to convert the DC supply from the PV modules to alternating current (AC). Whilst inverters comprising isolating transformers exist and utilize an earthed DC system, in the UK, solar inverters are predominantly transformerless, using an unearthed DC system, and are preferred since they generally have higher efficiency and improved earth fault sensitivity.
Insulated and sheathed cables, without metallic armouring or screens, such as those to BS EN , are commonly used to meet the requirements of Section 412 of BS :+A2:+A3: for double or reinforced insulation, and cables installed within buildings may need to meet additional requirements, such as the use of cables of limited smoke production.
The IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd Edition recommends that the cables shall not be directly buried in walls or otherwise directly encased in the fabric of the building, and where burial in walls cannot be avoided, they should be suitably protected from mechanical damage and a plan showing cable locations shall be provided.
It further recommends that solar PV DC cables buried in the ground shall be buried at a suitable depth and be protected against mechanical damage and impact in accordance with Regulation 522.8.10 of BS :+A2:+A3:, by installation within an underground conduit or duct meeting the classification of N750 according to BS EN -24: Conduit systems for cable management - Particular requirements. Conduit systems buried underground (incorporating corrigendum November ). Additional measures are to be taken to limit access to rodents, rabbits, etc. at the open ends of the conduit as they enter/leave the ground, by the use of suitable filling materials meeting the requirements of the manufacturers of the conduit or duct and the cables installed within it.
It is also important to ensure that ducts are sealed with waterproof fillers where required, since ducting can fill with water and cables left lying in water may absorb moisture into the sheath and insulation over time (see Figure 1).
Figure 1 Examples of sealing ducts. Photographs reproduced by permission of AC Cable Solutions showing Duct Seal LG Cable Duct Sealing System.
Regulation 712.521.101 of BS :+A2:+A3: also states:
Cables on the DC side shall be selected and erected so as to minimize the risk of earth faults and short-circuits. This shall be achieved by using:
(i) single-core cables having a non-metallic sheath, or
(ii) insulated (single-core) conductors installed in individually insulated conduit or trunking. Cable(s) shall not be placed directly on the surface of the roof.
Other types of wiring system providing an equivalent degree of safety are not precluded. NOTE: BS EN describes cables intended for use at the direct current (DC) side of photovoltaic systems.
There are, in fact, three things to consider in understanding why wiring systems (and other equipment) on the DC side are required to meet either the requirements for double or reinforced insulation, or alternatively, the requirements for separated extra-low voltage (SELV) or protective extra-low voltage (PELV):
SWA cable is used in AC systems where there is an increased risk of mechanical damage. The steel wire and the heavy-duty outer sheathing provides mechanical protection from minor abrasion and impact that might arise in environments such as being buried direct in the ground or where it may suffer from impact damage. The armouring does not protect against severe mechanical damage caused by puncturing or cutting through the steel, but it may provide protection from electric shock, since the armour is required to be earthed by Regulation 522.8.10 of BS :+A2:+A3:. The protective measure used in this scenario is automatic disconnection of supply (ADS). For example, if a metal spade is driven through a buried SWA cable, current will flow through a live conductor, through the metal spade and back along the steel wire, causing disconnection by a protective device.
Designers of unearthed DC solar PV systems may have to consider where there is a risk of mechanical damage to a string cable, for example, cables connecting ground mounted arrays may be buried. As such, an installer may opt for using SWA cable as a possible design solution, however, there are further considerations which need to be taken into account before such a decision can be made.
In the UK, Regulation 712.312.2 of BS :+A2:+A3:, does not permit the earthing of one of the live conductors of the DC side unless there is at least simple separation between the AC and DC side. As DC conductors within solar PV systems are typically unearthed, BS :+A2:+A3: denotes the protection method under Regulation 712.410.102 stating:
On the DC side, one of the following protective measures shall be applied:
(i) double or reinforced insulation (Section 412)
(ii) extra-low voltage (SELV or PELV) (Section 414)
In the case of SWA cable manufactured to BS , BS or BS , these do not meet the requirements of indent (i) above, since the single cores are not double insulated from each other. The insulated cores lay next to each other and additionally, the filler material (bedding) does not have any insulation properties (See Figure 2).
Regulation 712.412.101 of BS :+A2:+A3: also states:
The electrical equipment, for example PV modules, wiring system (e.g. combiner box, cables) used on the DC side (up to the DC connection means of the PV inverter) shall be Class II or equivalent insulation. NOTE: For wiring systems, see Regulation 412.2.4.
