People often ask ‘are solar panels safe?’ Let’s hear from Captain Richard Birt.

Retired fire Captain, Richard Birt is probably the best qualified person to answer this question.

Captain Richard Birt built a career keeping people safe. Along the way, he became a believer in the benefits of solar energy. He built his own home system and today, the 30-year veteran of the Las Vegas Fire & Rescue Department, spends his spare time traveling the world talking to fire departments and helping them understand solar and he advocates for the best solar standards in building codes. Are solar panels safe? Yes, if you use the right technology.

Original piece

What are DC arc faults?

An arc fault is the flow of electrical energy through an air gap by way of ionised gas molecules. Whilst air is normally regarded as a non conducting medium, a high potential difference, or voltage, between two conductors in close proximity can cause the air molecules to break down into their ionised constituents, called 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, but on a typical solar PV system, it is easily hot enough to melt glass, copper and aluminium, and to start the combustion of surrounding materials.

DC arc faults occur on the DC cabling of PV solar systems.

Schematic-diagram-DC-Solar

Schematic diagram – String inverter solar system with unprotected DC cabling. Cabling, joints and electrical equipment exposed to DC arc faults are shown in red.

What does a DC arc fault look like?

This shows just 4 x 250W solar panels connected in series (1kW).  The open circuit DC voltage for this demonstration was only 149.6V DC.  The average domestic solar system size is 3kW to 5kW with DC voltages between 200 and 600V depending on the panel configuration.

 

Is arcing an issue on AC electrical systems? NO

Arcing is not seen as a common hazard in regular AC electrical systems. This is partly because the electrical industry standards, practices and component designs have evolved over the last 100+ years to a point whereby modern AC installations are very safe.
Arcing is less of an issue in AC Solar PV 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. This does not occur with AC.

Do DC arc faults occur on all solar systems? NO

DC arc faults only occur on string inverter systems that have unprotected DC voltage circuits above approximately 80 volts DC. DC arc faults do not occur on solar systems that use microinverters, such as Enphase. Microinverters and DC optimised systems were developed more than 10 years ago to prevent solar fires caused by DC arc faults.

When we ask, are solar panels safe, the answer is YES they are when you don’t bring high voltage DC into you home or business. Why would you when there is a safe option. It just doesn’t make sense.

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IQ7

Where on the solar system do DC arc faults occur?

Faults can occur anywhere on the DC cabling that runs from the solar panels on the roof to the string inverter which is usually located in the loft or garage. There are approximately 28 connections in the DC circuit of an average domestic solar system. All of these connections are a potential point of failure. 
There are also additional connections inside the solar panels, DC isolators and the string inverter which are potential points of failure.

 

DC string system

What are the types of DC arc faults that occur on solar systems using string inverters?

There are three common types of DC arc faults:

Series – A series arc occurs when a connection is broken, or breaks, while the solar panels are producing current. Any intermittent connection in the DC circuit can produce a DC arc fault. These connections may include soldered joints within the panel, compression type wire connections, connectors used on the wire leads attached to the panels (MC4), connections in DC isolators, connections in the inverter, any DC circuits in the inverter or any of the DC cabling in the string circuit. Basically, any part of the DC system.

 

Series-DC-arc-fault

Parallel – Parallel arcs occur when there is a breakdown in the insulation and current flows between positive and negative. Two wires of opposite polarity in the same DC circuit are usually run in very close proximity to each other. The insulation between the two wires can easily become ineffective due to animals chewing on them, UV breakdown, embrittlement, cracking due to movement, moisture ingress and mechanical damage, again all due to movement of the DC cables. Parallel arc faults can continue along the conductors towards the array burning any and all materials along the way.

 

Parallel-DC-arc-fault

To ground – This fault only requires the failure of one insulation system to ground. This can be the solar panel frame, the solar array mounting kit, the roof or any other grounded surfaces.

 

A fault can often start out as one type of fault (e.g. series fault) but develop into a different type of fault (e.g. parallel fault).

Ground-DC-arc-fault

What can trigger a DC arc fault?

Breakdown of conductor continuity due to:

  • Loose connections due to poor installation.
  • Loose joints due to poor quality connections.
  • Corrosion of joints over time.

Breakdown of insulation system:

  • Insulation degradation over time due to UV exposure.
  • Insulation cracking over time due to changes in temperature (hot – cold).
  • Degradation of insulation due to ageing.
  • Damage to insulation by rodents, insects, birds.
  • Damage to insulation during installation.
  • Damage to insulation by future building works.
  • Water ingress to cables, connectors and conduits.
  • Water ingress to DC isolators from poor installation.
  • Water ingress to DC isolators due to degradation of seals over time.
  • Water ingress to inverter due to poor installation or leakage.
  • Water ingress to solar panels or junction box due to damage or wear and tear.
DC arc fault - fire

Are DC arc faults something to worry about? YES

DC arc faults have been identified as the root cause of over 400 domestic and commercial building fires in Australia alone. UK data is available from the link at the bottom of this page. DC arc faults also cause significant damage to solar equipment.

