ASK THE EXPERTS: Wire Management & Fire Prevention

Beginner
Unmanaged PV wiring
Unmanaged PV wiring can lead to damaged conductor insulation, increasing the potential for a DC arc to occur on the roof.
Poor planning and unmanaged array wiring.
Another case of poor planning and unmanaged array wiring: Here, rail alignment does not allow for PV wiring to be secured or supported by the racking system.
Unmanaged PV wiring
Poor planning and unmanaged array wiring.

With the generous PV incentive program currently available in Louisiana, there has been a rush of unqualified installation companies into our local market. We have seen examples of poor workmanship throughout our coverage area. Please see the attached photograph for one such example of poor rooftop wire management.

To promote better overall installation practices, we are trying to gather information and evidence related to rooftop fire hazards. In particular:

  • How can loose (unmanaged) wires spark a rooftop fire? What might initiate an arcing event?
  • How susceptible are shingles to fires caused by electrical arcing? Are there any documented cases of a connection between a rooftop fire and improper wire management? Any feedback you can offer is appreciated.

Corey Shalanski, Joule Energy • New Orleans, Louisiana

In the days before maximum power point tracking (MPPT) charge controllers (see “Buyer’s Guide to Charge Controllers” in HP146) and direct grid-tied inverters, PV arrays operated at “nominal” battery bank voltages of 48 volts DC or below. At such low voltages, fire hazards from rooftop PV arrays were minimal. MPPT technology has since revolutionized PV system design, allowing high-voltage PV arrays of up to 600 VDC, smaller and more economical wire sizes, and much greater overall system efficiency.

While roof fires directly related to PV arrays remain rare thanks to updated electrical codes for DC circuits and installer certification programs such as NABCEP, the downside of higher PV array voltage is increased fire hazard from arcing, which is directly related to poor rooftop wire-management practices. 

No roofing material can withstand the 3,600°C (6,512°F) temperature from a DC arc for very long. Worse, such arcs can energize unexpected and dangerous current paths in metal roofing, and flashing and conduit, posing an electrocution hazard to both firefighters and electricians called to the scene to repair the system.

New products are being introduced to the PV marketplace that can reduce the risk of DC arcs, such as transformerless inverters (see “Ungrounded PV Systems” in this issue) and DC arc-fault circuit interrupters (AFCIs). AFCIs are now required by National Electrical Code Article 690.11 on any DC circuit of more than 80 volts. They may be located in inverters, PV module-mounted electronics, DC-to-DC converters, or combiner boxes. 

While these new PV products will help reduce the risk of fire, damage to PV conductors and/or modules still needs to be avoided. Common causes of physical damage include accumulated snow, ice, leaves, and conifer needles under an array that is not inspected and maintained regularly.

PV array wiring is also vulnerable to wildlife damage, and to more gradual wear from the movement of rooftop wires from wind, and the natural expansion and contraction of metal conduit and array racking due to temperature changes.

Modern PV modules now also employ preattached, outdoor-rated wire leads instead of old-school screw terminal junction boxes that required watertight flexible conduit between the modules. These interconnection systems have drastically reduced PV array installation time and cost, but have also left many installers wondering how to safely and neatly manage the jumble of wires and connectors hanging from the back of a typical installation.

Taking care to avoid pinched or nicked wires during installation and liberally using S-clips and zip ties to avoid damaged wires after installation will mitigate most of the hazards posed by modern, high-voltage PV arrays. Be sure to research the newer PV racking systems that include integrated and protected raceways for DC conductors. And remember, just like back in the day—regular system inspection and maintenance will catch most potential problems before they can turn into disasters!

Dan Fink • Buckville Energy Consulting

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