How to choose safer string harnesses for solar projects
Improperly implemented PV string harnesses can cause down time, delays and replacement hassles. At their worst, they can present dire safety risks. Here’s what you should know to maximize their performance and safety.
Pressured by project economics, many folks try to cut solar power system costs where they can. Often overlooked, photovoltaic (PV) wiring string harnesses may appear to be a safe candidate to cut to size on the chopping block. But decision-makers don’t realize how risky that cut corner can be: due to their susceptibility to decay, improperly implemented string harnesses could lead to malfunctioning systems, costly repairs and, sometimes, considerable danger.
Here are some principles to keep in mind to maximize string harness safety and performance. Namely, it comes down to the importance of quality products, proper molding and bonding and rigorous testing.
What makes a good PV harness?
Depending on a person’s exact position in the solar supply chain, the answer may seem a little murky. Indeed, much of the risk of PV string harness failure can be mitigated by understanding the role of the harness and what separates some harnesses from other options.
First things first: what is a PV string harness? In brief, the harness provides a conduit to transfer direct current (DC) power from arrays of PV panels to the next stage of power transmission (a combiner box or inverter). Specifically, they provide environmentally sealed parallel connections between panels and the next stage.
As part of the DC collector system, string harnesses are built from cable and connectors designed for photovoltaic applications, augmented with molded junctions and fuses. Within a larger solar power system, a key performance metric of the string harness is minimizing power loss over the considerable length of cable. Given their size and scale, they often ship directly to sites without interim warehouses, pre-kitted in boxes or shipped on reels.
What makes a good PV string harness? Reliably molded junctions and fuses, a design life of 40 years, low resistance through all joints, and environmental sealing are all features to prioritize.
For system safety, prioritize molding and bonding
Like many solar system components, string harnesses are exposed to harsh environments and can be susceptible to decay. In the field, string harness components are subjected to UV exposure, temperature cycling, chemical cycling, humidity and precipitation. These environmental factors contribute to material degradation, including shrinkage of the cable jacket along the length of the harness.
If not prevented or detected, this decay can shrink cables enough to leave internal wires exposed, which can endanger solar system maintenance personnel or other end users, and ultimately lead to a failure in power transmission.
Solar professionals can reduce the susceptibility of harnesses to environmental damage by selecting products with proper molding and bonding, which preserves the protective sheath around the wires inside. Conversely, they should avoid harnesses connected with methods more vulnerable to environmental factors, including mechanical compression or heat shrink tubing.
Overmolds with permanent bonding to cable jackets have superior longevity. PV harnesses that use bonding instead of mechanical sealing eliminate jacket shrinkage and the potential failures from receding cable jackets.
In contrast, PV cables connected via other sealing methods can pull out of the junctions, showing copper strands — a malfunction that endangers personnel and can lead to arcing with any tall, dry grass growing near exposed wires. Additionally, wicking can easily occur if moisture gets inside an outer mold that is not fully bonded to the cable, compromising the performance of the fuse or junction.
For long-term durability, look to rigorous testing
To maximize string harness performance and safety, it is important to select products that have undergone thorough design and rigorous testing. Given the importance of material selection for product longevity, buyers should:
1. Choose products that select materials for high RTI value
An important parameter to consider for fuse holder material selection is the Relative Thermal Index (RTI). A temperature rating for plastic, the RTI assesses the temperature a material can safely withstand for extended periods of time without compromising critical material properties.
Materials with high RTI values are especially important for surfaces in direct contact with fuses.
Fuse holders constructed with high RTI polymers experience much greater longevity than those surfaced with rubber alone.
2. Look for solar components tested to the most stringent possible interpretation of standards.
Although an RTI standard exists, their writing is open for interpretation, meaning that a product may perform differently — even within the guidelines — depending on how the test is administered. For full confidence in material durability, solar component buyers should look to products tested by the most stringent standard interpretations. Meanwhile, scrutinize the ratings provided by third parties, which sometimes test interpretations that fulfill the letter of the standard but not the spirit — with durability consequences.
Durable PV harnesses, durable solar systems
Now, no matter where you may sit in the solar industry, you know a little bit more about the importance of PV string harnesses. Whether you’re an installer wondering how best to navigate your next project, an EPC contractor sourcing new material, or a site host wondering what questions to ask operations personnel, remember: when it comes to PV string harnesses, prioritize quality and proper implementation.