The highest quality DC cabling components can be selected and installed with the greatest care.  However, cable insulation and conductor degradation will occur over time, which can cause a DC arc fault.

DC arc fault - fire

Can switching off your inverter stop the arc? NO

DC arc faults can occur on the DC cabling under the solar panels, anywhere along the DC cabling from the solar panels on the roof to the string inverter and even inside the string inverter.  Switching off the inverter does not switch off the DC wiring. If its daytime then the panels are producing DC voltage. The DC wiring from the solar panels to the inverter is live whenever the sun is shining and there is no way to isolate this wiring.  Switching off the inverter will not quench a DC arc fault.  Depending on the location of the fault, operating the roof top DC Isolator may isolate the fault and extinguish the arc, however this will require accessing the roof and locating the appropriate DC isolator. Interrupting a DC arc fault with an isolator is not without its own dangers and would only be considered as a last resort.

Solar PV DC arc fault

Can DC arc faults be prevented? YES

There are a variety of solar technologies that have been available for many years which eliminate the risk associated with DC arc faults. These include microinverters manufactured by companies such as Enphase Energy, and also some DC optimised systems such as SolarEdge, which have a safe DC mode.

Elimination of dangerous DC voltages is a safer approach than implementing control measures such as protection for DC arc faults.  There is always a risk that a protection system that is not installed and maintained correctly will not operate when a fault occurs.

Why bring high voltage DC into your home or business when safe AC systems are available?

 

Solar PV DC arc fault

Do string inverters with built-in arc fault protection provide protection for all types of DC arc faults? NO

There are some conventional string inverters available with built-in arc fault detection. However, these DO NOT detect and isolate all types of DC arc faults listed above.

Inverters with built-in arc detection identity a DC arc fault using noise on the DC cabling produced by the arc. Once an arc is detected the DC circuit at the inverter will be isolated. This will extinguish a ‘series’ arc but WILL NOT extinguish a ‘parallel’ or ‘ground’ fault arc.  The DC cabling under the solar panels and down to the inverter will remain live when the DC circuit is isolated at the inverter.  A parallel or earth fault will continue to arc on solar systems using string inverters with DC arc fault protection.

Solar PV DC arc fault

Do rapid shutdown devices on the roof provide protection for all DC arc faults? NO

Rapid shutdown devices are available which can be installed on the roof next to the solar panels.  These are usually located at the end of a string.  They are designed to isolate the DC circuit when a DC arc fault is detected.  Unfortunately, they do not isolate any faults that occur on the many connections under the solar panels or provide protection for ground faults.

Why bring high voltage DC into your home or business when safe AC systems are available?

Solar PV DC arc fault

Do DC arc faults only occur on DC isolators? NO

There is a common belief among some solar installers that DC arc faults only occur at DC isolators.  This is not correct.  DC arc faults occur wherever there are joints . connections in the DC cables, a breakdown of cable continuity or a breakdown of insulation.  This is occurring on the solar panels, the DC connectors, the DC cabling, the joints in the DC isolators or inside the inverter.

Solar PV DC arc fault

Does the likelihood of DC arc faults increase as solar systems with string inverters age?  YES

The likelihood of a DC arc fault increases as solar systems age.  This is because joints in the DC wiring corrode over time increasing the impedance, causing heat and the breakdown of the continuity of the joint resulting in a DC arc fault.  Also, seals on isolators, conduits etc degrade over time allowing water increase which can cause a DC arc fault.

Rodents, insects, birds can damage insulation, future works can damage insulation or insulation can simply degrade over time.

Solar PV DC arc fault

The only way to totally eliminate the risk of DC arc faults is to eliminate the dangerous DC voltage itself from the solar system.

Why bring high voltage DC into your home or business when safe AC systems are available?

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Safe AC solar PV systems.

Solar PV DC arc fault

Are Solar Panels Safe? One of our friends in Australia made this comment about high voltage DC and the benefits of installing Enphase Microinverters

Hi Andrew, I own an installation company in Australia. I made the same decision [to goo 100% Enphase] 4 years ago, for the same reasons. Enphase has the reliability and support compared to string inverters and is so much better. Safety is the other big issue. Fires on DC string systems are pretty common here [in Australia], partly due to our stupid rules mandating rooftop isolators.
Being risk averse, running AC cabling from the roof protected by circuit breakers is a lot more appealing to me than running 600VDC with no protection at all. That might be OK for a homeowner with one system and no foresight but when I have thousands of systems on other people’s roofs I won’t risk it. Enphase ftw! happydaze, John Inglis (https://www.positronicsolar.com.au/)

Original DC arc publication

Of course not. Then why bring High Voltage DC into your home or business? It is simply not logical.