This is the third and final article in the series, looking at the AC side of a solar PV installation, and in particular the requirements of both Sections 551 and 712 of BS 7671.
The general requirements of Section 551apply to low voltage generating sets including, but not limited to, photovoltaic cells and batteries (551.1.1).Such forms of generation may be combined and used to provide an embedded renewable source of energy within an installation, while operating independently and/or in parallel with other sources, typically the public supply network. This type of arrangement, in which the employed embedded generation and energy storage system is able to both produce and consume electrical energy, is described in the new Chapter 82 of BS 7671 as a 'Prosumer's low-voltage electrical installation' (PEI).
Future issues of Connections will consider the operational characteristics and types of PEI in more detail.
Multiple sources of supply
Where multiple supplies are available within an installation, several characteristics must be determined for each energy source (132.2),for example, the prospective short-circuit current and prospective earth fault current (551.2.2).
Any chosen generating set falling within the scope of Section 551should be installed such that:
- It does not impair the safety or proper functioning of other sources of supply (551.2.1);and
- AnyRCDs installed within the installation, in accordance with Chapter 41, remain effective for every intended combination of sources of supply (551.4.2).
Current rating of equipment
In a domestic installation, connection of the PV supply cable from the inverter is typically via a dedicated 'way' in a consumer unit, or from an additional consumer unit provided specifically for the PV system and for those essential circuits requiring a back-up supply from the means of embedded generation.
The current rating of the overcurrent protective device for the supply cable to the inverter should be determined by taking account of the design current for the equipment. This may be either the AC maximum current provided by the inverter manufacturer, if available; or alternatively may be taken as1.1times the AC current rating of the inverter (712.433.104).
Additionally, where agenerating set is used in parallel with the public distribution supply system as a source of energy, the requirements given in Regulation Group 551.7 shall be met.
It is recognised that some generator sets may be connected on either the supply side or the load side of all overcurrent protective devices for final circuits of the installation (551.7.2). However, on the
AC side of a solar PV installation, the PV supply cable shall be connected to the supply side of the overcurrent protective device providing automatic disconnection of supply (ADS) for circuits supplying current-using equipment (712.551.7.2).
Additionally, where such PV equipment is installed, isolation shall be provided for both the AC and DC sides of the inverter (712.537.2).
It must be borne in mind that when selecting distribution boards and the like for such installations, it is important to differentiate between the rating of the main switch and the rating of the assembly, to ensure that the assembly is not
overloaded when taking multiple sources of supply.
Where a generating set is connected via a low voltage switchgear and controlgear assembly,such as a distribution board,it shall be selected such that:
InA -'In +Ig(s)
Where:
InA - istherated current of the assembly, In - istherated current or current setting of the incoming circuit overcurrent protective device either incorporated within the low voltage switchgear and controlgear assembly or upstream ofit, and Igcs>- is the rated output current of the generating set(s) (see Fig 1).
Furthermore,where such PV and battery storage systems are employed to provide an alternative source of supply, the installation design must ensure adequate availability of capacity to meet the expected energy demand for the connected load(s).
There are several different software tools and procedures available for estimating the performance expected from a grid-connected PV system. However, systems installed under the Microgeneration Certification Scheme (MCS) may need to use specific calculations and alternative methods for estimating system performance.
Fault protection on the ACside
Where the protective measure of automatic disconnection of supply (ADS) is used, it shall be provided for installations in respect of each source, or combination of sources of supply in use,and shall operate independently of other sources or combinations of sources (Section 411), except where modified by the requirements stated in Regulation Group 551.4.
In addition, where fault protection is to be achieved in different ways within the same or part of an installation in relation to the adopted source of supply, the effectiveness of the fault protection provision shall not be compromised or influenced by any such changing conditions within the installation (551.4.1).
For example, where a PV system combined with an EESS generating more than 16 A per phase is required to operate in isolation from the grid (island mode) as a switched alternative source to that of the TN public supply system, protection by ADS shall not rely upon the distributor's means of earthing (551.4.3.2.1).
The recommendations of G99 Requirements for the connection of generation equipment in parallel with public distribution networks published by the Energy Networks Association require a suitable earth electrode system independent from the earthing arrangement for the installation to be provided. In addition, the electrode shall be of a type listed in regulation 542.2.2.
However, any connection derived from the means of earthing from the public supply system need not be disconnected during island mode operation (826.1.1.2.1). 
Fault protection is generally provided for those essential circuits or parts of the installation supplied by the static inverter while relying on the automatic closure of the grid bypass switch (to be considered in a future article). However, under such conditions, protection against thermal effects in accordance with Chapter 42, and protection against overcurrent meeting the requirements of Chapter 43, shall remain effective in all situations (551.7.1).
Where fault protection and operation of protective devices may not be within the time required by Section 411, Regulation 551.4.3.3.1 requires supplementary equipotential bonding to be provided between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts on the load side of the static inverter (415.2).
Furthermore, the operation of protective devices shall not be impaired by DC currents generated by the static inverter or any associated filters (551.4.3.3.2).
RCD protection
Additional protection from an RCD meeting the requirements of Regulation 415.1.1 may be required where the supply cable to the inverter is installed in the fabric of the building at a depth ofless than 50 mm from the surface. The requirements ofboth 522.6.202 and 522.6.203 should also be considered where appropriate.
An RCD that is installed for additional protection, should disconnect all live conductors including the neutral conductor (551.7.1 (ii)).
Where the PV inverter does not have simple separation provided between both the AC and DC sides of the transformer, or where simple separation is not provided between the inverter and RCD, the RCD used to provide fault or additional protection must be suitable for operation in the presence of a DC component in the residual current. This will rule out the use of Type AC and Type A RCDs.
The RCD provided for protection of the PV AC supply circuit must be of Type B to BS EN 62423 or BSEN60947-2 (712.531.3.5.12).
However, where the manufacturer's instructions for a particular inverter state that a Type B device is not required, or where galvanic isolation is provided by a transformer separating both the AC and DC sides, this requirement need not be applied.
Protection against overvoltage
Where protection against transient overvoltage meeting the requirements of Section 443 is required for the installation, Regulation 712.443.101 also requires
provision of overvoltage protection for the DC side of the PV installation.
Typically, Type 2 SPDs shall be used.
However, where protection against effects from direct lightning strikes is specified and the requirements for a minimum separation distance between a lightning protection system (LPS) and a solar PV installation in accordance with BS EN 62305-3 is not achieved, a Type 1 SPD shall be used in conjunction with Type 2 devices (712.534.101and 712.534.102.1).
For both the AC and DC sides of the PV installation the copper conductors connecting the SPD to the main earthing terminal (MET) shall have a minimum cross-sectional area of 6 1111112 for Type 2 SPDs and 16 1111112 for Type 1 SPDs. Additionally, where such types of devices are used and contained within separate switch board assemblies, the separation distance between assemblies should be kept as small as possible (712.534.105).
Summary
This article considers some of the fundamental changes introduced in AMD 2 in respect ofrequirements for the AC side of a solar PV system. Where different sources of supply are available within an installation, it is important to ensure that the method of protection adopted remains effective for every intended combination of sources of supply and is not liable to impair the safety of proper functioning of the other sources.
