a
PV systems. While there is no reason to believe that the fire risk associated with PV systems is greater than that associated with other electrical equipment, the study, which involved on-site forensic investigations and a review of historical incidents, literature, standards and training, is helping inform the PV industry supply chain, fire and rescue services, technical standards writers and training companies. This article presents key findings from the study’s analysis of 80 fire incidents.
• Loose screw terminals within junction boxes or isolator switches • PV module junction box defect (e.g. poorly soldered joints) • Damage to a component (e.g. broken busbar within a PV module) While resistive heating (alone) is far less likely than arcing to be the only causative mechanism of fire, the breakdown of electronic components such as capacitors or transformers is possible. This is thought to be a likely cause of fires in inverters, although inverters were not a common origin of fires within the study.
b
ISSUES OF CONCERN PV systems are generally very safe; however, when incidents occur they can have far-reaching consequences. It is therefore important to understand the mechanisms by which a fire may ignite in a PV system. Electrical arcing is a significant factor; on a typical PV system, an electrical arc is hot enough to melt glass, copper and aluminium, and initiate the combustion of surrounding materials. Sources of heat, such as resistive heating in a corroded connection, could also be an ignition point for a fire, but the temperatures involved tend to be much lower than for an electrical arc-generated fire. Such heating can, however, still be a precursor to establishing an electrical arc. Arcing can occur where conducting parts become physically separated by mechanical movement or misalignment. Certain components, if incorrectly specified, poorly installed or faulty, can give rise to electrical arcs in PV systems. Arcing is not seen as a common hazard in AC electrical systems, partly because standards have evolved to a point where most installations are very safe. PV standards, practices and components are, however, relatively young and are still evolving. For an arc to be self-sustaining, the conditions for starting the arc have to be present continuously. AC arcs therefore tend to selfextinguish as the voltage alternates, passing through 0 volts 100 times per second, whereas the continuous voltage presented by a DC arc tends to support its continuation. Any evidence of arcing should act as an indicator of a problem within the system and should be urgently investigated. The study found that common causes of DC arcing included: • Moisture ingress degrading connections in connectors, junction boxes and switches • Incorrectly crimped connector contacts • The mating of incompatible DC plugs and socket connectors • Plug and socket connectors not being fully engaged
Remains of a DC connector ablated by arcing (a) and, by contrast, a connector with contacts still intact and engaged, merely damaged by the surrounding fire (b). In both cases the insulating body of the connector has burnt off
INCIDENT ANALYSIS The analysis of 80 fire incidents implicated the following PV components: DC isolators ( 26-28 incidents), DC connectors ( 5-12 incidents), inverters ( 6-9 incidents), DC cables ( 1-5 incidents), PV modules ( 2-5 incidents), and DC combiner box ( 1 incident). Approximately 36 per cent of incidents were attributed to poor installation practices, 10 per
‘Arcing can occur where conducting parts become physically separated by mechanical movement or misalignment’ The research on which this article is based was commissioned by the Department for Business Energy and Industrial Strategy (BEIS) and carried out by the BRE National Solar Centre. Further details are available in the interim published reports available at: bit.ly/BEISfire-solar-panels. Any views expressed are not necessarily those of BEIS
Colin Sinclair is principal consultant at BRE Scotland
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cent were attributed to system design errors and 5 per cent to faulty products. The causes of the remainder were unknown. The study found DC isolators presented the greatest fire risk, with approximately 30 per cent of the incidents being caused by issues with this component. The study also found separate evidence of fires originating within DC isolators with poor contact design (e.g. in components originally being designed for AC operation that have been re-designated as DC-rated by the manufacturer) and with incorrect internal wiring. Incorrectly specified DC isolators were also observed. The assessment and mitigation of potential fire risk is a vital element of PV design, installation and operation and maintenance processes. Ensuring the correct specification of components and applying appropriate installation methods, taking account of the environment in which the components are being installed, is also critical to a safe functioning system. Furthermore, PV is not a ‘fit and forget’ technology, and the study has also highlighted a need to educate installers and system owners on requirements for maintaining the safety of PV systems.
The number of PV installations 11 across the UK S C OT L AND S P RI NMG 2 018
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