Regulation 412.2.4.1 of BS :+A2:+A3: requires that in the case of wiring systems meeting the requirements of Regulation Group 412.2 relating to the requirements for basic protection and fault protection, where the protective measure of double or reinforced insulation is used, the mechanical protection is provided by the non-metallic sheath of the cable or non-metallic trunking or ducting. Users of the Standard may take from this the important point that it is best to avoid bringing unearthed DC wiring systems into close proximity to metal work that may become live due to a fault.
(Note: cables are often incorrectly referred to as double insulated, rather than insulated and sheathed. The reasoning is a wiring system is not classed as ‘equipment’, however, a wiring system which is insulated and sheathed is deemed to meet the requirements of 412.2. (See Wiring Matters Issue 75, May , Mythbusters #4.)
Figure 2 Steel wire armour and solar cable.
Where a manufacturer produces an SWA cable that replaces the single insulated conductors with insulated and sheathed cores, surrounded by steel wire armour and exterior sheath, whilst such a cable might have elements constructed in accordance with a relevant British or Harmonized Standard, the armoured ‘part’ of the cable would mean the final cable assembly is non-standard. If a designer selected such a cable, this would be a departure from BS :+A2:+A3: and they would need to state the design as being no less safe, despite its use in accordance with Regulation 133.1.3 of BS :+A2:+A3:.
Regulation 522.8.10 of BS :+A2:+A3: states that:
Except where installed in a conduit or duct which provides equivalent protection against mechanical damage, a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor.
Thus, the use of insulated and sheathed cables with an SWA exterior would require the armour to be earthed via one or more protective conductors when buried direct in the ground. Nevertheless, with unearthed DC solar PV systems, this would not be possible and any attempt to earth the armouring, either on the AC or the DC side, would not result in earthed armouring and may cause unintended consequences and introduce additional hazards. Regulation 522.8.10 of BS :+A2:+A3: would therefore require a suitable duct to be used, negating the need for armouring in the first place.
Regulation 21 and 22 of The Electrical Safety, Quality and Continuity Regulations (ESQCR) specifically state that installations operating as a switched alternative in parallel to a distributor’s network must fully comply with British Standard requirements (i.e. BS ). The use of a cable buried direct in the ground without an earthed metal sheath or armour does not meet the requirements and is therefore not permitted.
Installers of unearthed DC solar PV systems should adhere to the Energy Networks Association (ENA) engineering documents relating to connection procedures. For systems where micro-generating plant with an aggregate registered capacity of 16 A (3.68 kW) per phase, or less, are installed, this would be Engineering Recommendation G98 Issue 1 Amendment 6 September . For systems where generation is in excess of 16 A (3.68kW) per phase installers, this would be Engineering Recommendation G99 Issue 1 – Amendment 10, 4 March . Both of these Recommendations require compliance with BS .
The requirements of Regulation 712.521.101 of BS :+A2:+A3: state that:
Cables on the DC side shall be selected and erected so as to minimize the risk of earth faults.
The use of insulated and sheathed cables with an SWA exterior will clearly have been chosen as a consequence of a designer being concerned that mechanical damage could occur and thus, steel wire could become live in the event of damage which could turn an unearthed DC system into an earthed DC system and introduce other potential hazards (for example, no method of providing ADS).
Regardless, Regulation 712.312.2 of BS :+A2:+A3: states:
For more pv cable compoundsinformation, please contact us. We will provide professional answers.
Earthing of one of the live conductors of the DC side is permitted, if there is at least simple separation between the AC side and the DC side;
In the case of a transformerless inverter, this is not permitted. The fundamental rule for protection against electric shock is a requirement in Section 410 of BS :+A2:+A3::
The fundamental rule of protection against electric shock, according to BS EN , is that hazardous-live-parts shall not be accessible and accessible conductive parts shall not be hazardous-live, both under normal conditions and under single fault conditions.
This might lead a designer to assume that insulated and sheathed cables with an SWA exterior would be fine to use on unearthed DC solar PV systems because a single fault would only result in damage to the sheathing, but clearly there would be a risk that such damage would more than likely result in a fault to earth.
Given that solar PV cable is required to meet the requirements of the protective measure of double insulation, this regulation seems to suggest that installers should be wary of introducing a fault to metal work, even where two faults (damage to sheath and damage to insulation) would need to occur. Indeed, BS EN states:
If a single fault condition results in one or more other fault conditions, all are considered as one single fault condition.
With respect to wiring systems in general, BS :+A2:+A3: is clear with the requirements in Regulation 412.2.4.1:
Wiring systems installed in accordance with Chapter 52 are considered to meet the requirements of Regulation 412.2 if: (i) the rated voltage of the cable(s) is not less than the nominal voltage of the system and at least 300/500 V, and (ii) adequate mechanical protection of the basic insulation is provided by one or more of the following: (a) The non-metallic sheath of the cable (b) Non-metallic trunking or ducting complying with the BS EN series of standards, or non-metallic conduit complying with the BS EN series of standards.
