EV Solar & Battery Storage Ltd.

Arc Fault Protection Systems

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In this series

Solar fire protection

Protecting solar PV from Lightning

Arc Fault Protection

Firefighter’s Switch Installation

Fire Safety & Protection

Causes of Fire in Solar PV Systems

DC arc faults are the most common cause of fires associated with solar PV. They mainly occur with solar arrays connected to string inverters and very rarely with systems that use voltage optimisers or micro inverters. Some modern string inverters have built-in arc fault detection which can greatly reduce the dangers associated with DC arcs, but older inverters, at the cheaper end of the market, do not.

To reduce this risk and protect your system from arc faults we install arc fault detection devices (AFDD). These devices are capable of detecting arc faults and disconnecting the affected part of the DC circuit, which extinguishes the arc. The most common type of arc faults are the series type which these devices are good at detecting, but in some instances the arc fault can NOT be stopped.

EV Solar & Battery Storage Ltd. provide solar fire protection and arc fault detection services in south-east England. Our service area is Buckinghamshire, Cambridgeshire, Hertfordshire, Northamptonshire, Oxfordshire, and Bedfordshire. We work for both commercial and residential clients: from small domestic solar systems to industrial-size solar farms.

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Selected Products

This is an image of arc fault protection device in an enclosure. These are made bespoke depending on the size of the solar PV it’s protecting.

AFCI Combiner Box BHSZ

This is an image of arc fault protection device in an enclosure. These are made bespoke depending on the size of the solar PV it’s protecting.

AFCI Combiner Box

The Arc Fault Circuit Interruption Combiner Boxes monitor each string currents, voltages and the temperature in enclosure, the integrated automatic contactors triggered when there is arc fault detected, provides solar PV system safety.

Solar PV Fire Safety

DC Arc Faults: the most common cause of fire associated with solar PV

What does an arc fault look like? When an arc fault occurs in a solar PV system, electric current jumps a gap which creates an intense bust of heat and light. The heat created can be between 2,900 and 9,000 degrees Celsius.

A burnt out solar panel power connection caused by bird droppings

Do DC arc faults occur on all solar systems?

DC arc faults are a lot more dangerous in string inverter systems that have unprotected DC circuits and operate at high voltages. These can be in the hundreds if not over a thousand volts. DC arc faults are less of an issue on solar systems that use microinverters, for example Enphase, and systems that use DC voltage optimisers, for example SolarEdge.

Microinverters and DC voltage optimised systems were developed more than 10 years ago to prevent solar fires caused by DC arc faults. The two market leaders are SolarEdge and Enphase, both companies use a different approach to tackle this issue and differ by the design of their systems.

In a SolarEdge system each solar panel is connected to a voltage optimiser. In the event of a fault the system shuts down and reduces the voltage of each panel to 1v. This is known as SafeDC.

In an Enphase system each panel is connected to a micro-inverter. The only DC voltage present is between the solar panel and the microinverter where it is then converted to AC. If the microinverter detects a fault on the DC side it shuts down instantly. The DC voltage in an Enphase system doesn’t go above 80 volts.

Both these systems are very safe and worth the higher investment compared to string inverter based solar PV.

Is arcing an issue on AC electrical systems?

This isn’t as big an issue with AC systems. Electrical current can flow in two ways: alternating current (AC) and direct current (DC). The main difference between AC and DC is the way electrons flow. In an AC system electrons alternate between positive and negative, flowing back and forth in a circuit.

The associated voltage rises to, for example, 230 volts on the positive half of a cycle then rapidly drops, passing zero, before increasing again to 230 volts on the negative half of the cycle. This repeats very quickly which is known as the supply frequency. In the UK our electrical supply frequency is 50Hz; this means the AC voltage alternates between positive and negative 50 times per second.

Arcing is less of an issue in AC systems because they tend to self-extinguish as the voltage alternates passing through 0 volts. In a DC circuit current only flows in one direction and only on the positive half of the cycle, never passing zero. For an arc to be self-sustaining, the conditions for starting the arc must be present continuously. DC remains at a continuous voltage so once an arc has been established it will continue as long as the DC voltage is present and current is flowing.

