As rooftop photovoltaic arrays become a common sight in the UK, it's important to make them good-looking as well as safe and efficient.
You can picture the scenario – you're drilling into a wall to put up a shelf when there's a flash and the lights go out. Shorting the cables on a buried mains cable is best avoided, but at least the RCD or MCB in the feed to that cable keeps the event brief and hopefully minimises the hazard. Should the cable you inadvertently hit be one from a photovoltaic array, however, the result can be very different.
Shorting the DC cables from a PV array on your house roof won't cause a fuse to blow; in fact, the array will simply sit there merrily feeding current into the short circuit. Shorting the output of a PV array won't do any particular harm and has been used as a control strategy in some off-grid battery charging applications.
The short-circuit current of a PV array is not much greater than its operating current: for example, a typical domestic PV array in bright sunshine may have an operating current of 8A and a short-circuit current of 8.4A. This lack of a significant fault-current (for small arrays – it's slightly different for larger systems) means that adding a fuse or other overcurrent device to a circuit will not achieve any degree of protection in the case of a shorted cable. Hence, the vast majority of PV systems on UK homes will not have any fuses on the DC side as they serve no purpose and are not required by the installation standards. In the case of a short circuit, the only thing that will stop the fault is either isolating the cable or the Sun going down.
It is the job of PV installation standards and guidance to address potential hazards that such characteristics create. For example, standards will suggest that the DC cables from a PV array should not be buried in walls. There are a number of other key characteristics of a PV array that influence installation standards. For example, a PV array does not have a fixed voltage and current output, but one that varies considerably. Knowing what the maximum voltage the array could reach (which may be higher than the 'standard test condition' data) is vital when selecting system components. PV modules cannot be switched off: in daylight, the output of a PV module is always live. This has various ramifications, including a need to plan installations such that the risk of shock is mitigated during installation and maintenance.
I have been involved with PV for more than 20 years and have seen the industry change and grow. For a lot of this time I have been involved in installation standards and guidance. New applications and products mean that guidance needs regular updates. I am working with the IET on a new PV code of practice that will set out requirements for design, specification, installation, commissioning, operation and maintenance of grid-connected PV systems in the UK. It will address all parts of the system up to and including the connection to the AC mains and will cover both low-voltage and high-voltage systems.
While a PV array may have unique features to be covered by dedicated guidance, much of the content of documents such as the IET Wiring Regulations (BS761) is also vital in ensuring safe and reliable installations. The job of good PV design and installation guidance is to focus on the PV specific issues and then refer to other documents, such as BS7671, for everything else.
There are some issues that don't get addressed. PV is now everywhere, and I am pleased to say that an array is a common sight on UK roofs. I occasionally see some crazy or downright ugly systems on domestic properties. Aesthetics is not a typical component of technical guidance documents, though in the case of PV I wish it was. If you walk past an ugly array every day, it's not exactly going to encourage you to take the plunge. For long-term stability of our industry, I want systems to be both safe and aesthetically pleasing – it's not too much to ask is it?
Martin Cotterell FIET is founder of renewable energy company Sundog Energy (www.sundog-energy.co.uk). He chairs BSI and IEC solar PV technical committees. For details of the forthcoming IET Code of Practice for the Design, Installation and Operation of Solar Photovoltaic Systems see www.theiet.org/resources/standards/pv-cop.cfm
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