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Solar Photovoltaic Systems

INTRODUCTION


The installation of solar photovoltaic (PV) systems in the UK is growing at a fast rate, not just in the domestic field, but also in the commercial, industrial  and public property sectors, ranging, for example, from small installations in primary schools and office premises, to large scale projects such as those at Kings Cross Station, London and the Olympic Park.

The general consensus of engineering professionals is that PV systems are different, but no more hazardous, than traditional electrical installations.


PV technology and the various standards that apply to the design and installation of such products and systems have evolved significantly over the past decade; this Technical Bulletin aims to summarise some of the key risk control considerations which apply.


Detailed guidance on this subject is contained in the latest version (2023) of RISCAuthority publication RC62: Recommendations for fire safety with PV panel installations, which is posted in ATLAS.  Standards in relation to PV systems continue to evolve of which the revised and extended RC62 is viewed as a major development. 


(For the purposes of this Technical Bulletin, the terms “PV module” and “PV panel” are interchangeable and refer to an assembly of interconnected PV cells. A “PV string” applies to a number of PV modules connected in series to generate the required output voltage, and a “PV array” refers to a mechanically and electrically integrated assembly of PV modules and other necessary components, forming a DC power supply).


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Examples of Commercial and Industrial  PV Installations


ELECTRICAL HAZARDS


Solar power generation systems principally comprise of photovoltaic cells (normally between 100mm and 150mm square) formed into modules, power inverters and connecting cables. The photovoltaic cells utilise the power of sunlight to convert photons to clean DC (Direct Current) electricity. The electricity generated by the solar cells is then fed into a power inverter (PV inverter) that converts and regulates the DC source into usable AC power. The AC power can then be used locally or fed directly into the power grid. A DC isolator is installed above the inverter, although in some cases this will be incorporated into the inverter itself. Below the inverter an AC isolator is installed. Where required, systems can be equipped with battery storage, either as part of a true stand-alone (off-grid) system or part of a hybrid (e.g. grid-linked/ batteries) system. 


As indicated, PV panels generate DC, which is not always commonly encountered by electricians in the course of their work and cases have been reported, for instance, where lack of competence has resulted in incorrectly specified DC isolators being installed. In addition, as current limiting devices, a PV module single string circuit typical of many domestic installations cannot rely on fuse protection for automatic disconnection of supply under fault conditions, as the short-circuit current is little more than the operating current. Once established a fault may remain undetected for an extended period posing a potential fire hazard; fires in PV systems have occurred for this very reason.


In systems with multiple strings such as would be commonly encountered in larger commercial premises, some fault scenarios can result in the current from several adjacent strings flowing through a single string and the prospective fault current may be such that overcurrent protective devices are required. Fuses in compliance with the International Electrotechnical Commission (IEC) Standard 60269: Low voltage fusesPart 6: Supplementary requirements for fuse-links for the protection of solar photovoltaic energy systems, have been specifically designed and tested to safely protect PV systems with high DC voltages and low fault currents.


Irrespective of whether fuse protection has been incorporated, ensuring careful design and installation of the PV system, together with subsequent periodical inspection and testing is paramount.


As well as PV panels, solar tiles and slates are manufactured, designed to be used in place of ordinary tiles. A solar tiles system will typically cost around twice as much as an equivalent panel system, although some savings will accrue from the money that would have been spent on roof tiles or slates. Solar tile systems are generally less cost effective than panel systems and will normally only be considered in cases where panels are deemed inappropriate for aesthetic or planning reasons.

                                                          

SYSTEM DESIGN AND INSTALLATION


For UK projects , PV systems should be designed and installed in accordance with the following:



(The objective of the MCS document is to provide PV system designers and installers with information to ensure that a grid-connected PV system meets current UK standards and best practice recommendations. Whilst primarily aimed at typical grid connected systems of up to 50kWp (total combined DC output measured in kilowatts peak), most of what is contained in the document will also be applicable for larger systems.)

 

PRODCUTS AND INSTALLATION CONTRACTORS


International Electrotechnical Commission (IEC) standards are in place to provide safety and performance tests for PV modules including:


  • BS EN IEC 61215 series: Terrestrial photovoltaic (PV) modules. Design qualification  and type approval.

  • BS EN IEC 61730 series: Photovoltaic (PV) module safety qualification.


A certification scheme for PV modules (MCS005 or MCS017) and pitched roof installation kits (MCS012)  is operated by MCS, the use of certified products being one of the many key requirements of the MIS3002 standard.


