Electric arc risks in photovoltaic systems and safety solutions

The number of photovoltaic systems being installed in Brazil is increasing exponentially, as has occurred and still occurs in several other countries.

However, the electrical installations of photovoltaic systems have some particularities that differ from conventional electrical installations, which we will talk about in more detail below.

Among these particularities is the electric arc, which occurs differently in a specific part of the electrical installation of a photovoltaic system.

The three main catastrophic failures in photovoltaic arrays are ground faults, line-to-line faults and arc flash [1]. The heat and energy of an electric arc can cause electric shocks, burns and fires, the latter of which poses an even greater risk to people and property.

In this article we will talk about what an electric arc is, what its physical nature is, when and why it occurs, what types of electric arc are, what is the difference between an alternating current and direct current arc, what are the specific risks of the arc in photovoltaic systems, how to avoid and/or reduce the risks of electrical arcs, history in more mature markets, occurrences in Brazil and regulations that deal with the topic.

Lack or Failure? that is the question

In electrical systems, the words fault and failure cannot be used as synonyms, as they have different meanings. A fault is an impermissible deviation of at least one characteristic property of the system from the standard, acceptable and usual condition.

A failure is a permanent interruption of a system's ability to perform a required function under specified operating conditions [2].

When in an installation or equipment, two or more parts, which are at different potentials, come into contact accidentally, either due to insulation failure between them or with a grounded part, we have a fault.

A fault can be direct, when the parts actually touch, that is, when there is physical contact between them, or indirect when there is no physical contact but an arc between the parts [3].

Therefore, in this context, a failure of isolation caused a lack indirect, creating an electric arc. It is said, in this context, because an electric arc may not be accidental but on purpose.

For example, the electric arc can be intentionally applied to arc furnaces, arc welding, electric arc lamps, among others. In this article we will only deal with the electric arc as a lack.

What is an electric arc?

An electric arc is the circulation of current through the air, caused by any discontinuity in conductors or insulation failures in adjacent current-carrying conductors [4].

A large voltage difference between two points separated only by a small air gap can lead to an electric arc [5]. An arc is a “plasma discharge” [6].

In other words, an electric arc is the flow of electrical energy across an air gap via ionized gas molecules.

Air is normally considered a non-conducting medium, but a large potential difference (voltage) between two very close conductors can cause air molecules to break down into their ionized constituents (called plasma), which can then carry charges. electrical currents from one conductor to another.

This flow of charge (electrons), when sustained, results in a bright arc that generates heat, breaking the insulation of the wire and causing a fire.

What is the difference between a DC arc and an AC arc?

Although electrical arcs can occur in both alternating current and direct current circuits, direct current arcs are less understood and tend to remain longer [5].

Compared to an alternating current system, an arc in a direct current system can lead to a sustained arc because the current through the dc arc is not periodic and is also non-zero crossing [4].

In the wrong situation, dc arcs can be sustained for a significant period of time, and because they are so hot, they can cause almost any material in the vicinity to catch fire [5].

Why does electrical arcing occur in photovoltaic systems?

In photovoltaic systems, arcing typically occurs in string connections – in connectors between modules, in connections in the module junction box and in terminations in stringboxes (combiner boxes) [7].

Arcs can also occur in the photovoltaic module and inverters, as well as in the installation's alternating current circuits.

Modules that use certain types of encapsulants or modules that are constructed so that the cells are very close to the aluminum frame may experience arcing problems.

That is why qualification tests for modules such as damp heat tests and leakage current tests in humid conditions present in IEC 61215 are currently required.

Due to the requirement for the presence of a large voltage difference, individual PV modules and even PV systems operating at low voltage are not susceptible to arcing.

On the other hand, systems that operate at a very high voltage (>1000 V) are much more likely to experience arcing problems [5].

In traditional string systems, as long as the sun is shining, we will have the constant presence of high DC voltages on your roof.

With power being converted from dc to ac at the end of the string and not directly in the module, a voltage of up to 1000V is being produced and conducted through the roof. This can be a serious danger that can cause an electrical arc and consequently a fire [8].

What are the types of electric arc in photovoltaic systems?

There are three main types of electrical arcs in DC circuits of photovoltaic systems:

• Series – A series arc occurs when a connection opens circuits while the modules are producing current. Any faulty connection or broken conductor anywhere in the dc circuit has the potential to produce an arc.
• Parallel – parallel arcs occur when two conductors of opposite polarity in the same dc circuit come into contact. The voltage inside most modules is generally too low for a parallel arc to occur. In string systems, parallel arcs are more likely to occur in conductors, where higher voltages are often routed through the conductors.
• To earth – arcs to earth result from an insulation failure; in one place if the array is grounded or in two places if the array is ungrounded. In an ungrounded array, the first ground fault grounds the array. Typically, ground fault paths are not capable of carrying the high currents found in photovoltaic systems, therefore the paths overheat, leading to an arc.

