Caution: fire, termites, poor performance

Code agencies can specify PV cable, connector and raceway standards, while manufacturers can produce safer solutions and training organizations can encourage best practices. But installers can still cause a component failure if not foster a fire with substandard performance. Now that operations and management have moved into the forefront of PV industry concerns, wire management has come under the thermograph more frequently.
“Wire management as a whole is probably the largest risk area in a PV system,” says Chris Oestreich, the Director of Products for SnapNrack, a sister company and supplier of solar distributor AEE, in San Luis Obispo, California.
That risk is poor performance at least, and fire at worst. “When an in-line cable connector is disconnected from a hot system, it can release an arc that is hot enough to weld something,” claims Frank Rosenkranz, the Vienna-based solar product manager for Tyco Electronics’ Intelligent Building Systems unit.
But fires are not a phenomenon that the PV industry publicizes much. “There have been several prominent fires involving PV that have not received much press yet. When fires do get reported – at a rate of about one in five – then the case goes into a fire investigation process, which may not be public; the involved companies absolutely don’t want it public,” says Bill Brooks, the President of Brooks Engineering, of Vacaville, California. “It is frustrating that some powers that be don’t want to release the information, even though it could be hugely beneficial to the solar industry,” he laments.

U.S. cable practices lag Germany

“The U.S. PV industry hasn’t arrived on cable or wire management, so we are continuing to figure these things out,” says Brooks. “The struggle at the moment is that with costs coming down, any additional expense related to wire management – be it in hardware or labor – becomes a lot more visible,” he says. “So when the pressure is on to cut costs, in our industry – which has been employing minimalistic wire management in many cases for a long time – it is difficult to implement better practices, unless they directly affect the bottom line,” he explains. “The other problem is that there is really a long-term feedback loop for PV repairs, and financial people don’t know how to monetize those costs,” he surmises.
“In the United States, unlike Germany, there has been little development of wire management over the last 14 years as our industry has grown. It’s kind of left up to the discretion of the installer, so ultimately it becomes a training issue,” says Brooks. In Germany, the norm in the electrical industry is to manage wire exposure without necessarily resorting to metal pipe, which is the predominant solution in the United States, except in the PV industry. “So electricians accustomed to using pipe get up on a roof and look at loose wires and say, ‘Gee, this is new,’” he says.

The potential error list is long

The key reason that wire management is the greatest safety issue in a PV system is the long list of things an installer can do to botch a job. While greater installer training is one answer to this problem, the provision of proper tools can also aid in error reduction. For example, crimping a wire into a connector with the wrong pliers can increase resistance and enable the temperature of the wire to exceed specifications and cause a fire, Rosenkranz points out. The same poor crimping job can lower the threshold for a wire pull-out. The ready solution for this problem is to use the crimping tool – which may have a unique wire positioning design – recommended by the manufacturer.
Connector contamination – from chemicals or soilage – prior to connection can also reduce the strength of the connector. “The use of grease or a lubricant like WD40 can cause degradation that is visible after one day,” says Rosenkranz. A solution Tyco Electronics (TE) offers for contamination is connector caps, which some installers could find to be extra work and not worth the protection, if the EPC does not insist on them.
And cables that are too short can lead to an excessive bend radius for the wire, which can increase heat and degrade the plastic shielding. One solution to radius bending is a hard corner guide, like that used in the cross-linked polyethylene tubing (PEX) market, to limit the angle and to support the wire.

Pre-termination on the rise

“In free-field installs, there is a trend toward pre-cabling, but all the OEMs and the EPCs have their own system which they swear is the best, so there are no uniform parts to sell to all installations yet,” says Rosenkranz. “But on residential rooftops, the home run always will be different because the inverter can be placed either on the roof, or in the basement or near the breakers, so I don’t see a trend in pre-produced cables for the segment,” he adds.
The value of quality is not as perceivable as time savings, however. “Installers can save a lot of money by pre-configured solutions; but it’s not just cost savings, it’s factory-tested quality improvement,” says Oestreich.Beyond pre-termination of individual wires, more sophisticated plug and play systems are on the rise since Westinghouse pioneered the concept in the U.S. market years ago. SnapNrack recently unveiled a new wire retention clip to hold wires into the integrated wire channel of the company’s racking product. And TE is working with several module makers to come up with an integrated design for modules, connectors and wiring, but the final product will take several years to get through the certification and standards boards, Rosenkranz notes.