Under RC62 it is recommended that the PV system provider or installation contractor should, as a minimum:


  • Hold MCS Certification in PV systems and BS EN ISO 9001:2015

  • Be an SSIP (Safety Schemes in Procurement) registered member of an organisation, e.g. Acclaim or CHAS

  • Hold appropriate certification accreditation and competence for DC, low voltage AC, and (if the project will be HV connected) high voltage systems

  • Have membership of environmental schemes (e.g. ISO 14001:2015) and safety schemes (e.g. ISO 45001:2018) – desirable

  • Hold appropriate insurance including an appropriate level of professional indemnity to cover the scope of the project requirements.

  

FIRE RISK ASSESSMENT


Prior to and following the installation of the PV system, the existing fire risk assessment should be reviewed to consider fire and electrical hazards associated with the system and measures that may need to be taken to eliminate or mitigate identified risks.


As regards fire causation, a technical report published by the IEC International Electrotechnical Commission (IEC) in 2021 on fires involving PV systems, provides a clear indication that of the sample studied, 72% were caused by installation or product defects.

Careful consideration should be given when it is proposed to site PV panels on combustible roofs or roof decking incorporating a combustible overlay.

    

INVERTERS


Inverters generate heat and should be provided with sufficient ventilation. They should be mounted directly onto brick, concrete block or fire resisting boarding and clearance distances as specified by the manufacturer observed. Inverter locations such as in a boiler room, other plant rooms and roof spaces prone to high temperature should be carefully considered to avoid overheating. Housekeeping on roof spaces where inverters are installed should be monitored.


Inverters in large scale commercial systems will normally require fire compartmentation in the same manner as major switchgear and other plant items.  In these circumstances and where inverters are installed in lofts and other areas that are normally unoccupied, smoke detection should be considered.


Inverters shall comply with the requirements of BS EN 62109: Safety of power converters for use in photovoltaic power systems.

 

SURGE/TRANSIENT PROTECTION


Most grid connect inverters have some form of in-built surge suppression; however, discrete devices may also be specified where governed by the risk (Technical Bulletin 7 refers).


Where deemed necessary, surge protection devices (SPDs) should be installed on both the AC and DC sides of the inverter for which guidance is given in Application Note AN014 – Protection of Photovoltaic (PV) Systems published by ABB Furse - http://tnblnx3.tnb.com/emAlbum/albums//EU/Furse/ESP_PV_Datasheet.pdf

 

HANDOVER & DOCUMENTATION


The system user should be provided with the information as described in BS EN 62446-1: Grid connected photovoltaic systems — Minimum requirements for system documentation, commissioning tests and inspection. This will include:


  • Basic system information (parts used, rated power, installation dates, etc)

  • System designer information

  • System installer information

  • Wiring diagram

  • Module datasheets

  • Inverter datasheets

  • Operation and maintenance information

  • Warranty documentation for PV modules and inverters 

  • Documentation on any applicable workmanship or weather-tightness warranties


Test results and commissioning data is required to be provided.

 

SYSTEM MAINTENANCE


Fundamental to system safety and ongoing functional performance, PV installations should be inspected and tested at periodic intervals in accordance with BS EN IEC 62446-2: Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 2: Grid connected systems – Maintenance of PV systems. Whilst periodic inspection and testing is advocated in this standard, no guidance is given as to frequency.


Appended in MIS 3002 is an advisory maintenance schedule covering the following tasks:


  • Customer checks (visual only)

  • An intermediate maintenance visit by an MCS Contractor

  • A full maintenance visit with more involved tests requiring specialist equipment, again by an MCS Contractor.

  • MIS 3002 states that the actual checks required, and how frequent, will depend upon system size, the use/occupancy of the building, and ease of roof access along with specific requirements  of the customer and any other stakeholders, such as insurers. As general guide,  the following is advised for most commercial and industrial buildings:

  • Customer checks (visual only) - monthly

  • An intermediate maintenance visit by an MCS Contractor - annually

  • A full maintenance visit with more involved tests requiring specialist equipment, again by an MCS Contractor – every two years.


In respect or installations on buildings, the IET Code of Practice for Grid-connected Solar Photovoltaic Systems recommends that as a general rule the frequency for full periodic full inspection should be no longer that that required  by the AC system to which the PV system is connected.


Serious consideration (as recommended in MIS 3002) should be given to carrying out a thermographic examination of the panels in order to identify hot spots in the panels at the time of each full maintenance visit. Thermographic examination should also be considered of the inverters and other relevant parts of the installation.


Gaps between panels and external roof coverings should be routinely inspected for accumulated leaves and any such debris removed, and panel fixings checked for integrity. The cavity beneath the panels should not be enclosed as this will have a detrimental effect both on the condition of the roof and the operation of the panels.


It is essential that PV panels undergo routine cleaning, the frequency of which should be based on a risk assessment. Also, it is important for efficiency and safety to ensure that panels are only cleaned by personnel who have knowledge of the inherent risks associated with such activity and a robust approach to the mitigation of such risks (which a window cleaner is unlikely to have).


