It takes more than modules
Many articles about solar power tend to focus on the PV modules, which is hardly surprising as they are indisputably the key elements of the system. However, for a successful and safe PV installation, a lot more is needed than just the modules. Richard Molloy, Sustainability Segment Manager for Eaton’s Electrical Sector, and Martyn Berry, Principal Engineer at Solarcentury, explain further in this ten-point guide to PV systems for residential use.
Solar PV installations have to do more than simply produce energy. They have to transfer that energy in a suitable form to the loads and the grid, measure the amount of energy produced and, above all, operate safely under all conditions. To achieve this, a wide range of ancillary equipment is needed in addition to the PV array, as can be seen from this diagram of a typical residential installation (Figure 1).
The equipment is typically installed in three locations: the PV modules on the roof, the inverter (with its associated isolators and surge suppression components) in the loft space and the metering equipment and an emergency isolator near the consumer unit. Later in this article, we will look at each component of the ancillary equipment but, before doing so, it is useful to consider briefly the standards and regulations that apply to PV installations.
1. Standards and regulations
The most basic requirement for PV installations is that they must comply with the 17th Edition of the IEE Wiring Regulations, which is also designated BS 7671:2008. Additionally, however, the installations must be designed, installed and commissioned in accordance with the Microgeneration Installation Standard MIS 3002 Issue 2.0. This standard requires that the system be designed and installed in accordance with the DTI Guide, “Photovoltaics in Buildings – Guide to the installation of PV systems, 2nd Edition 2006.” PV installation work should therefore be carried out by contractors with MCS accreditation, who will guarantee compliance with the correct standards and legislation.
PV systems are also categorised as “special installations” in relation to Part P of the Building Regulations and are, therefore, subject to notification to and inspection by building control bodies. The DTI Guide specifically mentions the ancillary components used in PV installations and identifies some of them as mandatory and others as recommended. The MIS 3002 standard requires compliance with all mandatory items in the Guide and also expects compliance with items marked as recommended practice unless “reasonable justification can be given” for deviating.
Clearly, these ancillary components are important, so let’s look at them in more detail.
2. DC cables and connectors
Because the DC source (the PV array) will be live whenever there is light present, the use of specialist double-insulated solar cables to carry power from the array to the remainder of the installation is mandatory. Specialist solar connectors are strongly recommended as they further enhance safety. For residential systems that typically comprise fewer than four strings, string fuses are not mandatory, but the system and cable must be designed to withstand fault conditions.
3. DC isolator
A DC isolating switch to safely disconnect the DC source from the inverter is mandatory, and a switch specifically designed for solar applications must be used. Suitable switches are offered as loose components for mounting within an enclosure, or as enclosed versions that can be directly mounted on a wall or similar suitable surface.
4. DC Surge suppressor
This device protects the inverter against high voltage surges that may be produced in the PV modules when lightning strikes nearby. Its use is not mandatory, but it is a desirable addition to installations, particularly in exposed or isolated locations where there is a higher than usual risk of lightning strikes. Surge suppressors work by sacrificing themselves to protect the inverter, so they must incorporate an indicator to show whether they are still functional or whether they need replacing. Some types use replaceable cartridges.
5. Inverter
This converts DC power from the solar array to AC power suitable for connection to domestic loads and the supply network. It is not a mandatory product in regulatory terms, but it is nevertheless essential, as the installation won’t work without it! The UK feed-in tariff payment for domestic installations is at its maximum for retrofit installations rated up to 4kW, however systems larger than 2kW are less common as installations are often limited by the available roof area for the PV array
6. Local AC isolator
Mounted close to the inverter, the local AC isolating switch provides a safe means of disconnecting the inverter from the AC supply. By using both the DC isolating switch and the local AC isolating switch, the inverter can be completely disconnected, making it safe to work on or replace. Fitting a local AC isolator is not mandatory, but it is recommended.
7. AC surge suppressor
This protects the inverter against high voltage surges that might appear on the AC supply network, and provides a degree of protection for other electrical equipment in the premises against surges originating in the PV installation. AC surge suppressors are not mandatory, but are strongly recommended, as surges on the supply network are surprisingly common.
8. Switch and surge units
The DC isolator, the local AC isolator and the surge suppressors are all typically installed adjacent to the inverter. Separate components can be used for these functions, but that makes installation time consuming and often leads to an untidy appearance for the finished job. Combined switch and surge units offer a better solution by provided all of the necessary switching and surge protection components in a single enclosure, prewired and fully tested. Solar connectors are provided for the DC supply, with convenient screw terminals for AC connections. This gives a safer, neater installation at lower cost, as well as ensuring that the special labelling requirements for components in PV installations are properly met.
9. AC isolator for emergency switching
Whether or not a local AC isolator is fitted close to the inverter, it is mandatory to fit an AC isolating switch for emergency switching. This switch, which is a critical safety component, is usually sited close to the consumer unit and must carry appropriate labelling and be padlockable in the off position. This switch is needed to allow safe working on the domestic wiring system.
10. Metering
Metering for PV installations is mandatory and systems are required to be fitted with two meters. The first, which must be an MID (Measuring Instrument Directive) approved revenue class meter, measures the total amount of energy produced by the PV installation. The second meter is normally the ordinary household electricity meter that measures how much energy the property has consumed (imported) from the electricity supply network. Newer models measure both import energy and export energy and at some stage this is likely to become mandatory.
Interest in solar power is growing rapidly, with the response in the residential sector being particularly enthusiastic. To ensure that solar power reaches its full potential, however, it is important that solar installations operate safely and reliably. The best way to do that is to partner high quality PV modules with appropriate ancillary components from knowledgeable and reputable suppliers such as Eaton and Solarcentury.
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