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Optimum system tolerance is achieved with single-string source circuits and large
                          numbers of bypass diodes. Field studies indicate that the best approach in large
                          systems is not to replace modules containing failed cells, but to design the system to
                          be tolerant to such failures.


                          10.5 SAFETY
                          Many safety issues are common for building-integrated, building-mounted or central
                          station grid-connected PV systems.

                          Safety aspects that need to be considered (Florida Solar Energy Centre, 1987; Abella
                          & Chenlo, 2004) include fire resistance, correct wiring, placement, grounding, and
                          security against local weather conditions, particularly wind. Modules can be graded
                          according to their effective resistance against severe, moderate or light fires (Florida
                          Solar Energy Centre, 1987).
                          Simple disconnection of large (high voltage) arrays from loads or inverters does not
                          make them safe since they are live whenever illuminated. Protection of the DC side is
                          controversial and regulations vary between countries. Floating (unearthed) arrays and
                          inverters are commonly used in Europe but earthing is mandatory in the USA.
                          Commercial systems are only now available to meet the various specifications that
                          differ from place to place. Some of these are listed in Appendix E.
                          The Australian Standard (Standards Australia, 2005, 2002a, 2002b) defines the safety
                          requirements in Australia. The inverter system must be able to be isolated from live
                          conductors by a labelled and lockable switch, which is lockable in the off position to
                          interrupt all live conductors. The inverter must be connected on the utility side of any
                          residual current devices (RCDs) rather than the provision of an RCD on the output of
                          the inverter. An isolation device is necessary on each inverter input from the array
                          and various labelling requirements are specified (Standards Australia, 2005).
                          Protective features are described in an Australian Standard (Standards Australia,
                          2005):
                              1. Blocking diodes and over-current devices—As with any generating system,
                                 protection against large current flows must be in-built. Blocking diodes and
                                 over-current devices (e.g. circuit breakers or fuses) are used for protection in
                                 photovoltaic arrays. Blocking diodes protect against large current flows into
                                 shorts to ground, while over-current devices provide fuse protection in the
                                 event of failure of the blocking diode. Blocking diodes are not substitutes for
                                 over-current devices and are optional (Ibid.). Refer to Standards Australia
                                 (2005) or local standards for detailed recommendations and/or requirements
                                 for various array arrangements. The discussion in Chapter 6 and by Standards
                                 Australia (2005) also applies for grid-connected systems.

                              2. Array arcing—An open circuit in a high voltage branch of a solar array can
                                 produce voltages higher than the 70 V required to maintain an arc, as
                                 illustrated in Fig. 10.7. These can burn for hours, but can be prevented by
                                 introducing redundant connections, to prevent open circuiting and parallel cell
                                 connections.







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