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PV SYSTEM SAFETY IS A BURNING ISSUE

Phil Old, PV applications engineer at Seaward Solar, looks at the fire safety implications of rooftop solar PV systems.

Given the growth in solar PV installations in the last couple of years, news of what is believed to be the UK’s first PV system fire should not, perhaps, have been unexpected.

It was recently reported that Kent firefighters were called out to deal with a fire in a domestic property. The fire, which is thought to have been triggered by a faulty DC switch, was said to have “wreaked havoc” at a family home in Sittingbourne, Kent last month.

Thankfully there were no reported injuries to those in the property, but the family will not be allowed back into their home for at least two months.

Sittingbourne Fire and Rescue Services Watch Manager, Mat Barney, said: “All the time the sun is shining on those panels, they’re live and we’ve got an issue isolating that in the loft space, so they pose a significant problem for us fighting fires on roofs and inside roof spaces.”

The fire crew that dealt with emergency is now working with other teams across the UK to educate them on how to deal with any future solar fires.

The recent situation in Sittingbourne is not uncommon. In recent years there have been a number of international reports of fires and of unsafe installations in domestic solar PV installations that could have posed a fire risk.

Last year, in Australia, the NSW Government issued details of a survey of solar PV installations in western Sydney that found that 18.5% of the installations had major defects.  Of the 658 systems inspected, 122 were found to have significant safety issues and a further 418 (63.5%) were found to have minor defects.

These findings mirror that of a similar survey a year earlier in France when safety inspectors from the electrical safety certification agency Conseul found that 51% of all PV installations in the country posed a potential safety risk and did not conform to regulations.

There have also been reports in the USA of solar PV solar fires.  After a well-documented “thermal event” that occurred in Bakersfield, California, in April 2009, the cause was put down to an undetected fault-to-ground in a grounded current-carrying source circuit conductor at the site. A subsequent analysis of utility-owned and operated rooftop PV systems in North Carolina revealed the presence of undetected ground faults in approximately 10% of the systems surveyed.

Also in the USA, a report, ‘Fire Fighter Safety and Emergency Response for Solar Power System’s published in 2010, prepared by the Fire Protection Research Foundation, detailed several fires caused by PV systems. The believed causes included electrical malfunction, leaves and debris under the solar panels, electrical arcing and electrical faults in inverters.

Clearly all these cases highlight the fire risks that can be associated with PV systems and support the need for thorough commissioning and regular periodic electrical testing.

 
PV fundamentals
PV systems are unusual in that the energy source cannot be switched off. If there is daylight falling on a PV panel it will produce electricity and it is possible for a relatively small array of only a few panels to deliver a lethal shock.

Another important point is that PV panels generate DC voltage, which is not always commonly used by electricians in their normal work. In addition‚ because of the current limiting properties of PV cells‚ they are incapable of producing sufficient fault currents to operate over-current protection devices such as fuses. Once established a fault may remain undetected, not only posing a hazard for an extended period, but also wasting valuable energy generated by the PV system.

In many cases simple electrical faults or wiring failures can therefore cause a serious inefficiency in the ability of the system to produce power. This is particularly important for installers working on ‘roof rental’ schemes where installation has been provided free of charge in return for receipt of the FiT payments.

In this way undetected faults may also develop into a fire hazard over time. Without fuse protection against such faults, elimination of a fire risk can only be achieved by both good system design and careful installation alongside appropriate electrical inspection and testing.

In the main, proper electrical commissioning procedures are among the best defences against latent fire or electrocution hazards – although once installed, ongoing and effective electrical testing is also vital both to prove the continuing safe installation of a PV system but also to verify ongoing functional performance over extended periods.

IEC 62446
‘IEC 62446: 2009 Grid connected PV systems – minimum requirements for system documentation, commissioning tests, and inspection’, specifies the minimum requirements for PV system documentation‚ commissioning tests and inspections.

After the installation of a solar electrical system̦, subsequent building or electrical works in the vicinity of the PV array are likely and the ownership of a building with a system may also change. As a result‚ the standard recognises that only by providing adequate documentation at the outset can the long term performance and safety of the PV system be ensured.

The standard therefore sets out the information and documentation that should be provided to the customer following the installation of a solar panel system and also the initial (and periodic) electrical inspection and testing required.

In short the standard sets out measures to ensure that:
 
-    The PV panels and electrical supply connections have been wired up correctly
-    That the electrical insulation is good
-    The protective earth connection is as it should be
-    There has been no damage to cables during installation

This international standard was published in March 2009. When voting was taken on this standard, all member countries voted in favour, including the UK, USA, Russia, China and many other major contributors. The standard has subsequently been adopted as a European EN in many of the European member states and is generally regarded as a significant contribution to improving the quality and safety of PV systems.

Here in the UK the British Microgeneration Certification Scheme has adopted the principles of IEC 62446 as the basis for its testing and documentation regime. As a result, the fundamentals of the standard are effectively enforced because no feed-in-tariff will be paid to a consumer unless the installation has been installed by an MCS accredited installer.

It is interesting to note that the emphasis is on documentation, and this is in effect the evidence used to demonstrate that appropriate precautions and tests were undertaken prior to the handing over of a PV system to the property owner.  Such information not only provides evidence to the consumer that work has been performed correctly, but it also acts as a check list to an installer and ensures that best practice is followed with the work that is being undertaken.

Compliance testing
There are many instruments available that are sold under the title of ‘solar testers’ so it is vital to ensure that the instruments selected are capable of performing all of the tests required by the MCS.

The absolute minimum testing that needs to be undertaken involves continuity measurements (where applicable), open circuit voltage, short circuit current, insulation and irradiance.  

To meet the electrical test needs some contractors have used multiple instruments that typically include an earth continuity and insulation resistance tester‚ a multimeter, DC clampmeter along with various associated connectors and leads.

However, the danger with such ‘homemade kits’ is that not all of the tests required by IEC 62446 will be covered and, with different PV system electrical tests potentially requiring the use of different testers, using such an array of instruments can be cumbersome and time consuming.

When it comes to solar PV electrical test instrumentation, the choice for the installer is therefore between using general purpose individual items of equipment against all in one combination PV testers and dedicated MCS kits that enable measurements to be taken in a fast, safe and efficient fashion.  

In this respect, given the recent reductions to solar feed in tariffs, the ability of multi-function testers to help installers to work faster and more efficiently without reducing the integrity of testing is set to become even more important in terms of remaining competitive. In addition, all in one testers only require a single calibration service, which also reduces the ongoing cost of PV system testing when compared to using multiple instruments.

In terms of working more efficiently dedicated solar PV testers can also record and provide results in a format that is compatible with data recording programs that assist greatly in the creation of comprehensive system information folders for use in customer test certificates and system commissioning packs.

A better understanding and acceptance of IEC 62446 and the test instrumentation designed to meet this standard would therefore not only help safeguard the future integrity of solar PV installations – but would also help allay the concerns that are inevitably raised over the proficiency of the PV installers involved in unsatisfactory systems.

Seaward Solar

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