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Due to the prevalence of photovoltaic (PV) panels on residential rooftops throughout the UK, electricians working on domestic projects will no doubt have come across electrical installations connected to PV systems at some point. Whilst common, it is important to acknowledge the risks associated with such installations linked to an additional power source.
Qualified contractors registered to undertake microgeneration work (up to 16 A) are responsible for installing PV supply systems. These systems must comply with the relevant requirements outlined in BS 7671, particularly Section 712 (Solar photovoltaic power supply systems), and Section 551 (Low voltage generating sets). However, when conducting electrical work on an existing domestic installation with a connected PV system, the contractor may not be familiar with an installation linked to an additional power source. In such cases, it is crucial to exercise caution and ensure that all supply sources are securely isolated before commencing any work.
Supplying the electrical installation
A PV system serves as an additional power source that supplies electricity to an electrical installation. It can be configured as a switched alternative (standby) to the mains supply or operate as a standalone system for installations without a mains supply. Generally, PV systems installed in residential premises are designed to operate in parallel with the mains electricity supply. Whenever the PV system generates more power than the premises require, the excess electricity is exported to the utility grid.
A PV system can consist of several PV modules that convert solar radiation into electrical energy through the photovoltaic effect. These modules are connected in series to provide the necessary output voltage and are arranged in one or more arrays. An array may include multiple parallel-connected strings to deliver the required current or simply a single string.
To supply electricity to the electrical installation, the DC output from the PV modules is converted to AC by a power inverter unit known as a grid-tie inverter. This inverter operates in parallel with the incoming mains electricity supply and is connected to the load (outgoing) side of the consumer unit's protective device in the installation via a dedicated circuit (Regulation 712.411.3.2.1.1). If the PV supply cable is concealed within a wall or partition, additional protection is necessary in accordance with the requirements stated in Regulations 522.6.102 and 522.6.103.
Isolation and protection against electric shock
PV modules generate voltage whenever exposed to daylight. Therefore, PV equipment on the DC side of the inverter should be considered energised even when disconnected from the AC side (Regulation 712.410.3). Protection against electric shock on the DC side is typically achieved by using cables with double or reinforced insulation (Section 412). Automatic disconnection of supply (Section 411) is generally impractical on the DC side since fault currents are unlikely to be sufficient to trigger overcurrent devices.
Since a PV system serves as an additional power source, both the mains supply and the PV supply must be securely isolated before conducting any electrical work on the installation. For this reason, BS 7671 mandates warning notices at relevant points within the installation, such as at the origin, points of isolation of the additional source, and other specified locations (Regulation 514.15.1). Furthermore, warning labels should be affixed to junction boxes (Regulation 712.537.2.2.5.1).
Isolation between the mains supply and the PV supply requires treating the PV system as a load. Disconnecting the AC supply to the inverter will cause it to shut down. However, to enable safe maintenance work on the inverter, isolation must be provided on both the DC and AC sides (Regulation 712.537). It is essential to ensure that the PV system is securely isolated from the AC installation.
At a minimum, simple separation is necessary between the DC and AC sides of the PV supply system to prevent DC fault currents from reaching the AC side. If such separation cannot be achieved, any residual current device (RCD) installed for fault or additional protection of the PV supply cable must be of type B (Regulation 712.411.3.2.1.2).
Inverters used in mains-connected PV systems should comply with the Energy Networks Association's Engineering Recommendation G98 (for systems up to 16 A).