What are the risks?

Photovoltaic systems, particularly in direct current circuits, pose risks beyond those arising from conventional alternating current power systems, including the ability to produce and sustain electrical arcs with currents that are no greater than normal operating currents.

The type of electrical arc most likely to occur in a photovoltaic system is the series arc. Parallel arcs are more difficult to extinguish, but they are also less likely to occur.

However, if a series arc is not extinguished quickly, it can propagate and involve adjacent conductors producing parallel arcs [9]. Because series arcs maintain the normal current path, they are not detected by short-circuit or ground-fault protection devices [7].

Furthermore, electric arcs can be formed in a photovoltaic array with fault currents that would not trigger the actuation of an overcurrent protection device [9].

Typical safety issues in photovoltaic systems are [10]:

1. The power supply to photovoltaic modules cannot be turned off (as it is not possible to turn off the sun), therefore special precautions must be taken to ensure that live parts are not accessible or cannot be touched during installation, use and maintenance. In traditional string PV systems, a series of photovoltaic modules can produce a voltage of more than 1000 volts in direct current, in which case access must be restricted only to competent, qualified or instructed people;
2. Due to the potential presence of high direct current voltage in string photovoltaic systems, an arc hazard results in a high energy discharge that could lead to a fire;
3. Risk of electric shock due to direct and/or indirect contact with energized parts.
4. Lack of knowledge to work with direct current circuits in photovoltaic systems; It is
5. Risk of falls and injuries due to working at height and manual handling during the installation of photovoltaic systems.

Furthermore, as already mentioned, we have particularities that can generate problems that do not exist in conventional electrical installations.

For example, the source has limited power, which makes it difficult to identify faults in the installation using methods conventionally used in alternating current electrical installations.

Furthermore, direct current circuits remain energized when exposed to solar radiation, even when the photovoltaic system is disconnected from the electrical grid and even if the photovoltaic inverter is turned off;

These characteristics significantly increase the risk of electric shocks, which tend to be much more severe due to the high voltages and fire risks caused by the high temperatures generated by electric arcs.

What are the security solutions?

Solutions to mitigate the risks of electric shock, electric arcs and consequently fires are highly mature, including being standardized:

(i) DC circuit insulation monitoring system [11] and automatic interruption;
(ii) Leakage current monitoring system for photovoltaic installations [11] and automatic interruption;
(iii) Rapid shutdown system for the photovoltaic array [12];
(iv) Protection devices, protective fuses in photovoltaic arrays [9], and;
(v) DC arc detection and interruption system, known as AFCI (from English: Arc-Fault Circuit Interrupter) [13, 14, 15].

Series arcs are detected by devices that measure circuit currents and look for a high-frequency spectrum with a signature that is characteristic of ionization and plasma discharge in the arc.

If the series arc is detected, it can be extinguished by simply turning off the inverter and stopping the current flow [7].

Figure 3 shows the difference between the spectrum of a system with a detected electric arc and a system without the presence of an electric arc.

Although electrical and fire safety requirements are requirements for the photovoltaic system as a whole, the standards consider its inclusion mandatory together with the equipment. Based on this precept, the best location from a technical and economic point of view for items (i), (ii) and (v) previously listed are in the photovoltaic inverter.

Having the strings go straight to the inverter puts dc arc protection in the inverter, which is more convenient than having electronics detect dc arcs in the stringbox. Furthermore, DC arc monitoring and protection are easier. If there is a ground fault or electrical arc, the inverter must isolate both circuits from each other and from ground [16].

For this reason, the most advanced markets require that all inverters comply with the IEC 62109-2 standard (items (i) and (ii)), as well as increasingly demanding solutions for item (v). Local installation standards are also requiring suitability for item (iv). Many national electrical codes now require arc fault detection for photovoltaic systems [1, 17]. Below is a list of some countries and requested requirements:

• USA: NEC 2014, NEC 2017, NEC 2020 (Rapid Shutdown, Arc Flash);
• Canada: NEC 2017 (Rapid Shutdown, Arc Flash);
• Mexico: NOM-001-SEDE-2012 (Rapid Shutdown)
• Germany: VDE-AR-E 2100-712 (Fire safety)
• Italy: N.1324-2012 (Fire safety)
• Türkiye: Residential Requirement
• Shanghai – Technical Standard for rooftop photovoltaic installation DG/TJ08-2004B-2020
• Taiwan: NEC 2020 (Shutdown Process / Rapid Shutdown)
• Philippines: PEC 690.2 (Rapid Shutdown)
• Australia: Nova AS5033 (launch process)

As an example, the 2017 NEC states that “Photovoltaic systems operating at a voltage of 80 Volts dc or greater between two conductors must be protected by an indicated arc fault interrupter or other component with equivalent protection. The system must detect and interrupt electrical arcs in conductors, connectors, modules, or other components in the dc circuit of the photovoltaic system”.

