Updates to the UK Solar PV Guide

The Solar PV Guide was originally written for those working in the UK solar industry in 2002, with the second Edition released in 2006. Over the last 18 months, since the UK solar market has progressed to more than 1GW of installed capacity, the MCS PV working group has been working on a new version which will be released over the next few weeks. The following article summarises the main changes occurring.

Perhaps the most significant change in the document is a complete overhaul of the performance estimate method. The old system was based on SAP (Standard Assessment Procedure) and used one set of solar data and applied it to the whole of the UK. This methodology was considered both too crude and one that underestimated the performance of real systems.

The new procedure utilises a series of regional solar data tables – one for each of 25 defined postcode zones. Each data table provides kWh/kWp (kilowatt-hour generated - per kilowatt peak installed) data for that zone for variations in array orientation and pitch. Short versions of the tables will be provided in the guide but the full versions will be available as a download from the Microgeneration Certification Scheme(MCS) website.

For an unshaded site, the new procedure is very straightforward – multiply the size of the proposed system (kWp) by the kWh/kWp figure taken from the table. For un-shaded sites, this procedure is actually simpler than the original and was one of the first goals set when pulling the Guide together.

The final part of the new system sets out a procedure for calculating a shade factor. The previous method, based on SAP, was fairly crude and was also very hard to audit by the industry’s Certification Bodies. The new procedure defines a method for calculating a shade factor based on the use of a sun-path chart. This procedure is too complicated to describe in brief, but has been designed to be straightforward as it can be – without completely sacrificing accuracy. More details on this can be found in the updated Guide.

Earthing and Bonding

One of the most frequently debated PV topics, earthing and bonding, has once again been examined in depth in the new Guide. The most obvious change is that the new earthing and bonding decision tree does not differentiate between systems with transformer and transformer-less inverters(unlike the old one which required a very different approach). In most cases, users will probably find that protective equipotential bonding of the array frame will not be required.

RCDs

There has been no change to the intent of the RCD section; however new wording will hopefully clarify what is often a confusing area for many installers.

Aligning with IEC 62548

IEC62548 – Design Requirements for Photovoltaic (PV) Arrays is a draft international standard that is approaching its final version. While it may be a while until it is potentially issued as a BS standard in the UK, we have been working to make sure that the new UK PV Guide is aligned as far as possible with IEC62548. A particular example of this is the section on string fuses:

String fuses

While the old Guide required string fuses for systems with four or more parallel strings, the new Guide requires fuses where: (N – 1)×Isc> module maximum series fuse rating(where N is the number of parallel strings).

Like the old version, fuses must be installed in both positive and negative string cables. Fuses must be of a type gPV and must be selected with an operating current in such that:

•In> 1.5 x Iscstc
•In ≤ 2.4 x Iscstc
•In ≤ Maximum module series fuse value

Isolation

The requirements for the DC isolator have also been drawn from the draft of IEC62548. The new guide details various options for providing the DC isolator– including the exact circumstances that a DC isolator built into an inverter is an acceptable solution. As many inverters are now supplied with a DC isolator, this will provide a significant time and cost saving to installers (a separate isolator was required in all cases by the old guide). Requirements for DC isolators on systems using micro inverters are also detailed in the guide.

Aligning with new G59 & G83

Since the last guide, Engineering Recommendations G59 and G83, which cover the grid connection of PV systems, have been re-issued. The new PV Guide reflects the changes to these two key documents.

Structural calculations and roof works

While providing no new requirements, the updated Guide does provide clarification and more information on the means to determine the wind and other loads on a PV system and the structure it is mounted to. The Guide sets out a simplified method for determining wind loads on a PV system based on the Eurocode-1 (BS EN 1991-1).

The Guide also incorporates the changes that the release of MCS012 heralds, as well as providing clarification on various aspects of roof work, including a section that details in what (limited) circumstances bolt through fixings can be used on slate roofs.

The above only summarises some of the main changes – there are any number of others issues that could have been included here. For more information you will need to download the new guide.

This article first appeared in Solar Business Focus UK magazine. To read more like this subscribe today!