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PV & Safety Under (or Above) the Same Roof

By Danya Golan | May 27, 2010

So by now almost everybody knows why solar energy is good (e.g. these 10 good reasons), but not many consider the potential risks related to traditional PV installations. PV systems have been used for the last century with no fatalities and are considered very reliable and safe. However, it is important to remember that DC voltage is generated by PV panels as long as the sun is up, even when grid power is down. Therefore, electrocution can be a threat to installers, maintenance personnel and firefighters.

 

Not many fires have been reported so far that involved PV systems, and in most of the reported ones, the PV system was not the cause to the fire. However, when approaching a fire scene with PV system, firefighters must remember that the system is energized as long as the panels are illuminated. A typical solar PV installation for residential or commercial roofs is composed of dozens to a few hundreds of PV panels. Each panel typically has a voltage of several tens of volts. In the US a safe outdoor DC voltage limit is 30V, therefore just one or two light-exposed panels connected in a string create a dangerous voltage, if not handled carefully.

 

Last month, a fire broke out in a house in San Diego, California. The rooftop mounted PV system kept feeding the fire with electricity for hours, despite repeated applications of a household fire extinguisher and the efforts of the San Diego Fire-Rescue Department. The fire was terminated only after a few hours, when an electrician cut the wires leading from the panels. This whole unfortunate event could have been avoided if the PV system had measures to automatically disconnect the DC voltages in the case of a fire.

 

An interesting debate is taking place among US firefighters about the risks PV systems pose to them. For some, the bottom line is “let it burn…”.

 

European firefighters reached the same conclusion in many cases. In a fire in Schwerinsdorf, Germany last February, firefighters aware of the energized PV panels on the roof chose to let the roof burn completely rather than extinguish it for fear of electrocution, as can be seen in the following video clip

 

 

So was the case in a fire in Hohenaspe, Germany. When firefighters arrived at the scene, they found a burning storage house (see images below). Extinguishing was not an option due to the roof-mounted PV system, so they focused on controlling the damage to the surroundings. Marcel Hehr, Chief Firefighter at the Hohenaspe station, reported that when water finally arrived at the scene, the roof already collapsed and they were able to extinguish the fire from a safe distance.

   Hohenaspe warehouse burning 1  Hohenaspe warehouse burning 2  Hohenaspe warehouse burning 3

 

As PV systems gain popularity, the number of PV installations is growing exponentially, and so is the need for fire prevention and safety solutions for PV. In addition, PV systems, like all other things on our planet – age. Wiring and contacts deteriorate, increasing the occurrences of fire-causing failures.

 

Is there a better solution than “let it burn” in a PV system fire situation? The answer is yes. There is a way to safely extinguish the fire and save most of the houses even when PV systems are installed on the roof. And it is actually quite simple – shut down all PV modules in case of fire.

 

As stated in Photon’s December Issue, summarizing the “Solar on Fire” workshop held in Germany last August, the gold standard of safety dictates module-level signaling that will impose module bypass in case of a fire: “In the event of a fire, the fire department routinely cuts the electricity connection to the building. This would automatically deactivate the PV inverter – the signal then shuts off and all of the connected modules, and the DC cables connected to them, are rendered volt-free”.

 

None of the solutions suggested to date provided a complete solution for all safety issues and some of them even increased system cost or reduced yields. However, today the gold standard can easily be met – with implementation of the SolarEdge distributed PV architecture.

 

SolarEdge’s power optimizers are connected to each and every module. The power optimizers perform Maximum Power Point Tracking (MPPT) and monitoring per module, but they also play a crucial role in rendering the PV system completely safe.


Power optimizers sense disconnection of grid power or an inverter shutdown and in response immediately shut down power and voltage of each and every module. Once AC power is cut, all DC voltage shuts down automatically and the roof becomes safe. As a further precaution the power optimizers can shut down automatically when a certain temperature is exceeded, eliminating the need for special procedures before tackling a fire.


As a result, PV modules do not pose any risk, even when exposed to light, so firefighters need not fear climbing the roof or spraying water to extinguish the fire. For a demonstration of the power optmizers' safety voltage, watch the second part of the movie below.

 

Another risk factor is the many electric cables on the roof. If a connector or a wire fails, electric current can flow through the air, generating electric arcs, which produce a lot of heat which in turn can cause a fire and pose further danger of electrocution. Large PV installations can have thousands and thousands of contacts, each one a potential arc generator.


As electric arcs influence the electric characteristics of the DC flow, power optimizers can detect arcs* as well, and terminate the arc by shutting down the modules in response - see the first part of the following movie.

 

 

As the number of PV installations increases, so does the occurrence of PV safety events. Insurance companies have already started to respond to this trend by raising policy premiums. The SolarEdge PV  system ensures electrocution prevention and fire safety at all times, so insurance premiums are lower, and with the added energy gained with the distributed architecture, faster return on investment is guaranteed.

 

For further reading, please download our PV safety application note Safety application note

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