Photovoltaic (PV) panels are a common sight on the roofs of domestic properties, in towns and cities across the UK. So much so, it seems likely that most electricians who undertake domestic work will at some point encounter an electrical installation that has a PV system connected to it.
In such circumstances, the risks associated with an installation that is connected to an additional supply source must be recognised. This article gives an overview of a typical PV installation.
The installation of PV supply systems are carried out by contractors who are registered to undertake microgeneration work (systems up to 16 A). The systems being installed in accordance with the relevant requirements of BS 7671, particularly Section 712, Solar photovoltaic (PV) power supply systems, and those of Section 551, Low voltage generating sets.
However, where electrical work, such as the addition of a new circuit or the replacement of a consumer unit, is carried out on an existing domestic installation that has a PV system connected to it, the contractor may not necessarily be familiar with an installation connected to an additional source of supply. In such circumstances, particular care needs to be taken to confirm that all sources of supply have been securely isolated before starting work.
Supply arrangements
A PV system is an additional power source which supplies the electrical installation, and can be arranged to operate as a switched alternative (standby) to the mains supply, or used as a stand alone system to supply an installation that does not have a mains supply. However, generally those installed in domestic premises are designed to operate in parallel with the mains electricity supply whereby at any time the PV system generates more electric power than is being used at the premises the surplus will be exported to the utility grid.
As shown in Fig 1, the PV system incorporates a number of PV modules which convert the energy of solar radiation emitted by the sun into electrical energy by means of the photovoltaic effect. The modules are connected into series ‘strings’ to provide the required output voltage and arranged into one or more arrays. An array may include several strings connected in parallel to provide the required current, or just one string.
Connection to the electrical installation
To supply the electrical installation, the DC output from the modules is converted to AC by a power inverter unit which is designed to operate in parallel with the incoming mains electricity supply to the premises, and as such is commonly known as a ‘grid-tie’ inverter. The AC output of the PV inverter (the PV supply cable) is connected to the load (outgoing) side of the protective device in the consumer unit of the installation via a dedicated circuit (Regulation 712.411.3.2.1.1 refers). If the PV supply cable is concealed in a wall or partition, additional protection is required in accordance with the requirements of Regulations 522.6.102 and 522.6.103.
Shock risk on the DC side
PV modules will generate a voltage whenever subjected to daylight so PV equipment on the DC side of the inverter must be considered energised even when disconnected from AC side (Regulation 712.410.3 refers). The protective measure against electric shock on the DC side of the inverter is normally provided by using cables of double or reinforced insulation (Section 412).
The use of automatic disconnection of supply (Section 411) is not generally practicable on the DC side because the current flowing under fault conditions is unlikely to be sufficient to operate overcurrent devices.
A PV system is an additional source of supply, so both the mains supply and the PV supply must be securely isolated before electrical work is performed on the installation. For these reasons, BS 7671 requires warning notices to be provided at relevant points within the installation, such as at the origin, points of isolation of the additional source and at other relevant points listed in Regulation 514.15.1. In addition, warning labels should be provided on junction boxes (Regulation 712.537.2.2.5.1 refers).
Isolation
For the purposes of isolation between the mains supply and the PV supply, the PV system should be considered as a load. Disconnecting the AC supply to the inverter will cause the inverter to shutdown. However, to allow maintenance work to be safely carried out on the inverter a means of isolation should be provided on both the DC and AC side of the inverter (Regulation Group 712.537 refers). In all cases it is essential to ensure that the PV system is securely isolated from the AC installation.
At least simple separation is required between DC and the AC sides of the PV supply system to prevent DC fault currents from being fed to the AC side. Where this separation cannot be achieved, any RCD installed to provide fault or additional protection for the PV supply cable is required to be type B (Regulation 712.411.3.2.1.2 refers).
Inverters for mains-connected PV systems should be type approved to the Energy Networks Association’s Engineering Recommendation G83/1 (for systems up to 16 A).
NICEIC operates a Microgeneration Certification Scheme (MCS) which covers the design installation and testing of environmental technology installation work associated with dwellings. For contractors interested in becoming registered to install PV systems, or other microgenration systems, training courses are available from NICEIC.
For further information on the connection of microgeneration systems refer to: Best Practice Guide No. 3, Connecting a microgeneration system to a domestic or similar electrical installation, which is available to download from: www.electricalsafetyfirst.org.uk