In turn, UL1699B stipulated concrete test conditions and methodologies for detecting and interrupting electrical arcs. It is imposed that AFCI devices in photovoltaic systems need to detect an arc and interrupt it within a maximum of 2.5 seconds.

NBR 16690 itself states that “It is therefore desirable to have a rapid method of detecting and interrupting electric arcs in photovoltaic systems”. However, it does not present any requirement that effectively protects the photovoltaic system, people and property in cases where an electric arc occurs. In this aspect, only two sections of NBR 16690 present prescriptions related to electric arcs, both preventive, as follows:

1. All connections must be checked for minimum torque and polarity during installation to reduce the risk of faults and possible arcing during commissioning, operation and future maintenance;
2. Whenever possible, there must be separation between the positive and negative conductors within the junction boxes, in order to minimize the risk of direct current arcs that may occur between these conductors.

Furthermore, the effective solution to the problem is entirely the responsibility of the designer/installer, as NBR 5410 [18] prescribes that “the electrical installation must be designed and constructed in such a way as to exclude any risk of fire from flammable materials, due to high temperatures or electrical arcs”.

Many newer inverters are equipped with dc arc detection. If there is an arc, the inverter should shut down [6]. Ground fault detection and interrupting (GFDI) and arc flash interrupting circuit (AFCI) are often incorporated into the dc input circuit of directly grid-connected inverters [19].

The AFCI must disarm in any situation. AFCI provides additional protection beyond the protection provided by a GFDI.

While GFDI devices detect a difference in current between the positive and negative conductors of approximately 0.5 A for small systems, a difference in supply and return current is not necessary to activate an AFCI device.

The AFCI device electronically detects the electronic signature of an arc and responds by opening the circuit and providing a visual indication that the device has detected a fault [19].

Therefore, it is possible to purchase inverters that incorporate additional components so that they comply with the NEC code, with earth fault protection, arc flash protection, with disconnection of inputs and outputs, fuses for multiple string inputs and housing weatherproof [19].

Track record in more mature markets

The USA was a pioneer in city electrification. We can cite as an example that New York was the first city in the world to have public lighting, in 1882. Therefore, it is only natural that this country is a pioneer in several issues related to electricity.

That said, with regard to arc flash detection and interruption circuits, patent applications related to arc flash protection date back to the 1930s and today it is considered a mature technology [20].

The application of AFCI is not a restrictive measure, much less specifically aimed at protecting the fire department.

The Electrical Safety Foundation International (ESFI) stated that this is a technology that could save “hundreds of lives, reducing thousands of injuries and nearly US$1 billion in property damage annually” [21]. We are talking about lives and heritage. The need is clear and the technology is proven.

The NEC introduced AFCI requirements in 1999, and since then, the subject has been continually improved in all its revisions.

This improvement is the natural path, which allows standards to keep up with technological and societal evolution. The same process can be observed in standards and regulations in any other country.

There are several studies carried out by extremely reliable bodies addressing the problem of fires in places with photovoltaic systems.

To clarify these points, we present bibliography showing what happened in more mature markets, as follows:

1.United Kingdom

The UK's BRE National Solar Centre, through investigations and evidence, published a case study of fires involving PV systems, presenting a review of historical incidents, relevant literature, standards and training [22].

In summary, Figure 4 shows that, in all fire cases investigated, regardless of severity, the majority of fires were caused by the photovoltaic system.

BRE pointed out in Figure 5 that, among the cases in which photovoltaic system components were recorded as the most likely cause of the fires investigated, DC insulators came first.

However, it is worth remembering that DC insulators alone do not generate an electrical arc, but rather the DC voltage levels to which these components are subjected.

Thus, even though in 18 cases the DC insulators were identified as the probable cause of the fire, the study made a point of recording some evidence found that resulted in the following statement: “interpreting the data, there appear to be three separate problems with the DC insulators ”:

• Poorly designed or constructed products – 1 case
• Bad installation practice – 2 cases
• Incorrect specification of DC insulators – 9 cases

As previously stated, insulators by themselves do not generate an electric arc, and in some cases water ingress inside the insulators was detected. Based on the same premise, it is also incorrect to say that the connectors themselves cause fire, but rather the DC voltage levels that sustain the electric arc.