Arc fault detection devices (AFDD’s) interrupt the flow of current which in turn extinguishes the arc.

Solar PV Fire Safety

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 arcs, parallel arcs, and a ‘to ground’ arc faults.

Series Arc Faults

A series arc occurs when a connection is broken while the PV is producing current. Any intermittent connection in the DC circuit has the potential for producing a DC arc fault. These connections may include soldered joints within the module, compression type wire connections, connectors used on the wire leads attached to PV modules, connections in DC isolators, connections in the inverter, any DC circuitry in the inverter or any of the DC cabling in the string circuit.

Parallel Arc Faults

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

To Ground Faults

This fault only requires the failure of one cable to ground. This can be the solar module frame, the solar array racking, the roof, or any other grounded surfaces. Not so common in the UK due to the use of un-grounded arrays.

What can trigger a DC arc fault?

A Breakdown of conductor continuity due to:

Loose joints 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

Degradation of insulation due to aging

Damage to insulation by rodents, insects, birds

Damage to insulation during installation

Damage to insulation by future building works

Water ingress to cables, 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

Water ingress to solar module or junction box

N.B. Often a fault can start out as one type of fault (e.g. a series fault) and then develop into a different type of fault (e.g. parallel fault).

Customer FAQ

The most effective approach to prevent DC arc faults is to eliminate the presence of dangerous DC voltages using microinverters or the use of some DC optimised systems with safe DC.

Are DC arc faults something to worry about?

DC arc faults have been identified as the main cause of fires associated to solar PV. 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.

If you see or hear a DC arc fault, can switching off your inverter stop the arc?

DC arc faults can occur on the DC cabling under the solar array, anywhere along the DC cabling from the solar array on the roof to the string inverter, and even inside the string inverter. Switching off the inverter does not switch off the DC wiring. Installing AFDD’s will automatically disconnect the faulty circuit once an arc has been detected. Installing a DC rapid shutdown device will disconnect all DC cabling between the solar array and the inverter if the inverter is shutdown.

Can DC arc faults be prevented?

There are a variety of solar technologies that have been available for more than a decade which eliminate the risk associated with DC arc faults. These include microinverters manufactured by companies such as Enphase 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. Solar technologies are readily available that eliminate the risk of DC arc faults. These technologies have been readily available for quite a few years so any solar fire that occurs on a solar system was preventable.

Do string inverters with built-in arc fault protection provide protection for all types of DC arc faults?

There are some string inverters available now 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 identify 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 ‘to ground’ fault arc. The DC cabling under the solar array 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.

Do rapid shutdown devices on the roof provide protection for all DC arc faults?

Rapid shutdown devices are available which can be installed on the roof next to the solar array. They do isolate all cabling from the array but do not detect arc faults.

Do DC arc faults only occur on DC isolators?

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 in the DC cables, a breakdown of cable continuity or a breakdown of insulation. This is occurring on the solar modules, the DC connectors, the DC cabling, the joints in the DC isolators or inside the inverter.

Will removing DC isolators prevent DC arc faults?

Not installing DC isolators will certainly eliminate the possibility of them failing in future but as mentioned DC arc faults occur wherever there are joints in the DC cables, a breakdown of cable continuity or a breakdown of cable insulation. If a DC isolator was replaced with a junction box or DC combiner box, faults will continue to occur at this point as well as other points in the DC circuit. The most effective approach to prevent DC arc faults is to eliminate the presence of dangerous DC voltages using microinverters or the use of some DC optimised systems with safe DC.

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

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.

Is arc fault protection compulsory in the UK?

The potential dangers of arc faults have been officially recognised by countries around the world including Germany, the USA and Australia, but not the UK. The UK has now recognised the problem and is undertaking a detailed investigation into solar fires caused by DC arc faults. This is in partly due to a recent large fire in Peterborough.

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