Cable care

Cables were the culprits in two fires that happened in PV arrays, one in Bakersfield, California in 2008 and one in Mount Holly, North Carolina in 2011. Bill Brooks from Brooks Solar prepared a paper for the Solar America Board for Codes and Standards explaining the causes of the fires. In both cases, cable insulation damage was identified as the root cause. A significant amount of work goes into the complex process of designing and planning a PV power plant, whether on the rooftop of a building or ground-mounted on the field. This then translates into an efficient, working PV power plant in situ. With the rapid uptake of PV, it is also common to see homeowners who design and implement their own small PV rooftop projects. With so much already invested, it would be vexing if careless cable management after installation lead to losses. And dangling, untidy cables are simply unaesthetic.
Cables are subjected to thermal, mechanical and external loads. Just like the rest of the system, cables need to last the stipulated 25 years or more. Being exposed to harsh environmental conditions like temperature fluctuations and direct UV can damage unprotected cables, and in turn the wires in them that carry the power generated.
Lapp GmbH’s Bernd Leushake explains that the percentage of the system governed by cables and accessories is 3 to 4%. However these products are responsible for 6 to 10% of claims and problems caused. Therefore it starts with the selection procedure. Leushake says it is important to take note of who the supplier is and the supplier’s reputation in the market when selecting a cable. “You have to invest more in cables to be sure to have a good product in hand, rather than trying to save money wherever possible. Because when you have to replace the cables, then it gets expensive,” he adds.
After the selection process, the cables are installed and thereafter the installers are also expected to have a management system. Leushake says that as suppliers, cable companies provide recommendations on how cables can be managed. But it is up to the installers to follow the guidelines. “We have limited influence on what the installers do,” he adds.

Some common errors

Fastening: The fastening of cable ties, clips and other attachment tools should be carried out such that the electrical properties of the cables are not compromised. A PV installation is in the open and movement is something that installers have to factor into the equation of cable management. Sunlink’s Vice President of Products and Strategy Yury Reznikov says that cables can be expected to move a little. Under extreme weather conditions such movements are increased. “Installers have to make sure that cables are nice and tight and that they provide enough slack,” Reznikov adds. Too much slack can cause cables to be accidentally yanked out or tripped over. If the cables have little or no slack, and are pulled relatively taut, then the tensile strength of the cable would be called into question. Zerer adds that expansion and contraction of cables should be considered too.
Installers use different cable ties to attach cables to the rack or other supporting features. Normally such cable ties are UV-stabilized and available in a wide range of tensile strengths, bundle diameters and styles. Zerer explains that cheap cable ties that are not UV-rated can possibly fall apart after a few years. And they are not hardy enough to brave through 20 to 25 years of sun, rain and curious animals with a penchant for plastic. Metal wire ties, though not as appetizing, can also potentially cut into cables.
In North America, the NEC 2011 edition, Article 334 governs the cable routing requirements. Specifically in section 334.30, the guide for securing and supporting is provided. With regards to the support type that can used, the article states that the cables, “shall be supported and secured by staples, cable ties, straps, hangers, or similar fittings designed and installed so as not to damage the cable.” This article also calls for the requirement that conductors be secured within 12 inches of each box, cabinet, conduit body or other termination. On the contrary in Australia, the Australian Standard Compliance AS/NZS 5033:2012 explicitly states that plastic cable ties are not to be used as a primary means of support. In Germany, for example, plastic ties are a common sight in installations.
When too many cables are tied together with cable ties, overheating can happen, which in turn increases resistance. This translates to power losses and faults. Solarpraxis AG’s engineer Rajkumar Subbiah explains such overheating can lead cables, especially thin ones, if they are in the middle, to fail and even fires can eventuate.
With landscape orientations, things could get tricky with module leads, as Zerer points out. Module leads could be too short and more time and costs can be incurred. Installers have to note the planned orientation of the modules and prepare the necessary tools and material for such situations. On-site corrections and modifications may not follow codes or may even be counterproductive.
Sharp edges: Solarpraxis engineer Markus Zerer does PV plant assessments. One mistake that he sometimes sees on the field is the disregard of sharp edges and corners when routing and managing cables. When cables are laid over sharp segments of the mounting racks for example, there is a possibility that movement over time can result in the outer jacket of the cable getting cut. Leushake adds that sometimes cables are pulled through drill holes in the racking that cause damage to the outer sheath due to the sharp circumference. Attention also has to be paid that cables are not in contact with abrasive surfaces or compressed under the weight of modules.
However if the cable is TÜV approved then it has mechanical robustness. During TÜV certification, solar cables undergo – among other things – a number of tests like notch propagation and dynamic penetration tests to verify their mechanical robustness. This means that the cables are insulated twice: an insulation around the conductors and an outer jacket. So even if the outer jacket gets cut, there is another layer of insulation protecting the wires. Many companies, including Lapp, utilize the electron beam cross linking procedure for their insulation. These cross-linked insulation materials are said to improve shear and impact strength, as well as crack and chemical resistance thus protecting the wires more effectively. Nevertheless in the worst case scenario, the cut can eventually get deep enough to expose the conductors. This can cause a short circuit in the system or pose threats to anyone who comes in contact, so avoiding this error can save a lot of trouble and even lives.
Bend radius: Another issue is the bending radius of cables. Solar cables normally provide a bend radius in their product specifications. The bend radius is the minimum radius the installer can bend the cable without kinking it, causing it damage or leading it to have a shorter life. The installer should not go below the minimum radius. The over-bending of a cable can cause excessive heat at the bend and cause stress on the connection. This also increases conductor resistance.
“If you have a construction with, for example, 32 wires and you bend it too much, then 2 or 3 wires may break. Then you have higher conductor resistance and thereby problems,” Leushake adds. In Germany the DIN VDE 0298, Part 3 applies to this. If the bending radii are not maintained during the management of cables then this can cause the material to stretch and compress, leading to changes in the mechanical structure. This in turn can impair electrical properties.
In the U.S. case, the NEC provides Article 338.24 that states that the “radius of the curve of the inner edge of any bend, during or after installation, shall not be less than five times the diameter of the cable.” Again a damaged cable and exposed conductors can result in faults.
Water contact: Cables are also not meant to be in constant contact with water. Lapp’s Leushake gives an example, “On the string level, cables are laid in conduits underground. We had some complaints where people said that the conduits were permanently immersed in water. The conduits then fill with water and the cable is immersed in water”. The mistake here was that the ends of such conduits and pipes were not sealed. Water enters and the cables are then immersed in water and get saturated at some point thus giving no guarantee for insulation resistance anymore. Solarpraxis’ Zerer says that there are cases where the cables start to “rot.” This can cause current leakages and short circuits.
Weather exposure: Despite being rated as UV-resistant or able to withstand high temperature ranges to combat environmental factors, cables and connectors alike should be kept out of direct sunlight as much as possible. Materials that are exposed to high irradiation conditions, which are highly likely in countries like Australia or the MENA region, can face degradation over time despite ratings and having been tested. When cables are lying on rooftops or on the ground, they can also be subjected to snow cover or ice dams which can cause damage as well to cables.
Inter row cables: There are not many solutions for cables that hang between rows of panels. Inter row cable management can be tricky as cables are at times left drooping from one row to another. Not all mounting system manufacturers provide a solution for inter row cable issues and installers leave these cables as they are or use conduits to run them.
Overstuffed cable glands: Cable glands are often used to provide strain relief. However at times multiple cables are squeezed into a single cable gland. This can, contrary to the cable gland function, impose strain on the cables and cause overheating.