In the case that insulated and sheathed cables with an SWA exterior are installed using any other method of installation other than buried direct, whilst not specifically prohibited, this regulation implies that mechanical protection should be provided by non-metallic trunking or ducting and the introduction of metal armouring is therefore undesirable.
Traditional wiring systems, such as suitably impact rated insulated ducting to N750 and insulated and sheathed cable, meet the requirements of BS and provide adequate safety to human beings and livestock.
The use of single insulated SWA cable for unearthed solar DC cabling is not permitted by BS as it does not provide double insulation between the string conductors.
Insulated and sheathed cables with an SWA exterior may arguably be permitted for unearthed solar PV cabling where it is not directly buried in the ground. Where it is buried direct in the ground, the cable must be earthed in accordance with Regulation 522.8.10 which is not possible in an unearthed system and is therefore not permitted.
Other cables, such as insulated and sheathed cables wrapped in a “tough sheath”, could be used where there is a concern about cables being damaged as they are drawn into ducting or conduit.
According to the DOE’s Solar Futures Study, the United States will need to double the amount of solar energy installed per year between and to decarbonize the electricity sector by . Locating solar energy on farmland could significantly increase the available land for solar development, while maintaining land in agricultural production and expanding economic opportunities for farmers, rural communities, and the solar industry.
Read more about the potential of agrivoltaics for the U.S. solar industry, farmers, and communities. You can also read about our research on agrivoltaics through the National Renewable Energy Laboratory’s InSPIRE project, which includes an agrivoltaics primer, published research portal, and map of agrivoltaic sites in the United States.
The height of photovoltaic (PV) panels can be raised to allow for easier access to crops. Raising the height of PV panels, however, can increase the cost of the solar installation due to the need for additional steel for the foundational posts. The length of steel foundational posts underground may also need to be increased to accommodate the additional wind loading.
Another common way to adapt the design of a solar installation for agrivoltaics is to increase the spacing between panels and between rows, which allows for additional sunlight to reach the crops and increases the accessibility of the site to equipment. Increasing spacing, however, decreases the amount of electricity that can be produced on a given piece of land, so there is a trade-off between solar and agricultural productivity.
There is significant opportunity to produce large amounts of solar energy on farmland. Agricultural land in the U.S. has the technical potential to provide 27 terawatts of solar energy capacity. This is a quarter of the total U.S. solar energy capacity of 115 TW. Only 0.3% of farmland is expected to be used for solar energy by .
The Agrisolar Clearinghouse has an interactive map that identifies federal, state, and local financial incentives, tax breaks, and other relevant financial programs that support agrivoltaics. Federal tax credits like the 30% investment tax credit are available to homeowners and businesses, and you should check with a tax expert for financial guidance on how to access these incentives. For example, businesses are eligible for additional credits, such as the energy community bonus, which are not available to homeowners.
When considering the ground-mounted solar energy, it is also important to consider soil compaction and its effects on soil health. Heavy equipment used during installation can exacerbate compaction, which reduces pore space in the soil, limiting water infiltration and root growth. These effects can be reduced through the utilization of low-impact site preparation and construction techniques, as well as decompacting soils after construction. Developers can use software to model preconstruction grading needs and choose the appropriate torque tube height and racking systems for the specific area in order to minimize soil compaction.
Solar can be installed in flood plains, but all electrical equipment will have to be installed above the projected level of flooding. Raising equipment could increase the cost of installation and may negatively impact the project economics. Also, the cost of insurance could be higher for PV systems in a flooding area. Areas that do not flood may be better suited for PV installations.
Cable management—which is the organization of cables connected to electrical devices—is crucial for ensuring the efficiency of the solar installation and minimizing the impact on agricultural activities. The preferable cable management technique depends on your specific needs, aesthetic considerations, safety concerns, budget constraints, and local regulations. Options for cable management include:
Consulting with a solar developer, considering the impact on agricultural activities, and evaluating long-term maintenance requirements will help you determine the most suitable cable management approach to take.
Herbicide is currently sprayed at some solar facilities to prevent weed growth. Agrochemicals should not present an issue. Care should be taken to not spray panels themselves, but if it occurs, the panels can be washed off with water as they are made of glass and steel or aluminum and have been designed to withstand outdoor conditions.
