By Matthew Paiss
As we are trying to learn more about high-voltage electrical systems being installed on our roofs in the form of Solar Photovoltaic (PV) modules, they are fast becoming a key part of energy independence. In my May 2009 Fire Engineering article “Solar Electric Systems and Firefighter Safety,” I wrote about how to identify the presence of a PV system, what can hurt us, and what is safe. There are still dangers associated with PV that we hope will never be the reason for loss of property or, worse, firefighters’ lives. It is because of these dangers that we should continue to support technical innovation to reduce the hazards. That is the focus of this article.
Solar electric modules (photovoltaic, or PV) generate DC electricity in daylight hours only. This voltage is converted to AC at the inverter and then fed back into the grid (and your house) at the main service panel. Although you can shut down the inverter by securing the utilities, you cannot shut off the high-voltage DC power coming down from the roof arrays to the inverters. This is the area that poses the greatest fire and electrical hazards. There have been several reported fires of this nature in California alone but, thankfully, no injuries. An injury to a firefighter in Arizona who was severely shocked on a fence at a PV-equipped residential fire was initially attributed to the PV system but later was found to be because of the AC service drop contacting the fence. Regardless, it was a close call.
As the PV systems age, or wiring insulation becomes compromised because of poor installation or hungry rodents, fires may result from arcing. An arc in an outlet in your home will usually trip the circuit breaker before starting a fire. In a PV system, there are no breakers to trip as long as the sun is out. Currently there is little to prevent these arcs, which could lead to a fire. These are called “arc faults” and are part of the problem.
Some systems have incorporated a rooftop disconnect at the end of the array to kill the power from that point to the inverter. This does nothing about securing the high voltage in all the modules in the array (wired together, they can produce up to 600 vdc) and, in fact, can lead to a false sense of security if there are multiple arrays. If not all disconnects are shut off, then all are still live! This is the other area of concern.
There are products that can provide a greater level of safety in both of these areas, and that is the direction the industry must move toward. The “Gold Standard” of PV safety is arc-fault detection, which results in module level power isolation. What does that mean? Well, basically, if there is any arc, then each of the 40-vdc modules is shut down, not just at the end of a 600-volt string.
The United States is the third largest PV market in the world behind Germany and Italy. On August 27-28, 2009, the first workshop addressing these concerns was held in Aachen, Germany. Attending were top executives of the major PV equipment manufacturers, as well as fire service representatives of the three largest solar markets—Germany, Italy, and the United States. The event organizer was Photon International, the leading PV publication worldwide. It is concerned that these hazards, left unaddressed, could eventually lead to the loss of life or property. It recently ran an article on several large fires in Germany and Italy, some of which were caused by PV. The response from the fire service in a few cases was to go defensive because of the presence of the PV systems. This has the attention of not just the media but the insurance industry and the fire service.
(1) A photo from the conference. Photos by author.
Firefighters in Germany, like many in the European fire service (and some in cold U.S. regions), do very little vertical ventilation. This is primarily because of steep roof pitches, staffing levels, tactics, and (in Europe) roofs made of stone or poured concrete. This may explain the delay in their gaining experience in PV systems: They are just beginning to respond to fires in PV-equipped structures. Also, there are simply not that many rooftop PV systems yet.
(2) This building in Germany had a fire caused by PV. This incident was a turning point for the German fire service in regard to how they approach PV systems. The fire burnt one row of modules near the tower.
The workshop began with fire service reports from Andreas Kattge of the Hamburg Fire Department and German Fire Prevention Association, Salvatore Cacciatore of the Pisa Fire Department in Italy, and me from the San Jose (CA) Fire Department. The reports shared case studies of fires in PV-equipped structures, whether caused by the PV system or not. The basic message to the industry was that we are quite concerned about the HVDC. There were also presentations from insurance executives sharing their experience in covering these systems and the structures under them. They expressed concerns over future expenses if the concerns of the fire service led to more defensive operations and subsequent losses.
Lastly, there were presentations by innovative startup equipment manufacturers who have shown that the modules can become “smart” and be shut down individually in the presence of fires or shorts that create arcs. The “Gold Standard” of safety goal is to have every manufacturer incorporate these features into each module. This may take a few years, but if the message from the fire service is loud and clear that we are concerned, then it will be possible to avoid this green solution from being named in a line-of-duty death report.
Matthew Paiss is a captain in the San Jose (CA) Fire Department and the lead PV safety instructor for the “Nobody Gets Left Behind” Training Group, Lee & Associates Rescue, Inc.
Subjects: Firefighter hazards, building construction, systems, green energy, solar power