2.Italy

A study conducted by researchers from Tecsa SRL (a specialized consultancy company), the Polytechnic of Turin and the Italian National Fire Brigade presented an assessment of the fire risk of photovoltaic installations [23].

The abstract of the article is worth highlighting, which states the following:

“Photovoltaic plants have seen a sharp increase in the number and installed power in the last decade throughout the world. Along with this growth, the associated risks have also increased significantly. Among these, the risk of fire caught the attention of both Authorities, plant managers and any other interested parties (such as property owners) due to the high number of fires involving solar plants”.

Figure 6 shows the number of fires related to photovoltaic plants in Italy in the period from 2003 to 2014 (11 years).

“Available data on fires in PV plants include a wide variety of fire episodes, including connection box fires, fires involving only a few PV modules, and large fires (most) occurred in power plants located on the roof of the building, which spread inside through the skylights on the roof” [23].

The study presents some root causes and then states that one of the final effects of these phenomena is associated with a DC arc.

It is interesting to make a comparison with the Brazilian market, which effectively began after the publication of Normative Resolution No. 482 of April 17, 2012.

When making a parallel between the Brazilian market and the Italian market, where in 2012 almost 800 cases were recorded, it is possible to state that if regulatory or normative actions are not adopted to mitigate the risk of fires in buildings with photovoltaic systems, we will be risking repeating the same scenario of an exponential increase in fire cases related to photovoltaic plants.

In the case of Italy, the number of fire cases only began to decrease after the publication and application of two guidelines: Fire Safety of Photovoltaic Systems, Province of Trento, 2011 and Guidelines for PV plant Installation, Dipartimento dei vigili del fuoco, Dipartimento del soccorso pubblico della difesa civile, 2012 Edition.

3. Germany

A comprehensive study on fire hazards in PV systems [24], conducted by researchers from the Fraunhofer Institute for Solar Energy Systems, TÜV Rheinland Energy and Environment, Munich Fire Department and Bern University for Applied Sciences, has been published and presented at EU PVSEC, which is the largest international conference on photovoltaic research, technologies and applications.

The cases studied are part of a database that covers the period from 1995 to 2012 (17 years) and were limited to German territory. Out of a total of 400 incidents, 180 cases (45%) were caused by the photovoltaic system.

In this study, which is much broader and more detailed than that of the United Kingdom and Italy, draws attention the number of cases where the inverter was responsible for starting the fire, as can be seen in Figure 7 below:

We reiterate that the components alone do not generate an electric arc, but rather the DC voltage levels to which these components are subjected. It is worth noting that, starting on page 3 of [24], the study several times points to the electric arc as the fundamental risk, as shown in Figure 8 below:

Finally, the study states that “In critical applications, the use of arc detectors should be considered to reduce the risk of fire” [24].

We understand critical applications to be precisely places where there are no qualified and trained people to deal with the risks of electric arcs, for example, installations in homes.

4. Brazil

The electric arc problem has been taking on major proportions in Brazil. Several cases of fires caused by electric arcs have been recorded. Below are some images of some facilities in Brazil where the electric arc caused a fire:

Some of these cases have videos of the occurrence, which were obtained through discussion groups on the web.

Discussion 

More than 50% of the market share of inverters sold in Brazil comes from brands that also sell in markets that require both AFCI and Rapid Shutdown, therefore, they have mastery of these technologies.

However, some of these brands choose to bring simpler and consequently less safe versions to the Brazilian market, precisely because these devices are not mandatory.

The end consumer is normally unaware of the risks associated with photovoltaic installations. Added to this, many installers in Brazil are unaware of the risks because they have a low level of technical qualifications.

For this reason, it is essential that photovoltaic equipment has internal protections that can detect possible failures in the installations that cause very serious consequences, such as loss of life due to electric shock or fire.

ABNT NBR 5410 and 16690 standards are in the process of being revised. There is also an expectation of changes to NBR 5410 which will initially establish the recommendation for the use of arc detection and interruption devices (AFCI) in low voltage electrical installations. Consequently, AFCI should also be discussed in the review of NBR 16690 in photovoltaic system installations.

Conclusion

The subject of “electrical arc detection and interruption” is extremely important because it is a solution to meet the safety needs of the PV market, having reached sufficient maturity for its applicability.

Electric arcs are a serious problem that occurs in photovoltaic systems. Voltages of 600 volts, 800 volts and up to 1,500 volts in direct current can be found on roofs. When working with these voltage levels, the risk of electrical arcing will always exist.