Solutions available

The issues given above are not exhaustive. Installers are faced with a smorgasbord of challenges with different PV installations. However appraisals show that these mistakes are commonly noticed. Solutions are at hand though. Mounting system developers are realizing that cable management is not just an afterthought but an integral part of the entire system. There are an increasing number of solar mounting systems that are also offered with cable management solutions.
SnapNrack, for example, allows module leads to run through the channels not only keeping the cables protected but also providing a clean look for better aesthetics. Lorenz-Montagesysteme GmbH also took cable management into mind when they designed their mounting systems. Another example is Ecolibrium’s Ecofoot 2 Base Kit that tucks cables away on the base of the mounting system in an effort to streamline the installation process.
When we look at the individual issues and solutions, installers are able to find products to help them along the way. In the case of the cable ties, the UV-rated plastic ties are one of the most affordable versions. There are cable ties of other materials as well like metal ones that have UV-resistant vinyl jacketing and copper crimp sleeves for tightening and loosening. However installers should ensure that the cable ties will not cut into the cable jacket when fastening and if metal cable ties are used, this can happen. Most cable clips can be clicked on to module profiles and hold cables in place, off the ground. These click clips save a lot of time and enable easy cable routing. Würth Solar, who offers such clips amidst its PV wide offerings, states in its brochure “to prevent solar cables from sagging, they should be fixed in place every 40-50 cm with a cable clip.” Schletter’s mounting systems for example also offer different types of cable clips to help with cable management: rectangular ones that can be snapped into M8 channels or between upper and lower screw ducts (Schletter mounting system specific) and round clips that can be snapped onto the M10 screwing channels to enable better cable routing. There are other clips available as well that go along with the company’s systems.
Additionally companies like Cooper Industries also provide cable tray systems that can be used in lengths of panel arrangements. Cables can be run along these trays under the modules. But installers have to check local requirements. The NEC 2011 Article 392 for example does not allow single-conductor cables smaller than 1/0 AWG in cable trays.
Conduits are also seen as solutions to protect solar cables, especially when they are running inter rows. Conduits are normally flexible and provide an outer casing for cables to run through. Conduits are normally made of PVC or metal, the metal version being costlier. The PVC variation is flexible and allows cables to be routed through easily. PVC, however, does not age well. Conduits also have fill capacity limits that installers have to note. Just like overstuffing cable glands, overfilling conduits with cables to “neaten up” the array may not be the right move either.
Every installation is different and installers have codes and rules to follow. Nevertheless mistakes tend to happen. Perhaps with more practical cable management solutions integrated into mounting systems, installers will not have such a difficult time figuring out what to do with dangling cables or disintegrating plastic wire ties. The trend is moving towards such mounting system types that cuts costs and time of installations. Nonetheless, in the end the installer remains the one who decides.
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