If agricultural or grazing activities require equipment, machinery, or fencing, it is essential that these items are compatible with the solar design and configuration and will not lead to solar infrastructure damage. This not only includes consideration of panel heights and inter-row spacings, but also whether or not equipment would need to be attached to any part of the solar infrastructure, especially moving parts of tracking systems. Compact and low-profile machinery can allow for navigation between solar panels without causing damage. Automated or GPS-guided machinery can also be used to enhance precision and reduce the need for manual intervention, making it easier to navigate the layout of the solar panel system.
Adjacent agricultural activities can lead to increased soiling on panels from airborne dust and particulates generated during tilling, planting, or harvesting activities, or through pollen released by crops such as corn. Power generation loss due to soiling should be incorporated into PV system generation estimates. NREL’s PVWatts soiling calculator assumes that on average, 2% of power potential will be lost to soiling, but these losses are highly dependent on local weather and soiling conditions.
Land can be converted back to agricultural uses at the end of the operational life for solar installations, roughly 30 years. Depending on the solar operation, crops may or may not be present in the soil during the lifetime of the facility. Giving soil a rest can maintain soil quality and contribute to the biodiversity of agricultural land. On the other hand, implementing agrivoltaics and planting crops such as legumes underneath the solar installation can increase nutrient levels in the soil.
Sheep are grazed at some solar facilities in the United States and Europe. For grazing systems, most standard utility-scale solar panel heights can accommodate sheep grazing, but elevated panel heights are generally needed for cattle grazing. Research to facilitate cattle grazing under solar arrays is ongoing.
For all animals, wire management systems should be properly encased to avoid interactions with the animals. In many cases, traditional utility-scale solar infrastructure does not need to be modified significantly to support livestock grazing. According to data gathered by NREL’s InSPIRE project, as of November , over 4,000 megawatts of power generated by solar panels in the United States include sheep grazing underneath. Solar operators can benefit from sheep grazing through a reduced need for mowing, herbicide, and other vegetation management needs at the site. Local shepherds may also benefit through payment for managing the grazing.
Silicon-based PV cells are the most common solar PV technology. Most solar panels have a glass layer on top that protects the PV cell and an aluminum or steel frame. An Electric Power Research Institute report found that “leaching of trace metals from modules is unlikely to present a significant risk due to the sealed nature of the installed cells.”
Some solar modules use cadmium telluride (CdTe). Cadmium compounds are toxic, but studies show that such compounds cannot be released from CdTe modules during normal operation or even during fires. Industrial incineration temperatures, which are much higher than grassfires, are required to release the compounds from the modules.
In some cases, agrivoltaics can improve soil health. For example, grazing sheep beneath solar panels has been shown to increase the soil organic carbon uptake. Letting the soil sit can also improve soil health.
Microclimate effects depend on the design of the solar system and the surrounding environment. Air temperatures tend to be cooler under the panels during the day and warmer under the panels at night. One study found that soil temperatures under the panels were less than that of soil temperatures in full sun all day and higher at night. There have been no studies linking solar development with pest problems or invasive species, but studies have shown that crops and native plants can thrive underneath solar installations.
Agrivoltaics can enable farmers to grow shade-tolerant crops and to diversify crop selection, while also extending growing seasons and reducing water requirements. Solar panels can cool crops and vegetation underneath during the day due to shading and keep them warmer at night. Some studies have shown that these temperature differences cancel each other out, so that daily average crop temperatures are similar under panels compared to full sun crops. High temperatures are often detrimental to crop yields. One study found that shading from solar panels produced lettuce crop weight equal to or greater than lettuce grown in full sun. In other cases, depending on the crops and growing conditions, impacts on yields can be more nuanced and depend on system design.
Yes, solar installations can support native vegetation and pollinator habitat species. Low-height plants can thrive underneath solar panels, avoiding the need for mowing and keeping the panels unshaded. Fifteen states are using state-specific Pollinator-Friendly Scorecards to promote planting of pollinator habitat underneath ground-mounted solar projects. Over thirty states are using state-neutral scorecards. Pollinator habitat under solar arrays can benefit farms by increasing local agricultural yield and can also host beekeeping operations.
A total of 15 state scorecards and one nonspecific scorecard available
A SETO-funded project led by the University of Illinois Chicago is studying the economic, ecological, and performance impacts of pollinator plantings co-located at five solar PV facilities (10 megawatts or larger) in the Midwest and Mid-Atlantic regions. The project is developing guidance and decision-making tools, such as a pollinator planting manual, cost-benefit calculator, and native seed mix selection tool, for solar developers and landowners.
For more information about pollinator-friendly standards and practices for solar sites, visit the Center for Pollinators in Energy website.
Learn about the benefits of establishing pollinator-friendly plants under and around ground-mounted solar arrays.
If you are looking for more details, kindly visit thermoplastic compounds.