Therefore, we have seen that the risk of electrical arcs and, consequently, deaths, injuries, loss and damage to property is a serious concern. There are countries that adopt AFCI as mandatory and we have seen researchers from important institutions recommending the application of arc detectors to reduce the risk of fire.

It is possible to buy inverters with the AFCI circuit integrated, such as Huawei inverters that have AFCI integrated into their string inverters. Another option is to use technologies such as APsystems microinverters which, as they work with extra-low voltage, naturally do not require AFCI.

This is another consideration that every good photovoltaic system designer must take into account when selecting the inverter and photovoltaic system technology.

References

[1] M. K. Alam, F. Khan, J. Johnson, and J. Flicker, “A Comprehensive Review of Catastrophic Faults in PV Arrays: Types, Detection, and Mitigation Techniques,” IEEE J. Photovolt., vol. 5, no. 3, pp. 982–997, 2015.

[2] Isermann, Rolf, “Fault-Diagnosis Systems: An Introduction from Fault Detection to Fault Tolerance”, Germany, Springer Berlin Heidelberg, 2006.

[3] Prysmian Cables & Systems, “Prysmian Electrical Installations Manual”, 2010.

[4] Shiva Gorjian, Ashish Shukla, “Photovoltaic Solar Energy Conversion – Technologies, Applications and Environmental Impacts”, Academic Press, 2020.

[5] John H. Wohlgemuth, “Photovoltaic Module Reliability”, Virginia, USA, Wiley, 2020.

[6] Sean White, “Solar Photovoltaic Basics: a Study Guide for the NABCEP Associate Exam,” Second Edition, 2019.

[7] Reinders, A., Verlinden, P., Van Sark, W., & Freundlich, A., “Photovoltaic Solar Energy: from Fundamentals to Applications”, John Wiley & Sons, 2017.

[8] Ashok L. Kumar, S. Albert Alexander, Madhuvanthani Rajendran, “Power Electronic Converters for Solar Photovoltaic Systems”, Academic Press, 2020.

[9] Brazilian Association of Technical Standards, Brazilian Standard ABNT NBR 16690:2019 – Electrical Installations of Photovoltaic Arrays – Project Requirements, 2019.

[10] Regulation and Supervision Bureau for the water, wastewater and electricity sector in the Emirate of Abu Dhabi, “Installation of Solar PV Systems Guidance Document”, www.rsb.gov.ae., Publication No. EP/P04/101, January, 2017 .

[11] International Electrotechnical Commission, IEC 62109-2:2011 – Safety of Power Converters for Use in Photovoltaic Power Systems – Part 2: Particular Requirements for Inverters.

[12] 2014 National Electrical Code, ANSI/NFPA70, Published by the National Fire Protection Association, Quincy, MA, 2014.

[13] 2011 National Electrical Code, ANSI/NFPA70, Published by the National Fire Protection Association, Quincy, MA, 2011.

[14] UL Standard for Photovoltaic (PV) DC Arc-Fault Circuit Protection; UL 1699B, Underwriters Laboratories, Edition 1, 2018.

[15] International Electrotechnical Commission, IEC 63027 ED1 – DC Arc Detection and Interruption in Photovoltaic Power Systems.

[16] Bill Brooks, Sean White, “Photovoltaic Systems and the National Electric Code”, Taylor & Francis, 2018.

[17] Yongheng Yang, Katherine A. Kim, Frede Blaabjerg, Ariya Sangwongwanich, “Advances in Grid-Connected Photovoltaic Power Conversion Systems”, Woodhead Publications, 2019.

[18] NBR 5410

[19] Roger Messenger, Amir Abtahi, “Photovoltaic Systems Engineering”, Fourth Edition, CRC Press, 2017.

[20] White Paper – History of the AFCI – Siemens Download Center.

[21] Electrical Safety Foundation International, News Release, November 7, 2005.

[22] BRE National Solar Centre, UK, “Fire and Solar PV Systems – Investigations and Evidence”, Report #P100874-1004, Issue 2.5, July, 2017.

[23] Fiorentini L., Marmo L., Danzi E., Puccia V., 2016, “Fire risk assessment of photovoltaic plants. a case study moving from two large fires: from accident investigation and forensic engineering to fire risk assessment for reconstruction and permitting purposes”, Chemical Engineering Transactions, 48, 427-432.

[24] H. Laukamp et al. “PV Fire Hazard – Analysis and Assessment of Fire Incidents”, 28th EU PVSEC 2013, Paris.

The opinions and information presented are those of the author and do not necessarily reflect the opinion of Canal Solar.