scope:

Introduction

This guide is intended for installers, distributors, inspectors and anyone who has an interest in the correct specification of DC isolators - a critical component for the safe design and installation of photovoltaic (PV) systems.

The scope covers standard isolators for small/medium size PV systems, generally building-mounted.

Since the introduction of the Feed-in Tariff (FiT) in the UK in April 2010, PV power systems have become commonplace. In the vast majority of cases, these are safe, reliable systems, with no moving parts and the system owners can look forward to 20 to 30 years of electricity production from sunshine. The technology and installation techniques are quite mature since Germany has been leading the way for over 20 years.

One effect of the introduction of FiTs has been a rapid increase in the number of installers and distributors. This trend has been echoed worldwide and a similar expansion in numbers has occurred in the manufacturing sector, notably in China and the Far East. However, more recently many governments (with only a few exceptions) have made large cuts to their incentive schemes, leading to increased competition amongst installers and manufacturers, creating pressure to drive down the cost of PV systems.

This pressure has manifested itself in various ways, including the search for cheaper component parts, and in some cases, this 'value engineering' has resulted in some shortcuts being taken and a resulting effect on quality and safety.

For reasons of maintenance or fault-finding, isolators are required on both the direct current (DC) and alternating current (AC) sides of the system (PV systems are inherently DC and they use an inverter to convert the current to AC so that it can be used with standard mains electricity systems).

BRE was recently sent an example of a DC isolator which had malfunctioned and filled up a room with smoke in a few seconds. The electrical arcing inside the unit burnt through the side of the enclosure before it could be disconnected. The cause - most probably an incorrectly specified isolator. Fortunately, in this case, a solar engineer was on hand to deal with the dangerous situation, but it is unnecessary to describe in detail the potential consequences for life, property and the reputation of the solar industry from such incidents. This was not an isolated incident; BRE has been made aware of several others involving isolators used for switching DC currents in PV systems.

In the UK we are fortunate to have the Microgeneration Certification Scheme (MCS)^[3], which certifies both installers and products. The MCS requires installers to use appropriately rated DC components for all DC functions, but the MCS product scheme does not currently certify isolators. Moreover, it is difficult for MCS assessors, when inspecting installations, to ascertain the suitability of the DC isolators used since they are often not marked with a make and model number.

To make matters worse, there is anecdotal evidence that some suppliers have been re-designating some types of AC isolator as DC-rated in order to be able to supply a low cost item. Little wonder that some installers have been using incorrectly specified devices.