Detailed guidelines concerning safe and effective operation and maintenance (O&M) of roof-mounted solar power systems has been published by Solar Energy UK (SEUK) and is available here. 

 

FIREFIGHTER SAFETY


Fire and Rescue services across the UK and in other countries have expressed serious concerns regarding the potential electrical dangers posed to firefighters when tackling incidents in premises that have PV installations, whether caused by the PV system or by other means. The inability to “power-down” PV panels exposed to sunlight presents an obvious hazard, which also extends to the hours of darkness for systems equipped with battery storage. These hazards are particularly acute in respect of the DC supply from an array of modules running down to the inverter.


As a result, the IET Code of Practice requires that “A firefighter’s switch shall be provided to disconnect inverters from the PV array systems installed on buildings, where the DC system classified as high voltage”; this is endorsed in RC 62. (For the system to be classified as “high voltage”, the IET code states that the DC voltage must exceed 1,500 V DC between conductors, or 900 V DC between one or more conductors and Earth). These  switches should be placed in safe locations remote from the PV panel arrays and is additional to operational AC and DC isolation switches.


Fundamental to firefighter safety is the need to ensure that suitable electrical warning notices are provided which is covered in detail in the MCS Guide. This includes the following, positioned next to the main electrical intake: 


STATIC AND WIND LOADS


A key consideration for the safety of the PV installation is the need for the contractor to ensure that the roof structure is capable of withstanding the static and wind loads that will be imposed by the PV modules and their mounting arrangements. In the event of any doubt, a structural engineer must be consulted. If the static loading to the roof is increased by 15% or more, this is deemed a material alteration and formal Building Regulation approval is required.

 

THEFT


Theft of equipment from rooftop systems, though achievable, is made difficult due to the height of the equipment. However, there is often other equipment located at ground level, such as inverters and DC cabling that can be exposed to theft which will need to be considered as part of the overall protection of the premises proportionate to the risk.

(In cases where the theft of roof mounted panels is a genuine concern, additional physical security can be provided by the use of specially designed anti-theft screws or security bolts, combined, for example, with security marking strategies.) 


Conversely, ground mount solar systems which are often located remotely on predominantly unmanned sites pose an entirely different exposure for which reference should be made to RISCAuthority publication S33: Solar farm security, for guidance on theft protection.    

As well as the risk of theft, PV panels can also be exposed to malicious damage, the risk of which should be assessed.

 

SURVEY EXPECTATIONS


In cases where PV systems are encountered in the field, reference to their presence should be made in the survey report, together with information as to the design/installation criteria where available.  Investigations should include the following:


  • System compliance with guidance in force at the time of installation.

  • Arrangements for periodical inspection and testing.

  • Where access is obtained, inverter correctly installed and maintained clear.

  • Presence of appropriate electrical safety signage and, in the case of a high voltage system, “a firefighter’s switch”.


Standard risk improvement wordings relating to PV systems are available.


Prior to conducting a survey of a solar farm, the Consultant should refer to the Head of Risk Engineering and Surveys to discuss theft protection strategy.

 

Important Note: Research has identified serious fire risk concerns emanating in the Netherlands relating to approximately 650,000 Scheuten “Multisol” PV modules manufactured by Scheuten Solar Holdings and supplied across the world, including the UK, between August 2009 and February 2012. It is alleged that these have been manufactured with a design flaw in the junction boxes which have been the direct cause of a number of fires. The modules in question are the Multisol P6-48, P6-54, P6-60 and P6-66 versions of the Multisol module. The junction box is labelled “Solexus” and made by Dutch firm, Alrack. In October, 2012, Insurers acting for the now defunct Scheuten Solar Holdings decided that the matter should be dealt with by Suncycle, a German based company which specialises in test and repair services for the solar industry. Suncycle have been  charged with identifying potentially affected systems. Once the systems were identified, Suncycle would apply a TÜV (German) certified solution https://www.suncycle.eu/ 


In the event that these panels are identified in the field from the system specification on which no rectification work has been carried out since October 2012, the Consultant shall establish immediate telephone contact with the Underwriter, recommending that the system should be taken out of service until repairs are completed. At the same time the Head of Risk Engineering and Surveys shall be advised, in order that the issue can be monitored.

 

FURTHER REFERENCE


RISCAuthority RC62: Recommendations for fire safety with PV panel installations - The Joint Code of Practice for fire safety with photovoltaic panel installations, with focus on commercial rooftop mounted systems. This publication which has been developed jointly by the RISCAuthority and the MCS with the support of SEUK is essential reading and is available here.

 

In support of RC62, the RISCAuthority has subsequently published Need to Know Guide RE3: Rooftop-mounted PV Solar Systems which should be viewed as an initial point of reference as far as key risk control measures are concerned relative to such installations. This has been posted in ATLAS.


Extensive information is also available on the MCS and SEUK websites.

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