Harnesses replace wire spools

Some cable vendors compete better with harness offerings. “We’ve seen price pressure along with other balance of system component providers, but our focus is on the simplification of labor and the reduction of time,” says Mark Farrelly, the Director of Solar Business Development for Amphenol Industrial Products, based in Carlsbad, California.
“When I go in to a developer to make the case for using pre-terminated strings and wiring harnesses, they understand the labor savings, which they say is up to 70% of total labor cost for DC stringing,” he says.
“We do the layout for PV cable harnesses for our clients for free, so what they get are factory assembled pre-terminating strings with connectors on each end of 8 to 10 wires in a harness, zip tied and labeled, so the installer knows exactly where it goes,” he says. The company recently released its Trunk & Drop harness for string installations.
The use of such plug-and-play components in a PV system also reduces the expertise level required for an installer. While a licensed electrician may be required in California to strip the end of a wire and torque it down in the correct position, a mere technician can plug in pre-terminated cables in a harness, notes Farrelly. “Some of the junction box and connector box manufacturers have begun producing their products with built-in connectors compatible to ours, so the labor savings flow through the BOS chain more smoothly,” he says.
Another reason that cable harnesses are gaining favor among developers and installers is easier accounting and theft protection. “Our clients say that they have a hard time doing the site-specific accounting on huge spools of wire that may be used on more than one job, but a single custom harness is easier to associate with a specific job,” Farrelly says. “Theft prevention is another issue that harnesses help with. Some companies in one region we supply say that they routinely plan for the theft of 10 to 15% of the total copper that may be used on a job,” he asserts.

Cable enhancements

Several sources expressed satisfaction that the current U.S. standard for the protection of PV cable from ultraviolet light degradation is satisfactory to last 20 or more years. Cross-linked polyethylene (PEX) has been adopted by the PV industry after years of testing by the petroleum and automotive industries.
But other hazards for cable exist. In Australia, for example, tough species of termites routinely chew through common PV cable. So specialists like Nexans Olex, based in Tottingham, Victoria, produce PV wires with additional protective sheathing made from hard nylon or other materials to combat termites. Similarly, rodents have been found to munch through cabling with ease. “We’ve looked at making rodent-proof PV wire, but the cost would be four times normal PV wire, so the demand won’t be too heavy,” says Amphenol’s Farrelly.
A possible change in materials use practice that could lead to U.S. wire cost reduction is the greater use of aluminum, or copper-clad aluminum wiring in place of pure copper. While aluminum is outlawed in Europe for PV wiring, it is permitted in the United States by Underwriters Laboratories (UL), Rosenkranz asserts. Alcan Cable, based in Atlanta, for example, offers Stabiloy SunPlus aluminum cable to the U.S. market. TE also is working with automotive OEMs to switch from copper wiring to aluminum, and once that process is complete, the technology could transfer to PV, he points out. The switch could cut wiring costs in the U.S. market by 20% to 30%, he reckons.
One area where new code guides will help PV wire management is in ground fault and arc fault protection, Brooks points out. The U.S. National Renewable Energy Laboratory, in Golden, Colorado, is reviewing the final version of a handbook for the field analysis and repair of PV ground and arc faults that is expected to go public by this summer, he notes.
Other standards work that will take place this year includes PV working groups led by Underwriters Laboratories and the National Fire Protection Association, points out Ken Boyce, a principal engineer manager at UL, in Northbrook, Illinois. “Apart from our working group suggestions, the NFPA will take proposals this year for consideration to change the 2017 National Electrical Code content,” he says.
“It is always important to consider experience from the field and perhaps offer more specifics, for clearer or more useable guidance for PV installation,” he concludes.