What causes solar system fires
Coromandel Valley Country Fire Service responding to a house fire caused by a DC arc fault on a solar system. January 2019
Solar system faults and fires are often attributed to “poor workmanship” or “faulty components”, however all faults and fires can be traced back to one root cause – DC arc faults.
So what are DC arc faults and how can they be prevented?
The technical definition of an arc fault is the flow of electrical energy through an air gap by way of ionized gas molecules. Whilst air is normally regarded as a non-conducting medium, a high potential difference (voltage) between two conductors in close proximity can cause the air molecules to break down into their ionized constituents (called a ‘plasma’), which can then carry a charge from one electrode to the other.
The temperature of an electrical arc depends on a number of factors such as the level of current flow, but on a typical PV system it is easily hot enough to melt glass, copper and aluminium, and to initiate the combustion of surrounding materials.
This video shows a DC arc fault from four 250W solar modules (total 1kW) connected in series.
The open circuit DC voltage for this demonstration was only 149.6V DC.
The average domestic solar system size in Australia is 3kW to 5kW with DC voltages between 300 and 600V depending on configuration.
Faults and fires caused by DC arc faults can occur at any point within the high voltage DC wiring on conventional DC Solar Systems. This wiring runs from the solar modules on the roof down to the string inverter which is usually installed adjacent to the main switchboard.
There are approximately 50 joints in the DC circuit of a typical domestic DC solar system (5kW) which are connected on site by the solar installer. All of these connections undertaken by the solar installer on site are a potential future point of failure. There are also additional connections inside the solar modules, DC isolators and the string inverter which are potential points of failure.
Arcing is less of an issue in AC systems because AC arcs tend to self-extinguish as the voltage alternates, passing through 0 volts one hundred times per second for standard grid supplies. For an arc to be self-sustaining, the conditions for starting the arc must be present continuously. DC remains at a continuous voltage and once an arc has been established it will continue as long as the DC voltage is present.
There are 2 main causes of DC arc faults:
1. The insulation on one or more of the DC cables is compromised,
2. A connection anywhere in the DC circuit is compromised
Contributing factors that lead to DC arc faults:
- Animals (Insects, rats, possums, birds) damaging cables
- Extreme weather events (cyclone, flood, high winds, hail)
- Accidental damage (crawling through a roof space, drilling through a wall to hang a picture, contact by a vehicle, water spray when cleaning)
- Intentional damage (vandalism – rocks on panels, loose cabling pulled out)
- Water ingress due to ageing of plastic fittings, conduits, rubber seals
- Failure of plastic fitting and enclosures due to UV exposure
- Moisture build up within sealed components due to thermal cycling
- Poor installation workmanship
- Inappropriate component selection
- Poor quality equipment.
Given the wide range of factors that can lead to DC arc faults, and that solar systems by their nature are exposed to the weather and operate under harsh environmental conditions, it is simply not possible to eliminate the potential of a DC arc fault in conventional DC solar systems.